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Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2021, Tutor: Domènec Espriu Climent, In this work we study the possibility that the recently discovered particle at the Large Hadron Collider (LHC) with similar properties to the ones of the Standard Model (SM) Higgs boson and a mass around 125 GeV could be related to the dilaton, the Goldstone boson that appears when scale invariance is spontaneously broken. We will focus on the Coleman-Weinberg (CW) mechanism, in which the mass term from the Higgs potential is turned off, thus obtaining a theory classically invariant under dilatations. It is due to quantum effects that the spontaneous breaking of this symmetry occurs. We will study the viability of this scenario treating with the CW potential and doing numerical testing as well as proposing possible modifications of the SM

[eng] Recently lepton flavor universality tests in colliders have reported tensions with the Standard Model, providing an experimental hint at low energies of the effect of a new high energy theory. These tensions are known as flavor anomalies and that is why in this thesis we focus on the study of the impact of the flavor anomalies on the flavor sector of the Standard Model of particle physics. We introduce the Standard Model of particle physics and list the relevant tensions of the theory with the experiments in B decays, commenting in a brief manner the possible solutions involving new physics, as well as the main advantages and inconveniences of each new physics scenario. We first focus in a model with new heavy scalars and fermions were we only account for left-handed couplings to the Standard Model particles. We consider two possible models: one with an additional scalar and two vector-like fermions and another with two additional scalars and one vector-like fermion. The purpose of this model is to solve the tensions in FCNC decays, together with the anomalous magnetic moment of the muon, which are induced at loop-level in both models. We list the relevant constraints and show that the models are able to solve the anomalies, albeit a relatively large coupling of the muons is required. In the case of semi-leptonic decays, stringent constraints arising from B oscillations can be relaxed if the new fermions are considered Majorana particles. Then, since we cannot explain the muon magnetic moment with only left-handed couplings, we construct a model with new scalars and fermions allowing also for right- handed couplings, were we list the relevant Wilson coefficients for b quark decays to a strange quark and two muons, as well as the relevant observables acting as constraints for any number of new scalars and fermions. In order to illustrate this generic approach we supplement the Standard Model with a fourth generation of quarks and leptons. With this model we can explain FCNC anomalies and the magnetic moment of the muon avoiding all the constraints. The second kind of model that we explore is an extension of the Standard Model with scalar leptoquarks. In this case, we compute the Z- and W-decays to leptons for each one of the scalar leptoquarks at next-to-leading-logarithm approximation, and show that the finite terms can account for 20% of the total contribution for leptoquark masses of below 1.5 TeV. We also show their phenomenological relevance in a model with a singlet and a triplet, where our computation pushes the fit towards a better explanation of data. Besides, we comment on the fact that the B mixing has to be implemented carefully as it is one of the main constraints that was missing in earlier studies of these kind of models, and we also illustrate its key role in a particular model model, since it spoils the pure left- handed scenario with 2018 data. Finally, we scrutinize the FCNC process of b quark dcaying into an s quark and two leptons, in the framework of a two Higgs doublet model. We compute all the relevant Wilson coefficients performing the matching of the full theory with the low energy theory showing that it is necessary to keep the external momenta for the scalar and pseudo-scalar operators. This is the first time computation of a proper matching including all the relevant operators. We perform a phenomenological analysis of the model with the strange B meson decaying to two muons and the B meson decaying into two muons and a K meson at high momenta, where we have control of the hadronic uncertainties., [cat] Des del 2012, una serie de mesures en experiments com BaBar, Belle i LHCb, han presentat tensions respecte el model estàndard en decaïments de mesons B. Aquestes desviacions són conegudes com les anomalies de sabor. En aquesta tesi interpretem les anomalies de sabor com a possibles efectes de nova física i proposem alguns models simples per poder acomodar les dades experimentals que difereixen del model estàndard. En primer lloc, proposem un model amb nous escalars i fermions pesats que només s’acoblen als fermions tipus esquerra del model estàndard. En aquest escenari intentem explicar una part de les anomalies ensems amb el valor anòmal del moment magnètic del muó. El resultat són uns acoblaments relativament grans de valor 2 aproximadament. Pel que fa a resoldre part de les anomalies, tenim la possibilitat de considerar els nous fermions com fermions de Majorana, que permet reduir el valor dels acoblaments. Per tal de poder relaxar el valor dels acoblaments d’una forma més general, en el següent capítol proposem un model semblant però amb acoblaments tipus dreta amb el model estàndard. En aquest model calculem totes les fórmules d’una manera genèrica i posteriorment ho particularitzem en un model de quarta generació. Amb la introducció d’aquests acoblaments tipus dreta combinats amb els esquerra podem explicar part de les anomalies de sabor i també el moment magnètic del muó. Seguidament, construïm un model amb leptoquarks escalars on estudiem alguns dels lligams més importants, com els decaïments de Z o les oscil·lacions del mesó B. Posteriorment, proposem models fenomenològics de 2 leptoquarks escalars on veiem l’impacte dels lligams estudiats anteriorment. També discutim com els models quedarien obsolets si no fos perquè les dades experimentals van canviar subtilment després de Moriond 2019. Per últim, fem un estudi de l’impacte de dos decaïments: del mesó B estrany a dos muons i del mesó B al mesó K i dos muons dins d’un model de dos doblets de Higgs. Aquí no ens centrem en les anomalies, sinó que mirem l’impacte d’aquests observables en el model i realitzem una comparació adequada entre la teoria efectiva i la teoria a altes enegies.

[eng] At the end of the 20th century, experiments studying neutrinos coming from the Sun and cosmic rays hitting the atmosphere revealed that these particles can change their flavour, behaving in a way that the Standard Model (SM) explicitly forbids. The quest for understanding the properties of neutrinos, which may reveal the next underlying structure of Nature, has since then led thousands of scientists. The scenario when this thesis was initiated was driven by the latest experimental surprise that neutrinos provided: the initial hints towards their strong violation of the particle-antiparticle CP symmetry. Characterising the statistical significance, robustness and physical origin of that hint is the main goal that this work pursuits. For that, there is a rich programme of present and future long baseline (LBL) accelerator neutrino experiments. At the beginning of the development of this thesis, the LBL accelerator neutrino experiment NOvA released its first data. To obtain a global picture, this work combines it with the results of other relevant neutrino experiments. The status of leptonic mixing and CP violation is quantitatively assessed as LBL accelerator neutrino experiments kept releasing data. As the unknowns start getting clarified, the data points towards maximal CP violation. This hint is driven by an excess of electron neutrino appearance events in the LBL accelerator experiment T2K. In the three massive neutrino paradigm and with the other leptonic mixing parameters accurately measured by different experiments, the excess can only be accommodated by large CP violation. Nevertheless, three massive neutrinos is just a minimal extension of the SM: other new physics could be present, masking the results as direct leptonic CP violation has not yet been conclusively observed. This thesis confronts with data the scenario that is less bounded by other experiments: neutral current Non-Standard Interactions (NSI) among neutrinos and matter. Physically, they are generated by new interactions among neutrinos and matter mediated by potentially light particles. Due to the large parameter space involved, first just CP-conserving NSI (i.e., their moduli) are explored. Current bounds are evaluated, as well as the synergies and complementarity among different experiments. Thanks to the experiments working with various neutrino energies and travelled distances, the determination of leptonic mixing parameters is found to be quite robust. Thus, it is possible to move on and evaluate the current sensitivity to leptonic CP violation assuming the most generic CP-violating NSI are present. CP violation induced by neutrino masses and leptonic mixing is found to be quite robust, due to the large amount of neutrino flavour transition data collected along three decades. Nevertheless, the next generation LBL accelerator experiments are aimed at precision measurements that could be more severely affected. Luckily, in the last years the COHERENT experiment has provided independent constraints on NSIs. This experiment measures neutral current coherent neutrino-nucleus elastic scattering, a low momentum transfer process quite sensitive to NSI induced by potentially light mediators. In the final chapter of the thesis, the data from the COHERENT experiment is analysed and integrated into the global analyses from previous chapters. Particular attention is paid to how the results depend on the assumptions about the experiment background, nuclear structure, and detector response. Combining COHERENT data with flavour transition experiments unveils its incipient role in increasing the robustness of their interpretation. These first results could be greatly improved by increasing the statistics of the signal and/or by performing the measurements with different nuclei sensitive to different NSI models. For that, the European Spallation Source is an ideal future facility. It will produce a neutrino beam one order of magnitude more intense than the one used at COHERENT, and as it is still under construction there is potential space for various modern detectors. Its prospects for bounding NSI are also explored. In summary, this thesis deals with the current experimental hint for large CP violation in the leptonic sector. It first quantifies its global significance, and then moves on to checking its robustness against the framework in which the experimental data is interpreted. For that, complementary experiments on neutrino-nucleus coherent scattering play, and will keep on playing in the future, a significant role. Thus, a global approach is taken to rigorously assess whether cutting-edge leptonic flavour measurements are pointing towards a new strong violation of a symmetry of Nature., [spa] A finales del siglo XX, una serie de experimentos que estudiaban neutrinos provenientes del Sol o de rayos cósmicos que colisionaban contra la atmósfera terrestre mostraron que estas partículas pueden cambiar su sabor, comportándose de una manera que el Modelo Estándar prohíbe explícitamente. El camino hacia la comprensión de las propiedades de los neutrinos, que podría revelar la siguiente estructura subyacente de la naturaleza, ha guiado desde entonces a miles de científicos. El escenario al inicio de esta tesis estaba impulsado por la última sorpresa experimental de los neutrinos: los primeros indicios que apuntan hacia su fuerte violación de la simetría materia-antimateria o CP. El objetivo de este trabajo es abordar el problema desde una perspectiva global para evaluar rigurosamente si las medidas punteras de física de sabor leptónico están apuntando hacia una nueva violación fuerte de una simetría de la naturaleza. Para ello, se combinan los resultados de todos los experimentos de neutrinos relevantes. Se evalúa cuantitativamente el estatus de la mezcla leptónica y de la violación de CP, que los datos apuntan a que podría ser máxima. Este indicio está dominado por un exceso de neutrinos electrónicos en el experimento de neutrinos con acelerador a larga distancia T2K. Dentro del paradigma de tres neutrinos masivos, y con el resto de parámetros de mezcla leptónica medidos con precisión en varios experimentos, el exceso solamente se puede acomodar mediante una violación de CP grande. A pesar de esto, tres neutrinos masivos es solamente una extensión mínima del Modelo Estándar: podría haber otra nueva física enmascarando los resultados, ya que la violación de CP leptónica aún no se ha medido de manera directa y concluyente. Por ello, esta tesis confronta la nueva física que podría afectar a los experimentos de transiciones de sabor de neutrinos con datos experimentales. La violación de CP inducida por las masas de los neutrinos y por la mezcla leptónica resulta ser bastante robusta. Tal y como se explora en este trabajo, los experimentos complementarios de interacción coherente entre neutrinos y núcleos juegan, y continuarán jugando en el futuro, un papel importante a este respecto.

[eng] This thesis studies Dark Energy, a central topic of Modern Cosmology, from different points of view. It covers from models and parametrizations to likelihood approximations for the observations of the Large Scale Structure (LSS) of the Universe, which are vital for model constraining, as well as for our study of the S8 tension. The studied model is the α-attractors dark energy model, which reproduces current observations thanks to its capability to mimic a cosmological constant. It is inspired on the Planck-favored inflationary class of models α-attractors and links the inflationary and dark energy periods with the same scalar field. Fortunately, next-generation surveys might be able to distinguish it from ΛCDM by up to 3σ of confidence. Nevertheless, in order to make the most of forthcoming data, we need a formalism that allows us to test models in an efficient way. This could be a observational and theoretical constrained parametrization. We show the case of w0 -wa for thawing quintessence. This parametrization is able to reproduce the observables from recombination to the present when chosen smartly. In addition, their probability distributions built from purely theoretical considerations allow us to restrict the observational bounds to the theoretically motivated area. This work shows how we can work with very general theories such as Horndeski. However, the observational constraints also depend on the correct estimation of their likelihood. Therefore, we have developed (and implemented in NaMaster, from LSST DESC) the Narrow Kernel Approximation (NKA) for the Gaussian (not causally connected) part of the LSS pseudo-Cl covariance matrix. This allows to reproduce accurately the cosmological parameters’ posterior distribution while reducing the computational cost from O(lmax6) (exact computation) to O(lmax3). For sure, the exact computation will be intractable for the next-generation LSS observations, which will resolve really small scales. The NKA has allowed us to start our study of the origin of the S8 tension reported by KiDS-450. With data of LSS from DES and data of CMB from Planck, we want to obtain the temporal evolution of S8 and compare it with that given by KiDS-450 data. Summing up, this thesis has covered dark energy models and developed accurate methods to test their viability in an efficient way. Furthermore, it has gone closer to data and developed the NKA that allows to accurately estimate the LSS likelihood, making it possible for us to study the origin of the S8 tension., [spa] Esta tesis estudia la energía oscura, un tema central en la Cosmología Moderna, desde diferentes puntos de vista. Cubre desde modelos y parametrizaciones, hasta aproximaciones de la likelihood de observaciones de la Estructura a Gran Escala del Universo (LSS), vitales para constreñir los modelos, como para nuestro estudio de la tensión en S8. El modelo estudiado es el de energía oscura de α-attractors, que reproduce las observaciones actuales al poder asemejarse a una constante cosmológica. Inspirado en la clase de modelos de inflación favorecidos por Planck, α-attractors, enlaza el periodo de inflación y energía oscura mediante el mismo campo escalar. Afortunadamente, las misiones futuras podrían diferenciarlo de ΛCDM con hasta 3σ de confianza. No obstante, aprovechar al máximo los datos futuros requiere un formalismo que nos permita comprobar la viabilidad de los modelos eficientemente, como una parametrización acotada observacional y teóricamente. Mostramos el caso de w0-wa para quintaesencia de tipo thawing, que reproduce los observables desde recombinación hasta hoy, si se eligen apropiadamente. Construidas sus distribuciones de probabilidad bajo condiciones puramente teóricas, nos permiten restringir las cotas observacionales a la zona teóricamente motivada. Este trabajo es una muestra de cómo podemos trabajar con teorías muy generales como Horndeski. Pero las cotas observacionales dependen también de una correcta estimación de su likelihood. Por ello, hemos desarrollado (e implementado en NaMaster, del LSST DESC) la Narrow Kernel Approximation (NKA) para la parte gausiana (causalmente no conectada) de la covarianza de los pseudo-Cl de LSS. Ésta permite obtener con exactitud las cotas en los parámetros cosmológicos, al tiempo que reduce el coste computacional de O(lmax⁶) (cómputo exacto) a O(lmax3). Claramente, el cómputo exacto es inviable para las observaciones futuras de LSS que resolverán escalas muy pequeñas. La NKA nos ha permitido empezar a estudiar el origen de la tensión en S8 reportada por KiDS-450. Con datos de LSS de DES y del CMB de Planck, pretendemos obtener la evolución temporal de S8 y compararla con la dada por los datos de KiDS-450. En resumen, esta tesis ha trabajado en modelos de energía oscura y desarrollado métodos precisos para comprobar su viabilidad de manera eficiente. Además, se ha acercado a los datos y desarrollado la NKA que permite estimar precisamente la likelihood para LSS; permitiéndonos estudiar el origen de la tensión en el parámetro S8.

[eng] The proper understanding of the equation of state (EoS) of highly asymmetric nuclear matter is essential when studying systems such as neutron stars (NSs). Using zero-range Skyrme interactions and finite-range interactions such as Gogny forces, momentum-dependent interactions (MDI) and simple effective interactions (SEI), we analyze the properties of the EoS and the influence they may have on the calculations for NSs. We start by studying the convergence properties of the Taylor series expansion of EoS in powers of the isospin asymmetry. Next, we analyze the accuracy of the results for β-stable nuclear matter, which is found in the interior of NSs, when it is computed using the Taylor expansion of the EoS. The agreement with the results obtained using the full expression of the EoS is better for interactions with small-to-moderate values of the slope of the symmetry energy L. The mass and radius relation for a NS is obtained by integrating the so-called Tolman-Oppenheimer-Volkoff (TOV) equations, where the input is the EoS of the system. We have studied the mass-radius relation for Skyrme and Gogny interactions, and we see that that very soft forces are not able to give stable solutions of the TOV equations and only the stiff enough parametrizations can provide 2M0 NSs. We also notice that none of the existing parametrizations of the standard Gogny D1 interaction is able to provide a NS inside the observational constraints. Because of that, we propose a new parametrization, which we name D1M∗, that is able to provide NSs of 2M0 while still providing the same good description of finite nuclei as D1M. A parametrization D1M∗∗ is also presented, which is fitted in the same way as D1M∗ and provides NSs up to 1.91M0. Moreover, we estimate the core-crust transition in NSs by finding where the nuclear matter in the core is unstable against fluctuations of the density. To do that, we employ two methods, the thermodynamical method and the dynamical method. In the case of finite-range interactions, such as the Gogny ones, to use the dynamical method we have had to derive the explicit expression of the energy curvature matrix in momentum space for this type of interactions. We observe a decreasing trend of the transition density with the slope L of the symmetry energy, while the correlation between the transition pressure and L is much lower. Finally different NS properties are studied. The crustal properties, such as the crustal mass, crustal thickness and crustal fraction of the moment of inertial have lower values if one computes them using the core-crust transition density obtained with the dynamical method instead of the one obtained with the thermodynamical method, pointing out the importance of the accurate evaluation of the transition density when studying observational phenomena. We have also studied the moment of inertia of NSs, which is compared to constraints proposed in the literature. Finally, the tidal deformability for NSs is also calculated and compared with the constraints coming from the GW170817 event detected by the LIGO and Virgo observatories and which accounts for the merger of two NSs in a binary system., [cat] El coneixement de l’equació d’estat (EoS) de matèria altament densa i assimètrica és essencial per tal d’estudiar les estrelles de neutrons (NSs). En aquesta tesi s’analitzen, utilitzant interaccions de camp mig no relativistes, les propietats de l’EoS i la seva influència en càlculs de NSs. Primerament, s’estudia la convergència del desenvolupament en sèrie de Taylor de l’EoS en potències de l'assimetria d’isospí. Seguidament, s’analitza l’exactitud dels resultats per matèria β-estable, la qual es troba a l’interior de les NSs, quan es calcula utilitzant el desenvolupament de Taylor de l’EoS. La relació entre la massa i el radi obtinguda integrant les equacions Tolman-Oppenheimer-Volkoff (TOV) també és estudiada. A causa de que les interaccions de Gogny de la família D1 no aconsegueixen donar NSs compatibles amb observacions astrofísiques, en aquesta tesi proposem dues noves forces de Gogny, anomenades D1M∗ i D1M∗∗, les quals poden donar, respectivament, NSs de 2 i 1.91 masses solars. Una altra part de la tesi es dedica a l’estudi de la transició entre l’escorça i el nucli, buscant la densitat a la qual la matèria uniforme al nucli és inestable contra fluctuacions de densitat. Ho estudiem amb dos mètodes, el mètode termodinàmic i el mètode dinàmic. Finalment, s’analitzen diverses propietats de les NSs, com són la relació entre la massa i el radi de l’estrella, les propietats de l’escorça, el moment d’inèrcia, així com la deformació deguda als corrents de marea (tidal deformability) que està relacionada amb l’emissió d’ones gravitacionals en sistemes binaris d’estrelles de neutrons.

[eng] Double § hypernuclei are bound systems of several nucleons and two § hyperons. These systems are unstable with respect to the weak interaction and chiefly decay through two-body processes that conserve neither parity, isospin or strangeness. Among such processes we can find those mediated by the §N ! NN, §§ ! §n and §§ ! ßN reactions but also, thanks to the baryon channel mixing (§N ° ßN, §§ ° •N,§§ ° ßß) made possible by the strong interaction, the weak ßN ! NN, •N ! §n, •N ! ßN, ßß ! §n and ßß ! ßN channels. This work completes the study of the decay of double-§ hypernuclei by taking into account the microscopic effects of the strong interaction on two baryon states in strangeness sectors -2, -1 and 0, pertaining to the possible baryon-baryon states appearing in the initial and final states, as previously stated. The initial strangeness °2 wave function is obtained from the solution of a G-matrix equation with the input of realistic strong baryon-baryon potentials, while the final hyperon-nucleon wave functions are derived analogously from a microscopic T-matrix calculation. Consequently the opening of the channel induced by the •N pair (with a lower energy threshold than that corresponding to the ßß pair) had to be considered. In order to evaluate these new decay channels the potential that governs the •N interaction with a strangeness variation |¢S|= 1 had to be derived for its incorporation in the study of the decay of 6He. This potential is based on the meson exchange model and includes fundamental states of the pseudoscalar and vector octets. v The inclusion of •° p and •0 n states required the derivation of new coupling constants using chiral Lagrangians and SU(3) flavour symmetry (extended to SU(6) spin-flavour symmetry for vector mesons), as well as the construction of new operational structures to account for the different isospin transition channels. The new §§ ! YN decay rate studied in this work, with YN = §n, ß0 n and ß° p, represents 3-4% of the total one-baryon induced non-mesonic decay and is remarkably affected by strong interaction effects. In particular, the relative importance of the partial decay rates, encoded in the ratio °§n/(°ß0 n + °ß° p), gets inverted when the mixing to •N states is incorporated in the initial correlated §§ wave function. This sensitivity can be used experimentally to learn about the strong interaction in the strangeness °2 sector., [cat] Els hipernuclis doble-§ són estats lligats de nucleons i dos hiperons  quests estats són inestables en front de la interacció feble, i es desintegren majoritàriament a través de processos a dos cossos que no conserven paritat, isospín ni estranyesa. Entre aquests processos tenim els que venen donats per les reaccions NNN, n i N, però també, i gràcies a la interacció forta que permet la barreja de canals bariònics (NN, N, ), pels canals febles NNN, Nn, NN, n i N. En aquesta tesi s’ha completat l’estudi d’aquests hipernuclis, tenint en compte els efectes microscòpics de la interacció forta per als estats de dos barions en el sector d’estranyesa -2, -1 i 0, corresponents als possibles sistemes barió-barió que apareixen als estats inicial i final esmentats al paràgraf anterior. Per al cas de les partícules lligades en un nucli s’ha resolt una equació de matriu-G mentre que per als sistemes finals s’ha resolt una matriu-T, ambdos casos sota la consideració de potencials barió-barió moderns i realistes, i que ens proporcionen la funció d’ona acoblada per al sistemes inicial  i final respectivament. S’ha hagut de considerar doncs l’obertura del canal induït per la parella N, amb un llindar energètic més baix que el corresponent a la parella . Per tal de considerar els nous canals que apareixen en tenir en compte aquesta parella s’ha hagut de derivar el potencial feble a dos cossos que regeix la interacció ΞN amb canvi d’estranyesa |N amb canvi d’estranyesa |S|=1, per tal d’incorporar-lo en el càlcul de la desintegració del 6He. La inclusió de les transicions febles corresponents als estats intermedis ΞN amb canvi d’estranyesa |0 n i ΞN amb canvi d’estranyesa |- p necessita del càlcul de noves constants d'acoblament, les quals s’han derivat utilitzant Lagrangians quirals i simetria de sabor SU(3) (estesa a simetria de sabor-espín S(6) per als mesons vector), així com la construcció d’estructures operacionals que donin compte de les noves transicions que apareixen a l’espai d’espín isotòpic. La nova taxa de desintegració corresponent a YN representa el 3-4% del decaïment no-mesònic induit per un sol barió i es veu afectada considerablement per la interacció forta. En particular la importància relativa dels rates parcials es veu invertida quan s’incorpora l’acoblament a estats N en la funció d’ona correlacionada inicial. Aquesta sensibilitat pot ser usada per a estudiar la interacció forta en el sector d’estranyesa -2

[eng] For ninety years we have known that our universe is in expansion. Cosmological data favor an unknown form of intrinsic and fundamental uniform energy contributing approximately 68% of the total energy budget in the current epoch. The simplest proposal in accordance with observations is the standard cosmological model consisting of a small but positive cosmological constant producing a gravitational repulsive effect driving the accelerated expansion. In standard cosmology general relativity is assumed as the theory for gravity, which in turn predicts that a sufficiently compact mass can deform spacetime and form a black hole. At a mathematical level, these objects are considered vacuum solutions described by very few parameters. For instance, a stationary black hole solution is completely described by its mass, angular momentum, and electric charge; and two black holes that share the same values for these parameters, are indistinguishable from one another. On the basis of the usual metrics describing black holes, it is generally believed that all contained matter is localized in the center or, if rotating, on an infinitely thin ring. Recent approaches challenge this unintuitive assumption and consider matter just spread throughout the interior. Clearly, this begs for a quantum description in curved space. In past years, a novel approach established a new bridge between quantum information and the physics of black holes when an intriguing proposal was made: black holes could possibly be understood as Bose—Einstein condensates of soft interacting but densely packed gravitons. The aim of this thesis is to discuss how to construct a graviton condensate structure on top of a classical gravitational field describing black holes. A necessary parameter to be introduced for this analogy is a chemical potential which we discuss how to incorporate within general relativity. Next we search for solutions and, employing some very plausible assumptions, we find out that the condensate vanishes outside the horizon but is non-zero in its interior. These results can be extended easily to a Reissner—Nordström black hole. In fact, we find that the phenomenon seems to be rather generic and is associated with the presence of a horizon, acting as a confining potential. In order to see whether a Bose— Einstein condensate is preferred, we use the Brown—York quasilocal energy, finding that a condensate is energetically favourable in all cases in the classically forbidden region. The Brown—York quasilocal energy also allows us to derive a quasilocal potential, whose consequences allow us to suggest a possible mechanism to generate a graviton condensate in black holes. On the contrary, this is not the case for any kind of horizons; for instance, this mechanism appears not to be feasible in order to generate a quantum condensate behind the cosmological de Sitter horizon. Furthermore, when a pair of black holes merge, an immense amount of energy should be given off as gravitational waves. Their wave forms have been recently confirmed to be perfectly described by general relativity. We discuss why for low frequency gravitational waves aimed to be detected by astrophysical PTA observations the fact that propagation should take place over an expanding (approximately globally de Sitter) spacetime should be taken into account. In this manner, harmonic waves produced in such mergers would become anharmonic when measured by cosmological observers. This effect is tiny but appears to be observable for gravitational waves to which PTA are sensitive. Therefore we have characterized modifications to the expected signal, and how it is related to the source and pulsar characteristics that are employed by the IPTA collaboration. If the cosmological constant were an intrinsic property, this experiment would be capable of confirming the relevance of lambda at redshift z < 1., [spa] El objetivo de la presente tesis es profundizar en diversos aspectos de la física de los agujeros negros. Tanto en lo que respecta a sus características constitutivas fundamentales, su "estructura" interna, como a la posibilidad de observar o detectar mediante observaciones astrofísicas ciertos efectos producto de su dinámica. Por un lado, hemos seguido las ideas de Dvali, Gómez et al. quienes han sugerido la posibilidad de que un agujero negro sea un condensado de Bose—Einstein de gravitones débilmente interactuantes. En nuestro caso hemos estudiado la existencia de este tipo de soluciones sobre diferentes métricas de agujero negro (Schwarzschild y Reissner— Nordström) que actuarían como potencial confinante para dichos condensados. Un parámetro necesario para ello, es el equivalente a un potencial químico que debe ser incorporado a la relatividad general. Cabe destacar que la solución encontrada puede ser interpretada como la función de campo medio del condensado. Además resulta fuertemente ligada a la estructura clásica de la métrica que la sustenta. Por otro lado, es bien sabido que la aceleración de cuerpos muy masivos producen perturbaciones de tipo onda en el espaciotiempo. Son de nuestro interés las ondas gravitatorias de baja frecuencia, provenientes de la colisión de agujeros negros supermasivos y que deberían poder ser detectadas mediante sistemas de púlsares (Pulsar Timing Arrays). De acuerdo a una línea de investigación desarrollada por Espriu et al. la presencian de una constante cosmológica podría tener un efecto en la propagación y por lo tanto en la detección por parte de la colaboración IPTA de estas ondas. En la presente tesis hemos generalizado el método para incluir diferentes tipos de materia (relativista y no relativista) además de la constante cosmológica. Del análisis se deriva que el efecto depende sensiblemente del valor de la constante de Hubble (que engloba todos los tipos de materia presentes). Continuando dicha línea, hemos caracterizado detalladamente el efecto en su dependencia con los parámetros cosmológicas y las distancias involucradas, y cómo podría ser hallado. Esperamos que nuestros resultados puedan contribuir a una definitiva detección por IPTA.

[eng] In this thesis, we study the physical properties of several ultracold few-boson systems depending on the interactions between their constituents. Nowadays, experimentally, it is possible to have great control with high precision over the geometry and the interactions between the particles, making them an excellent setup to test directly the principles of quantum mechanics. A very interesting point is to study the evolution of their properties with the number of particles. The theoretical study of these systems pretends to microscopically understand the current experimental results and give support to new experimental developments. The method that will be used is the exact diagonalization of the Hamiltonian of the system. As we will see, in spite of the attempts to improve it, the method is limited by the fact that, in practice, it is only useful to study few-particle systems. The method has several advantages. First of all, one has access to both the ground and the excited states. In second place, the method is variational and converges to the exact solution as long as the Hilbert space in which we diagonalize is enlarged. Moreover, since we have access to the states of the system, it is possible to calculate any observable quantity of interest. First, we will study a system of spinless bosons trapped in a two-dimensional harmonic potential. The effect of the trap is to keep the system bound. It will be seen how the presence of a repulsive interaction changes the energy spectrum and other properties of the system. For instance, the density profile, which is usually measurable, and also the two-body distribution function, which is intimately related to the existence of correlations. Afterwards, the focus will be on the particular case of having only two bosons in the system interacting through a strong repulsive force. Inspired by the one-dimensional case where the fermionization phenomenon takes place in the strongly-interacting limit, we will study whether in two dimensions there is a resembling reminiscent effect. In other words, we will analyze if there are properties of the two strongly-interacting bosons in two dimensions that are like the ones of two noninteracting fermions. After that, we will tackle the localization phenomenon in a one-dimensional system that is caused by an external speckle potential that introduces disorder in the system. We will show that the localization is a robust phenomenon against repulsive contact interactions. Finally, we will study the influence of the spin-orbit coupling in a system of bosons with two possible pseudospin components, associated, for instance, to two hyperfine levels, confined in a two-dimensional harmonic trap. We will present an exhaustive analysis of the combined effects of the interaction and the spin-orbit coupling in the spectrum and the properties of the system. In particular we show the existence of a crossover in the ground state of the system susceptible to be experimentally identified., [spa] En aquesta tesi, estudiarem les propietats físiques de diversos sistemes de pocs bosons ultrafreds depenent de les interaccions entre els seus constituents. Avui dia, a nivell experimental, es té un gran control amb una gran precisió de la geometria i les interaccions entre les partícules, fet que fa aquest sistemes excel·lents per comprovar de forma directa els principis de la mecànica quàntica. Un punt d'interès és comprovar l'evolució de les seves propietats amb el nombre de partícules. L'estudi teòric d'aquests sistemes pretén entendre a nivell microscòpic els resultats experimentals actuals i donar suport pels nous avenços experimentals. El mètode que farem servir serà la diagonalització exacta del hamiltonià del sistema. Com veurem, malgrat les millores que es poden implementar, ens trobarem amb la limitació de no poder estudiar sistemes de més d'unes quantes partícules. Els avantatges d'aquest mètode són diversos. En primer lloc, podrem obtenir no només l'estat fonamental del sistema sinó que també els primers estats excitats. En segon lloc, el mètode és variacional i sabem que convergeix cap a la solució exacta a mesura que ampliem l'espai de Hilbert en que diagonalitzem. A més a més, en tenir accés als estats del sistema, podem calcular qualsevol quantitat observable que sigui d'interès. Primerament, estudiarem un sistema de bosons sense espín atrapats en un potencial harmònic bidimensional. L'efecte de la trampa és de mantenir el sistema lligat. En haver-hi una interacció repulsiva, veurem com canvia l'espectre d'energia del sistema i també altres propietats. Per exemple, la seva densitat, que habitualment es pot mesurar, i també la funció de distribució de dos cossos, que va íntimament lligada a l'existència de correlacions. Tot seguit, ens centrarem en el cas particular de tenir només dos bosons en el sistema interaccionant a través d'una gran força repulsiva. Inspirats pel cas unidimensional en que té lloc el fenomen de la fermionització en el limit d'interacció molt forta, estudiarem si en el cas bidimensional hi queda cap reminiscència d'aquest efecte. En altres paraules, analitzarem si hi ha propietats dels dos bosons fortament interactuants en dues dimensions que siguin com les de fermions no interactuants en el mateix sistema. A continuació, tractarem el fenomen de la localització en un sistema unidimensional en el qual hi ha un potencial extern de tipus speckle que introdueix desordre en el sistema. Veurem que la localització és un fenomen robust en front de les interaccions repulsives. Per últim, estudiarem la influència de l'espín-òrbita en un sistema de bosons amb dues components de pseudoespín, associades, per exemple, a dos nivells hiperfins, atrapats en un potencial harmònic bidimensional. Presentarem un anàlisi exhaustiu dels efectes conjunts de la interacció i l'espín- òrbita en l'espectre i en les propietats del sistema. En particular, mostrarem l'existència d'un encreuament en l'estat fonamental del sistema susceptible de ser identificat experimentalment.

[eng] This thesis describes some results in AdS/CFT duality for quantum field theories in 2+1 and 1+1 dimensions. In first place, a one-parameter family of supergravity solutions is described, obtained as a deformation of a D2-branes system. These solutions are dual to a family of field theories in 2+1 dimensions. All the backgrounds in this family present a geometry that ends smoothly. But, except in a particular case, there is no confinement. The reason for this is that the string theory description of these solution is not valid and they are only regular solutions in the context of M-theory. In this description quarks (i.e. fields in the fundamental representation) are represented by membranes wrapped around a circle that can end smoothly in a cigar-like tip. This allows a disconnected configuration. Consequently, in the field theory there exists a mass gap due to the end of the geometry at a finite value of the radial coordinate, but there is no confinement. This unexpected lack of confinement is not the only remarkable feature of this family of solutions. One of the dual theories has a renormalization group (RG) flow that ends in a fixed point, that is, a conformal theory. Varying slightly the parameter that labels the solutions we end in solutions dual to theories whose RG flow passes near the fixed point, thus exhibiting a quasiconformal behaviour. All these theories also flow to the gapped phase at lower energies. The existence of a quasiconformal regime in a RG flow implies that there is an approximated conformal symmetry. When the flow leaves the quasiconformal region this approximated symmetry is broken and this breaking can be either spontaneous or explicit. By the Goldstone’s theorem, when a symmetry is spontaneously broken a massless scalar Goldstone boson must appear for each of the generators of the symmetry. If the symmetry is not exact, the Goldstone boson is not massless but light. In the case of the conformal symmetry, the Goldsonte boson associated to its spontaneous breaking is a dilaton, a light pseudo-dilaton for the approximated symmetry. In chapter 3 we compute the spectra of spin-0 and spin-2 states. The result is that we did not find such pseudo-dilaton. The reason for this is that the spontaneous breaking of the symmetry is of the same order than the explicit breaking, Nevertheless, it is possible to simulate a spontaneous breaking of the conformal symmetry by putting a hard-wall cut-off in the infrared. By tuning the energy scale at which we impose the cut-off we can explore different energy regions of the flow, finding the quasiconformal behaviour in the appropriate solutions. Finally, we consider a new system, consisting in D1-branes in a space that has as transverse manifold a space with special holonomy. This implies that the maximal supersymmetry corresponding to the D1-branes in flat space is partially broken and reduces to a number of supercharges that depends on the particular holonomy of the space considered. The internal manifolds that we study are tri-Sasaki, Sasaki-Einstein and weak-G2 , that reduce supersymmetry from N=(8, 8) to N=(0, 3), (0 ,2) and (0, 1) respectively. Furthermore, we introduce D5-branes sharing the gauge directions with the D1-branes, which in the gauge theory translates into the presence of matter in the fundamental representation. For this reason they are called flavour branes. In these flavoured supersymmetric theories we have found renormalization group flows that connect the brane-like ultraviolet with a fixed point., [spa] En la presente tesis se exponen una serie de resultados en el marco de la dualidad AdS/CFT concernientes a teorías de campos en 2+1 y 1+1 dimensiones. En primer lugar se detalla una familia uniparamétrica de soluciones de supergravedad duales a teorías 2+1-dimensionales basadas en conjuntos de N D2-branas con D4-branas fraccionales con una variedad transversa de holonomía Spin(7). Estas teorías tienen la particularidad de no ser confinantes a pesar de tener un mass gap. Además la presencia de un punto fijo en el flujo del grupo de renormalización de una de las teorías hace que exista un comportamiento cuasiconforme para las que tienen un valor del parámetro cercano. Se calcula el espectro de modos de spin 0 y spin 2 para toda la familia de soluciones descrita y se estudia el comportamiento cuasiconforme de las teorías que lo presentan en función de la escala de energía usando una hard-wall en el infrarrojo. Finalmente, se encuentran ecuaciones BPS que describen soluciones duales a teorías de supersimetría reducida (N=(0,3), N=(0,2) y N=(0,1)) en 1+1 dimensiones con grados de libertad de sabor. El sistema se construye considerando D1-branas con un espacio transverso de holonomía especial (Sp(2), SU(4) y Spin(7) respectivamente) y añadiendo D5-branas que comparten con las D1-branas las direcciones de la teoría gauge. En todos los sistemas considerados se encuentra una solución que fluye desde el ultravioleta de tipo D1-brana hasta una solución AdS3 en el infrarrojo.

[eng] The aim of this thesis is to study the quantum entanglement and, in particular, under which circumstances is maximum. In the first place, Bell's inequalities are analyzed from an operational point of view. In particular, we focus on those involving qutrits. The states that maximally violate these inequalities are of the GHZ type, for inequalities involving qubits, and little deformations of GHZ state, for those involving qutrits. This result shows that, although maximum entanglement and non-locality are very close concepts, they are not equivalent. In the second place, multipartite entanglement in spin chains is studied. We use as a figure of merit the hyperdeterminant and two polynomial invariants, S and T. These figures quantify a specific type of quadripartite entanglement, as demonstrated by an analysis of well-known quantum states such as the GHZ or W. In an Ising spin chain, we observe a pronounced peak near the quantum phase transition. Similar results are observed for the XXZ and Haldane- Shastry models for the case of the S and T invariants. For that reason, we conclude that these figures of merit are sensitive to quantum phase transitions. The second part of the thesis focuses on the field of quantum computing. This field has experienced a significant expansion in recent years. Due to this growth, several companies have started to develop prototypes of quantum computers. For this reason, one line of research in quantum computing is to propose methods to benchmark these machines. One of the methods presented in this thesis consists of the exact simulation of a XY spin chain. We propose a circuit that diagonalizes the XY Hamiltonian, which allows simulating time evolution and thermal states as well. Since this model is exactly solvable, the results obtained from a quantum computer can be compared with the theoretical solution. After running this circuit for four qubits on two IBM computers and one from Rigetti computing company, we obtain worse results than expected. With this, we conclude that there are error sources that, in general, are not being taken into account and that become relevant even in such small circuits. Moreover, we propose another method to test quantum computers performance: the simulation of absolutely maximally entangled states. The implementation of these circuits is a hard but necessary test for a quantum computer since the advantage of quantum algorithms lies in the generation of entanglement. Finally, we study the generation of entanglement at the most fundamental level: particle physics. We obtain that the QED interaction at tree-level is dictated by the imposition of maximal entanglement in outgoing particles. We apply the same philosophy in processes involving weak neutral currents obtaining that the weak mixing angle must be pi / 6, very close to the experimental value., [cat] La motivació d’aquesta tesi és estudiar l’entrellaçament en general i sota quines circumstàncies és màxim en particular. Primerament, estudiem i deduïm noves desigualtats de Bell en termes d’operadors, focalitzant-nos en aquelles que involucren qutrits. Les desigualtats per qubits són violades màximament pels estats coneguts com a GHZ mentre que les de qutrits, per estats que són una deformació dels GHZ. Seguidament, estudiem l’entrellaçament multipartit i la seva aplicació en la detecció de transicions de fase quàntiques. Com a figura de mèrit, utilitzem l’hiperdeterminant i els invariants S i T. Obtenim un pic pronunciat de l'hiperdeterminant al voltant de la transició de fase en el cas del model d’Ising. En el cas del model XXZ, el valor dels invariants canvia bruscament en els punts on hi ha transició de fase, resultat similar al obtingut en el model de Haldane-Shastry. En una segona part de la tesi, ens centrem en el camp de la computació quàntica. Per una banda, proposem i testegem un mètode que consisteix en la simulació exacta del model d’Ising. Proposem un circuit quàntic que diagonalitza l’Hamiltonià d’Ising i que, per tant, fa possible la simulació en el temps i la preparació d’estats tèrmics. Testegem aquest circuit pel cas d’una cadena de quatre espins en tres ordinadors quàntics. Els resultats difereixen notablement del valor teòric esperat. Això fa pensar que encara hi ha moltes fonts d'error a tenir en compte en aquest tipus de dispositius. Per altra banda, proposem la simulació d'estats absolutament màximament entrellaçats. També analitzem com l’entropia de cada bipartició majoritza i utilitzem aquesta propietat per trobar els circuits més òptims. Finalment, estudiem quina ha de ser l’estructura de la interacció de QED per tal de poder generar estats màximament entrellaçats en termes de les helicitats de les partícules sortints. El resultat demostra que, a primer ordre, la interacció de QED es recupera imposant màxim entrellaçament. També estudiem quines implicacions té aquesta imposició en processos que involucrin corrents dèbils neutres. El resultat a primer ordre és que el valor de l’angle de Weinberg ha de ser de pi/6, molt proper al valor experimental.

[eng] In physics, it is common to find different phenomena being described by similar equations. A good analogy can make us look at a problem from a different point of view. In that way, ideas may be transferred from one field of science to another, allowing to model new phenomena after previous, well-studied ones. In the case of the field of analogue gravity, systems that mimic certain aspects of the physics of curved spacetimes are studied. In this thesis, we are interested in the analogy between geometry and media. It has been known for several decades that light propagation in a gravitational field is formally equivalent to that in a bianisotropic medium. On the one hand, ray paths are bent due to spacetime curvature. On the other hand, spatial variations of the permittivity and permeability of a material can make light follow curved trajectories. These two phenomena can be related mathematically in the context of transformation optics, which provides the tools to determine the medium parameters necessary to mimic a certain coordinate transformation. Materials with these specific properties are not naturally occurring, therefore, the emergence of metamaterial science at the beginning of the century was needed to realize them. Metamaterials are artificial composite materials with sub-wavelength constitutive elements that exhibit exotic properties. They have been one of the hot topics of the past years given the variety of opportunities they offer: negative refraction, superlenses, indefinite dispersion, invisibility, among many others. In this thesis we study the analogues of two static spacetimes from the point of view of transformation optics: one with spherical symmetry and one with conical geometry. Both cases are inspired by solutions to Einstein’s equations: the Schwarzschild black hole and the cosmic string, respectively. For each case, we derive the permittivity and permeability of the analogous material using Plebanski’s formulation of the electromagnetic constitutive equations. We solve numerically the wave equation in the metamaterial and compare the results with analytical theories. We find that the spherically symmetric spacetime can be mimicked by either an anisotropic or isotropic medium due to its rotational symmetries. This is achieved by performing a coordinate transformation of the general metric to a conformally flat form. We obtain the medium parameters for both cases and apply the results to the case of the Schwarzschild black hole. We simulate the propagation of a Gaussian beam in the two materials and compare the numerical results with the null-geodesics in the Schwarzschild spacetime, finding a good agreement. The cosmic string is an example of a topological defect with conical geometry. A conical space can be interpreted as flat space with a wedge removed. We make use of this transformation to study the wave equation in the cosmic string background. We apply asymptotic diffraction theories to obtain analytical models that describe wave propagation of electromagnetic or gravitational waves (in a certain gauge). We find that our expressions reproduce accurately the results of the numerical simulations in the analogous metamaterial. Moreover, with our models, we can understand the observed diffraction pattern as the interference of four characteristic waves. With this interpretation we can introduce the Fresnel observation zones, which are related to the diffraction maxima. They help localize the regions – in either space or frequency – where the wave effects are more significant. In fact, in the diffraction by a non-compact object such as the cosmic string, we find that the contribution to the field of wave effects such as interference or diffraction can be of the same order as the geometrical optics terms. Furthermore, the conical topology also appears in condensed matter systems as disclinations or wedge dislocations, therefore we expect our results to be applicable in those systems as well., [spa] La investigación en gravitación análoga consiste en el estudio de sistemas físicos donde se pueden reproducir algunos de los fenómenos propios de relatividad general. Esta tesis se centra en la conocida propagación análoga de las ondas electromagnéticas en un espacio-tiempo curvado y en un medio con una permitividad y permeabilidad generalmente anisótropas. Para la realización de este tipo de medios, fue necesaria la aparición de los metamateriales, materiales artificiales diseñados para tener propiedades electromagnéticas fuera de lo común. En este contexto, se estudian modelos análogos a dos objetos estáticos con distinta simetría: uno con simetría esférica y otro con topología cónica. Ambos casos están motivados por soluciones a las ecuaciones de Einstein: el agujero negro de Schwarzschild y la cuerda cósmica, respectivamente. A través de las ecuaciones de óptica de transformación, determinamos los parámetros de los medios análogos a estos espacio-tiempos. Estudiamos la propagación de ondas electromagnéticas en los materiales obtenidos mediante simulaciones numéricas y comparamos los resultados con teorías analíticas, encontrando muy buen acuerdo. Por un lado, los contrastamos con las geodésicas en los espacio-tiempos considerados a través del formalismo Hamiltoniano. Por otro lado, desarrollamos modelos analíticos para describir la difracción de una onda (en principio tanto electromagnética como gravitatoria) debido a la cuerda cósmica. Para ello, usamos teorías asintóticas de difracción en un espacio virtual plano con un déficit de ángulo, ya que es una representación equivalente a la geometría cónica de la cuerda. De este modo, se obtienen expresiones que nos permiten explicar detalladamente los fenómenos ondulatorios de interferencia y difracción que se producen en este espacio-tiempo. Observamos que estos efectos pueden ser comparables a los términos de óptica geométrica: añaden una modulación en la amplificación del campo relacionada con la formación de imágenes dobles propia de la topología de la cuerda. Cabe destacar que estos fenómenos son conceptualmente distintos a los que se podrían esperar en la difracción sobre un objeto sólido como una barra. Utilizando nuestros modelos analíticos, obtenemos el patrón de difracción característico de la cuerda cósmica, que podría ser de interés para su detección.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2017, Tutor: Domènec Espriu, The discovery of the Higgs boson in 2012 was a huge moment of achievement: the particle postulated more than 50 years ago was at last discovered. Even so, the particle acted as it was expected and, paradoxically, gave no new clues about to where to look next. In this report we brie y discuss what we call the Higgs mechanism, which gives mass to the W and Z when they interact with an invisible field, the Higgs field, that pervades the universe. Moreover, the Electroweak Chiral Lagrangian is presented, which allows us to consider an alternative way to perform an analysis of the model. A brief outline and some final reflections are exposed in the conclusion section.

[eng] The cosmological constant appearing in Einstein’s equations is a key element of the ΛCDM standard cosmological model. This term performs, effectively, as pure vacuum energy, and is considered the easiest explanation of the positive acceleration of the current Universe. But despite the successes of the concordance model in explaining a large variety of high precision data, some tensions are also present at the observational level, and there are also some important problems arising on the theoretical side. The most severe one is the so-called “cosmological constant problem”, which is caused by the gigantic discrepancy between the predicted value of the vacuum energy density in Quantum Field Theory and the measured one. This huge difference is considered as one of the most profound (unsolved) problems of theoretical Physics and its solution probably will come hand in hand with a change of paradigm. But at this moment, there is no clear hint pointing to the aforesaid solution. In view of the current status of the problem, those phenomenological studies that are able to shed some light on the nature of the dark energy (DE) component that is dominating the current Universe, are very welcome. In this thesis are presented detailed studies (at the background and perturbations levels) on various running vacuum energy models, which are motivated from the renormalization group equation formalism of Quantum Field Theory in curved spacetime. In these models, the cosmological term is not a rigid Λ, it depends explicitly on the Hubble function and its time derivative. Thus, it varies with the cosmic expansion. Upon the study of the capability of these models on fitting the experimental data, we can determine whether this dynamical behavior is favored by observations or not. Is the vacuum energy density (or in more general terms, the dark energy density) dynamical? One of the main conclusions of this dissertation is that we have indeed strong evidences in favor of the variability of the DE throughout the cosmic history. It has been shown that this variation can also be traced through other (purely phenomenological) dynamical vacuum models, together with different parameterizations of the DE (as the XCDM and CPL), and scalar field models as the Peebles-Ratra one. The statistical confidence level with which these evidences are obtained reaches in some cases the 4σ c.l., something that is unprecedented in the literature. An exhaustive explanation on the reasons why large collaborations as BOSS or Planck have not been able to detect such signal in favor of the DE dynamics is provided in this thesis too. This is mainly due to the fact that they do not make use of a complete enough data set that include more large scale structure (LSS) and baryon acoustic oscillations (BAO’s) data points. The analyses carried out in this thesis have also served e.g. for: 1) ruling out some running vacuum models (and DE models inside the D-class) without a well-defined ΛCDM limit (with no constant term in the expression for the vacuum energy density); 2) showing the potential of the Press-Schechter formalism in the characterization of the various vacuum models; 3) pinpointing the importance of the data set BAO+CMB+LSS in front of other data sets as those formed by Hubble function data points and the luminosity distance versus redshift points of type Ia supernovae., [cat] La constant cosmològica és un element clau del model ΛCDM, el model estàndard cosmològic. Aquest terme actua, de manera efectiva, com energia pura de buit, i representa l’explicació més senzilla de l’acceleració positiva amb la que s’expandeix l’Univers actualment. Ara bé, tot i que el model estàndard cosmològic és capaç de desciure dades observacionals provinents de fonts molt diverses amb molta precisió, es coneixen també algunes tensions a nivell observacional, així com també greus problemes teòrics associats. El més sever és el que es coneix com el “problema de la constant cosmològica” i té a veure amb la gran diferència (de més de 55 ordres de magnitud) entre l’estimació teòrica de la densitat d’energia de buit que es fa a partir de la Teoria Quàntica de Camps i el valor mesurat de la mateixa. Aquesta discrepància és exageradament gran i representa un dels problemes més importants de la Física teòrica actual. Ara per ara, no sembla que disposem de les eines teòriques per solucionar aquest problema. En aquest sentit, estudis fenomenològics que puguin ajudar a caracteritzar millor l’energia fosca que domina l’expansió del teixit còsmic són molt benvinguts. En aquesta tesi es recullen els estudis detallats (a nivell de background i pertorbacions) de diferents models de buit dinàmic que neixen del formalisme del grup de renormalització en Teoria Quàntica de Camps en espais corbats. En aquests models, el terme cosmològic no es pren constant, sinó com una funció explítica de la funció de Hubble i la seva derivada. Per tant, la densitat d’energia de buit varia amb l’expansió de l’Univers. Estudiant la capacitat d’aquests models per ajustar les dades experimentals podem veure si realment la dinàmica d’aquesta component còsmica està o no afavorida per les observacions. És la densitat d’energia de buit (o, en termes més generals, la densitat d’energia fosca) dinàmica? Una de les conclusions més importants a les que s’arriba en aquesta dissertació és que hi ha indicis grans a favor d’aquesta variabilitat de l’energia fosca en el temps i que aquesta pot ser traçada també a partir d’altres models de buit dinàmic purament fenomenològics, així com amb diverses parametritzacions de l’energia fosca (com la XCDM o CPL) i models de camps escalars com el de Peebles- Ratra. El nivell trobat d’evidència a favor d’aquesta dinàmica no nul·la no té precedents a la literatura, arribant en alguns casos a les 4σ de nivell de confiança.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2017, Tutor: Domènec Espriu, In this work we place ourselves in the framework of an holographic approach to QCD. We study the mass spectra of a family of dilaton backgrounds more general than the usual solvable one in a Soft-Wall model. Taking a particular limit of this family we recover the mass spectra of the Hard-Wall model.

The production of a Standard Model Higgs boson in association with two hard jets in the forward and backward regions of the detector, frequently quoted as the “vector-boson fusion” (VBF) channel, and the production of a Higgs boson in association with a W or Z boson, known as “VH production” or “Higgsstrahlung”, represent cornerstones in a comprehensive study of Higgs boson couplings at the LHC. These production channels do not only provide valuable information on the couplings of Higgs bosons to the massive gauge bosons by themselves, but also allow for the isolation of the Higgs boson decays into -lepton or bottom-quark pairs, whose investigation is essential in the Higgs boson couplings analysis. In the previous reports [7–9], state-of-the-art predictions and error estimates for the total and differential cross sections for pp ! H + 2 jets and pp ! HV ! H + 2 leptons were compiled (with V = W; Z), but the process of improving and refining predictions is still ongoing, even within the Standard Model. In this contribution we update the cross-section predictions for VBF and VH production, covering integrated total and fiducial cross sections as well as differential distributions. In detail, the presented state-of-the-art predictions include QCD corrections up to next-to-next-to-leading order (NNLO), electroweak (EW) corrections up to next-to-leading order (NLO), and contributions from specific partonic channels that open at higher perturbative orders, such as photon-induced collisions or gluon-fusion contributions. Apart from collecting numerical results, we give recommendations as to how to combine the individual components and to assess conservative estimates of remaining theoretical uncertainties. Moreover, issues connected to the matching and the impact of parton showers (PS) are discussed.

Màster Oficial d'Astrofísica, Física de Partícules i Cosmologia, Facultat de Física, Universitat de Barcelona, Curs: 2016-2017, Tutors: Domènec Espriu Climent, Federico Mescia, Discrepancies with the Standard Model (SM) have recently been observed in b s transitions, which hint at a possible description in terms of four-fermion operators. In this work, we seek to describe the anomaly by introducing new heavy fermions. Firstly we build four-fermion operators out of SM and new fermions and introduce the hypothesis of Minimal Flavor Violation. Next we integrate out the heavy degrees of freedom to form operators of four SM fields and obtain an effective coupling constant.

In this thesis we have studied how high energetic excitations propagate through a non- abelian strongly coupled plasma. This new state of matter is produced at heavy ion collisions in our accelerators and allows us to study a stage of the evolution of our Universe that occurred during the first microseconds after the Big Bang. In this extreme conditions of temperature and density the ordinary matter that we are made of behaves as a an almost perfect fluid, the most perfect known by mankind up to now in fact. The theory of strong interactions is tested at an energy scale that even though it is high enough to melt hadrons, it does not get to the point where the coupling constant is low enough to allow a perturbative description. In the plasma, the partonic field content, the quarks and gluons, cease to be the relevant degrees of freedom and a microscopic description in terms of quasi-particles is not possible. A very useful tool to put to test the actual behaviour of this strongly coupled fluid is the analysis of jet modifications as a result of their interactions with the plasma. In a first introductory part we have given the concepts needed to picture how heavy ion collisions develop as we are able to understand it today. At weak coupling, the main mechanism responsible for energy loss is induced gluon emission and interesting interference phenomena occur that lead to a dependence on path length of as the squared distance. These are known as coherence effects and their study becomes richer by considering multi-gluon emission, as it is done done in Part III. The strongly coupled picture uses holography to map a dressed excitation moving through a strongly coupled plasma into a string propagating in a higher dimensional space containing a black hole. Since the non-abelian theory in which the calculation is done is not QCD, but N = 4 SYM, we take these results as an insight to describe energetic parton propagation in a model of jet quenching in heavy ion collisions. Even though we assume that the exchanges with the medium are soft enough to include non-perturbative effects, as described by gauge/gravity duality, the energetic partons that are produced in the collision generally have a high virtuality which they relax by successive splittings. The latter occur at length scales that are not resolvable by the medium, and they should proceed as in vacuum. This observation motivates us to adopt a hybrid description for the interplay between the multi scale jet and the QGP, using each description at the scale it is supposed to be valid. This phenomenological description has proven to be very successful in describing dijet and photon-jet data at different centralities, and predictions have been made for a wide range of observables for the coming data from run 2 of LHC, including a new observable, the ratio of the fragmentation functions of the leading and subleading jet in a dijet pair, which is highly sensitive to the specific energy loss mechanism. In the next part of the work we extend our hybrid model by the inclusion of two effects, broadening and medium response, which should help us better describe intra-jet observables. The first effect, broadening, is due to the Brownian motion that probes experience in a thermal bath, and it will tend to broaden the distribution of particles within the jet. As it turns out, the observable quantifying such modifications, the jet shapes, are rather insensitive to the inclusion of this effect. However, by restricting the range of the tracks entering this analysis,we have been able to produce a new observable which shows a remarkable dependence on the precise strength of the broadening mechanism. The second effect involves overall energy-momentum conservation. The rapidly thermalized energy deposited by the energetic partons modifies the plasma, inducing temperature and velocity fluctuations in the surrounding fluid cells. This perturbation propagates long distances in the form of a wake and eventually decays into soft hadrons, whose orientations keep a correlation with the jet direction and therefore produce a net effect even after background subtraction. The observable consequences are best noticed in intra-jet measurements such as jet shapes and fragmentation functions, where it is clearly seen that the inclusion of such physics is in good agreement with the observed experimental trend, and it becomes simply unavoidable when comparisons against global measurements are performed. Finally, we compute the inclusive two gluon stimulated emission within the context of perturbative QCD. By studying the full answer in different kinematical limits we arrive to the conclusion that jet propagation is perceived from the point of view of the plasma as a set of effective emitters depending on the resolution power, which for a thin plasma it is of the order of the Debye screening mass. This physics is a missing piece of the Monte Carlo jet quenching model presented in this thesis and its inclusion is expected to have important consequences for the more differential observables, a task that will be undertaken in future work. These are very exciting times for the physics of strong nuclear interactions. We have seen how the very fundamental questions about the nature of the high temperature, strongly coupled phase of ordinary matter can be addressed by the study of jet quenching and its observable consequences. This thesis represents an effort in the confrontation of the seductive ideas of holography with experiments. Having the means to quantitatively confront new ideas, as we have done throughout the presented work, new observables, and new data is critical if we are eventually to understand the properties of the strongly coupled liquid quark-gluon plasma that Nature has served us., En aquesta tesi estudiem com excitacions d’alta energia es propaguen a través d’un plasma no abelià fortament acoblat. Aquest nou estat de la matèria es produeix a les col·lisions de ions pesats als nostres acceleradors i permet l’estudi d’una etapa de l’evolució del nostre Univers que va tenir lloc durant els primers microsegons després del Big Bang. En aquestes condicions extremes de temperatura i densitat, la matèria ordinària de la qual estem fets es comporta com un fluid perfecte, de fet el més perfecte conegut per la humanitat a dia d’avui. La teoria de les interaccions fortes és posada a prova a una escala d’energia que tot i ser prou alta per fondre els hadrons, no arriba al punt en què la constant d’acoblament és prou feble com per permetre una descripció pertorbativa. Al plasma, el contingut en camps partonics, els quarks i els gluons, deixen de ser els graus de llibertat rellevants i una descripciò microscòpica en termes de quasi-partícules no és possible. Una eina molt útil per posar a prova l’autèntic comportament d’aquest fluid fortament acoblat és l’anàlisi de les modificacions dels jets com a resultat de la seva interacció amb el plasma. Tot i que assumim que els intercanvis amb el medi són prou suaus com per incloure efectes no pertorbatius, tal com descriu la dualitat gauge/gravetat, els partons energètics que es produeixen a la col·lisió generalment tenen virtualitats altes, les quals relaxen a través de particions successives. Aquestes últimes ocorren a escales de longitut que no són resolubles pel medi, i per tant haurien de procedir com en el buit. Aquesta observació ens motiva a adoptar una descripció híbrida per a la interacció entre el jet i el QGP, fent servir cada descripció a l’escala on es suposa que es vàlida. Aquesta descripció fenomenològica ha demostrat ser molt exitosa en la descripció de les dades de dijets i fotó-jet per diferents centralitats, i hem produït prediccions per a un ample ventall d'observables a l'energia del run 2 del LHC, incloent un nou observable molt sensible al mecanisme específic de pèrdua d'energia. També incorporem els efectes d'eixamplament i resposta del medi per descriure els observables intra-jet tot obtenint importants millores qualitatives en la descripció de les correlacions a llarg abast entre el jets i el plasma. Finalment, calculem l’emissió estimulada de dos gluons en el context de QCD pertorbativa. Estudiant la resposta completa en diferents límits arribem a la conclusió que des del punt de vista del plasma la propagació del jet és percebuda com una col·lecció d’emissors efectius depenenent del poder de resolució, el qual per un plasma prim és de l’ordre de l’invers de la massa d’apantallament de Debye. La seva inclusió en el model híbrid s’espera que tingui conseqüències importants pels observables més diferencials, una tasca que s’emprendrà en un treball futur.

Màster Oficial d'Astrofísica, Física de Partícules i Cosmologia, Facultat de Física, Universitat de Barcelona, Curs: 2014-2015, Tutor: Domènec Espriu, In this work we introduce and combine an effective Lagrangian formalism with the inverse amplitude method to unitarize WLWL ! WLWL and ZLZL ! ZLZL amplitudes in an extended electroweak symmetry breaking sector. We derive the unitarisation method via dispersion relations, and discuss different ways of fnding the possible resonant states. We compare the two amplitudes calculated with the equivalence theorem approximation and with an exact computation. Moreover, since a diboson excess has been observed near 2 TeV |albeit with very limited statistical significance| in WW, WZ and ZZ final states at the LHC experiments, we constrain the Lagrangian low-energy parameters by limiting the region of the parameter space where a resonance around 2 TeV may appear, and also derive the width of these resonances. Computing the cross-section for two processes we get an idea of how the signal varies along the parameter space, and from the amplitudes we introduce unitarized form factors and vertex functions to allow for a proper treatment of the resonances in Monte Carlo generators and a more precise comparison with experiment

[eng] In this thesis we study axions, a byproduct of the Peccei-Quinn solution to the strong CP problem, which are also a viable candidate for the dark matter content of the Universe. In the first part of the thesis, we revisit the Dine-Fischler-Srednicki-Zhitnisky axion model in light of the recent Higgs LHC results and electroweak precision data. This model is an extension of the two-Higgs-doublet model incorporating a PQ symmetry which leads to a physically acceptable axion. For generic values of the couplings, the model reproduces the minimal Standard Model, with a massless axion and all the other degrees of freedom at a very high scale. However, in some scenarios, the extra Higgses could be relatively light. We use the oblique corrections, in particular Δρ, to constrain the mass spectrum in this case. Finally, we also work out the non-linear parametrization of the DFSZ model in the generic case where all scalars except the lightest Higgs and the axion have masses at or beyond the TeV scale. In the second part, we study the relevance of a cold axion background (CAB) as a responsible for the dark matter in the Universe. We examine indirect consequences of its presence through its effects on photon and cosmic ray propagation. First, we study the axion-photon system under the joint influence of two backgrounds: an external magnetic field and a CAB. Their effect consists in producing a three-way mixing of the axion with the two polarizations of the photon. We determine the proper frequencies and eigenvectors as well as the corresponding photon ellipticity and induced rotation of the polarization plane that depend both on the magnetic field and the local density of axions. We also comment on the possibility that some of the predicted effects could be measured in optical table-top experiments. Then, we consider the case in which no magnetic field is present. Here, circularly polarized photons are energy eigenstates, with a modified dispersion relation. This enables the emission of a photon by a charged particle, such as a cosmic ray, which is forbidden in regular QED due to energy-momentum conservation. We study the energy loss of a cosmic ray due to this process and compute the energy flux of photons emitted in this way, which depends on the cosmic ray spectrum.

Màster Oficial d'Astrofísica, Física de Partícules i Cosmologia, Facultat de Física, Universitat de Barcelona, Curs: 2014-2015, Tutor: Domènec Espriu, In the Minimal Standard Model (MSM) there is no degree of freedom for dark matter. There are several extensions of the MSM introducing a new particle - an invisible axion, which can be regarded as a trustworthy candidate at least for a part of the dark matter component. However, as it is extremely weakly coupled, it cannot be directly measured at the LHC. We propose to explore the electroweak sector indirectly by considering a particular model that includes the axion and deriving consequences that could be experimentally tested. We discuss the Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model, which extends the two-Higgs doublet model with an additional Peccei-Quinn symmetry and leads to a physically acceptable axion. The non-linear parametrization of the DFSZ model is exploited in the generic case where all scalars except the lightest Higgs and the axion have masses at or beyond the TeV scale. We compute the oblique corrections and use their values from the electroweak experimental fits to put constraints on the mass spectrum of the DFSZ model.

In this thesis we study the possibility that QCD breaks parity at high temperatures and densities, a scenario that may be tested in heavy ion collisions. We present two different approaches to this problem. On the one hand, analytical studies with effective models suggest that QCD may break parity in dense systems due to condensation of pseudoscalar mesons in the isotriplet channel. On the other hand, P- and CP- odd bubbles may appear in a finite volume due to local large topological fluctuations in a hot medium. The presence of a large magnetic field or angular momentum may lead to the so-called Chiral Magnetic Effect. We assume topological fluctuations as the source of parity violation in heavy ion collisions throughout. This effect may be treated in a quasi-equilibrium description by means of a non-trivial axial chemical potential. We consider the 'two flavour' Nambu—Jona-Lasinio model in the presence of a vector and an axial external chemical potentials and study the phase structure of the model at zero temperature. The Nambu—Jona-Lasinio model is often used as a toy replica of QCD and it is therefore interesting to explore the consequences of adding external vector and axial chemical potentials in this model, mostly motivated by claims that such external drivers could trigger a phase where parity could be broken in QCD. We are also motivated by some lattice analysis that attempt to understand the nature of the so-called Aoki phase using this simplified model. Analogies and differences with the expected behaviour in QCD are discussed and the limitations of the model are pointed out. We also consider the low energy realization of QCD in terms of mesons when an axial chemical potential is present; a situation that may be relevant in heavy ion collisions. We demonstrate that the presence of an axial charge has profound consequences on scalar/pseudoscalar meson physics. The most notorious effect is the appearance of an explicit source of parity breaking. The eigenstates of strong interactions do not have a definite parity and interactions that would otherwise be forbidden compete with the familiar ones. We focus on scalars and pseudoscalars that are described by a generalized linear sigma model. We comment briefly on the screening role of axial vectors in formation of effective axial charge and on the possible experimental relevance of our results, whose consequences may have been already seen at RHIC. Finally, we investigate how local parity breaking may affect vector physics. A modified dispersion relation is derived for the lightest vector mesons rho and omega. They are characterised by a mass splitting depending on their polarization. This effect predicts a natural overproduction of lepton pairs in the vicinity of the rho-omega resonance peak as well as a polarization asymmetry around this peak. The dilepton excess seems relevant to explain the anomalous dielectron yield quoted by PHENIX/STAR. We present a detailed analysis of the angular distribution associated to the lepton pairs created from these mesons searching for polarization dependencies. Two angular variables are found to carry the main information related to the parity-breaking effect. Possible signatures for experimental detection of local parity breaking are discussed., En esta tesis estudiamos la posibilidad de que QCD rompa paridad a altas temperaturas y densidades, un escenario que se puede examinar en colisiones de iones pesados. Estudios analíticos con modelos efectivos sugieren que QCD puede romper paridad en sistemas densos. Además, pueden aparecer burbujas con P y CP impar en un volumen finito debido a grandes fluctuaciones topológicas locales en un medio caliente. Este último efecto puede ser tratado en una descripción cercana al equilibrio mediante un potencial químico axial. Consideramos el modelo de Nambu—Jona-Lasinio con ‘dos sabores’ en presencia de un potencial químico vectorial y uno axial y estudiamos la estructura de fases del modelo a temperatura cero. Estas magnitudes podrían abrir una fase donde la paridad podría romperse en QCD. También nos motivan algunos análisis en el retículo que intentan entender la naturaleza de la llamada fase de Aoki usando este modelo simplificado. Se discuten analogías y diferencias con el comportamiento esperado en QCD además de las limitaciones del modelo. Consideramos también la realización de QCD a bajas energías en términos de mesones en presencia de un potencial químico axial. Demostramos que la presencia de una carga axial tiene profundas consecuencias en la física de mesones escalares y pseudoescalares. Los autoestados de la interacción fuerte no tienen una paridad bien definida y en consecuencia, algunas interacciones antes prohibidas, ahora compiten con las más familiares. Nos centramos en escalares y pseudoescalares que se describen mediante un modelo sigma lineal generalizado. Finalmente, investigamos cómo la ruptura de paridad local puede afectar la física de mesones vectoriales. Se deriva una relación de dispersión modificada para los mesones más ligeros rho y omega. Este efecto predice una sobreproducción natural de parejas de leptones en las inmediaciones del pico de resonancia rho-omega así como una asimetría en la polarización alrededor de este pico. El exceso de dileptones parece ser relevante para explicar la producción anómala de dielectrones citada por PHENIX/STAR. Presentamos un análisis detallado de la distribución angular asociada a las parejas de leptones creadas de estos mesones en busca de dependencias en la polarización. Se encuentran dos variables angulares que transportan la información principal relacionada con el efecto de ruptura de paridad.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Any: 2014, Tutor: Domènec Espriu, Using the equivalence theorem we study the scattering of longitudinally polarized gauge bosons in extensions of the Standard Model where anomalous Higgs couplings to gauge sector and higher order O(p4) operators are considered. Forcing the amplitudes to be unitary we nd new resonances for di erent values of the coupling constants. Hereby we would be able to narrow the range of values of the chiral parameters with new experiments at LHC.

[eng] In this thesis a viable theory of gravity resulting from the spontaneous symmetry breaking of some more fundamental theory is developed. This fundamental theory does not include any notion of a metric or geometry. All the corresponding degrees of freedom are obtained dynamically from the symmetry breaking. The low energy effective theory corresponds to the Einstein-Hilbert theory naturally equipped with a cosmological term. The appearance of such term makes the cosmological constant interpretable as an intrinsic property of space time rather than an effective description valid at cosmological scales. The effect of this cosmological constant different from zero in the propagation of gravitational waves is assessed. In the presence of an cosmological constant gravitational waves propagate in a de Sitter space time and not in a minkoskian at space time. This fact brings about noticeable effects in the propagation of the waves. Finally we study the relevance of such effects due to the cosmological constant in the gravitational waves targeted at the PTA (Pulsar Timing Array) projects. We conclude that the effects can be measured in local systems (low redshits). This could represent an alternative way to set bounds to the value of the cosmological constant and at the same time could even facilitate the first detection of gravitational., [cat] Generació espontània de la gravetat i possibles conseqüències observacionals. En aquesta tesi hem desenvolupat una teoria en què la gravitació d'Einstein s'obté dinàmicament mitjançant el mecanisme de ruptura espontània de simetria d'una altra teoria més fonamental. En aquesta teoria fonamental no existeix cap noció mètrica o de geometria. Tots els graus de llibertat associats a la geometria de l'espai temps emergeixen tan sols després del trencament de la simetria. El resultat final correspon a la teoria d'Einstein-Hilbert naturalment equipada amb una constant cosmològica. El fet d'obtenir aquesta constant ens porta a la seva interpretació com a una propietat intrínseca de l'espai temps en contraposició a la interpretació com a descripció efectiva tan sols vàlida a escales cosmològiques. Aprofitant el fet d'obtenir una constant cosmològica diferent de zero estudiem l'efecte que aquesta té en la propagació d'ones gravitatòries. En la seva presència les ones es propaguen en un espai temps de De Sitter i no en un espai temps pla de Minkowski. Els efectes poden, en principi, ésser mesurats en observacions en sistemes locals (“redshifts” petits). Finalment estudiem la rellevància que poden tenir aquestes modificacions, degudes a la constant cosmològica, en les ones gravitatòries que podrien ser detectades mitjançant els “pulsar timing arrays” en un futur proper. La conclusió és que la inclusió de la constant cosmològica té efectes notables que podrien significar un mètode alternatiu a la mesura del valor mateix de la constant cosmològica. De fet, aquests efectes podrien facilitar la primera detecció de les ones gravitatòries.

[eng] The gauge/gravity duality has proven to be a very useful tool in the understanding of quantum field theories outside the perturbative regime. In particular, holography has been able to shed light not only on generic mechanisms of strongly coupled theories, but also on processes occurred in experimental set-ups, such as the heavy ion collisions. Experimental observations such as small viscosities or fast hydrodynamization find a natural explanation when the problem is expressed in terms of gravity and black holes. Despite the successes, however, it is important to bear in mind that holography provides computational tools for toy models rather than for QCD itself, and that these models are usable only under certain assumptions. Nature is very often far more nuanced than the models physicists use to describe it. In the case of heavy ion experiments and QCD there are many features that are commonly coarse grained in the holographic computations. For instance, non-trivial RG flows or baryon currents have not been included in the holographic models until very recently, although these are very relevant to experiments, and fundamental in critical phenomena. In this thesis we present a series of works in the topics field theory and heavy ion collisions that use applied holography and numeric GR as computational tools. The unifying factor among them is that they consider gravitational set-ups beyond pure gravity to describe the physics of conserved currents, non-trivial RG flows and phase transitions. In chapter 2 we use an Einstein-Maxwell set-up to compute the collision of two shock-waves with a conserved current and the hydrodynamization of the subsequent plasma. This conserved current is used to model the baryonic charge deposition by rapidity, observed in the experiments. The simulations are done with and without including the backreaction of the Maxwell field into the metric, which corresponds to the quenched approximation for the effects of the baryon charge on the gluons. In chapter 3 we present a one parameter family of non-conformal models. By adding an scalar field with a polynomial potential to the pure gravity set-up, we can achieve a non-trivial RG flow between two fixed points in the dual field theory. In this work we compute the thermodynamics and the quasi-normal modes spectra for the homogeneous states, being the latter one of the main results of the chapter. In chapter 4 we present the first holographic shock-wave collisions in a non-conformal model. To do so, we use the model introduced in chapter 3. In non-conformal models the average pressure in equilibrium is not fixed by symmetry, but by the equation of state. Out of equilibrium the average pressure might take any value, giving a new probe for the equilibration of the system. When the plasma's average pressure is well approximated by the equation of state value, we say that the system has “EoSizied”. In this chapter we show that the EoSization can indeed happen before the plasma has hydrodynamized. Finally, in chapter 5 we explore a holographic model that can contain phase transitions. This model is the same as the one presented in chapter 3, but now taking pure imaginary numbers for the controlling parameter. In an effort to understand the instabilities present in models with phase transitions, we trigger and evolve a spinoidal instability to its inhomogeneous end state. This is done by adding a small perturbation to a uniform black brane in a locally unstable branch, triggering a Gregory-Laflamme type instability in the gravity side. The most remarkable result found in the simulation is that both the evolution and the final result are well described by second order hydrodynamics., [cat] La cromodinàmica quàntica (QCD), la teoria que descriu la força nuclear forta, és cas paradigmàtic de teoria quàntica de camps amb fases fortament acoblades. Amb l'objectiu d'entendre en profunditat la QCD i la seva dinàmica, és va iniciar a la dècada de 1970 el programa de col·lisions de ions pesants. Aquest programa experimental té com a objectiu crear, mitjançant acceleradors de partícules, fases de QCD de-confinades i estudiar-ne les seves propietats. Entre els formalismes utilitzats per a descriure sistemes fortament acoblats, com les col·lisions d'ions pesants, hi ha la dualita “gauge/string” o holografia. L'holografia és una correspondència entre dues teories -- una teoria gauge i una teoria de cordes -- que permet fer càlculs en una de les dues teories per mitjà de la seva dual. La correspondència es pot fer servir per relacionar un plasma fortament acoblat i el seu dual, forats negres en un espai asimptòticament anti-de-Sitter (AdS), on els càlculs resulten factibles. Així, per simular la col·lisió d'ions pesants s'evoluciona numèricament la col·lisió d'ones gravitatòries en AdS, i la subseqüent relaxació del seu horitzó d'esdeveniments. En aquesta tesi s'hi presenten un seguit de treballs emmarcats en el camp de l'holografia aplicada, on s'hi considera dinàmica en models més enllà de gravetat pura. En el capítol 2 es presenta la primera simulació hologràfica de col·lisions amb càrrega bariònica, un observable accessible en els experiments. Els capítols 3, 4 i 5 estan dedicats a una família de models no conformes. En el capítol 3 se n'estudia la dinàmica prop de l'equilibri per mitjà dels modes quasi-normals. En el capítol 4 s'hi estudien col·lisions hologràfiques. Finalment en el capítol 5 es genera una inestabilitat espinoidal en un model amb transició de fase i se segueix fins a un estat final inhomogeni.