Search

The Standard Model (SM) of particle physics is a tremendously successful explanation of the nature we observe, both in Earth-based experiments and cosmological observations of the past history of the universe. However its failure to account for gravity, dark matter and dark energy, as well as for various hierarchy and naturalness problems, motivates the formulation and investigation of Beyond-the-Standard-Model (BSM) theories. In this thesis we will first review the evidence for and current understanding of dark matter (DM), including the mechanisms of freeze-out and freeze-in, before discussing the idea of 'secluded' dark matter without non-gravitational interactions with the SM. This will form the motivation for chapter 2, where we consider the emission of such secluded DM from an early population of primordial black holes, and for chapter 3, where we present a new mechanism by which an initially underpopulated secluded DM population may account for the observed DM in the universe today. We will also discuss the hypothetical axion particle, which arises from an elegant so- lution to one of the problems with the SM (the Strong CP problem), and its generalisation, the Axion-Like-Particle (ALP). We will also introduce the one-dimensional topological defects called cosmic strings, and show how they can arise from ALP fields. ALPs serve as a candidate DM particle, and, further, are expected to appear in great numbers in the 4-dimensional effective theories arising from string theory (ST), a multiplicity known as the 'string axiverse'. We shall review the foundations of ST with a focus on the creation of the string axiverse by the compactification of extra dimensions; finally in chapter 4 we construct global cosmic string solutions arising from string axiverse ALPs and analyse aspects of the resulting physics. In particular, we include the physically-crucial presence of a stabilising potential.

Q-balls are an example of non-topological soliton that can appear in the spectrum of scalar field theories. They are semi-classical states that minimise the energy for a given Noether charge. If isolated from other sectors, these spherically symmetric objects are kept absolutely stable by energy and Noether charge conservation. Though the properties of Q-balls cannot be determined analytically in general, progress can be made in two opposite limits: the thin-wall (large charge) limit; and the thick-wall (small charge) limit. Q-balls are both interesting phenomenologically and as formal objects in their own right, as evidenced by the large body of work on this subject. This thesis represents a study on the formal properties of field theories, with the aim to determine the subclass of models that admit Q-ball solutions. To that end, its goals are two-fold. Firstly, to constrain the theories that possess stable Q-ball states by: (i) determining the differential equations that govern the VEV of the constituent scalar in the thin-wall limit, together with the physical properties of the resulting Q- ball; (ii) constraining the small-field expansions that lead to stable thick-wall Q-balls by demanding that the minimum of energy be classically stable against decay to the underlying quanta of the scalar field. Secondly, to determine whether stable Q-balls, in both the thick- and thin-wall limit, can form from scalar mesons, arising from the breaking of an approximate chiral symmetry, in the SM and in a specific BSM theory. We will not discuss the phenomenology of these theories in this thesis. The content of this thesis is laid out in three parts. Part I is concerned with Q-balls in theories possessing only one scalar field, and comprises Chapters 2 and 3, whereas Part II is concerned with theories with multiple scalar fields, and comprises Chapters 4-6. In these parts, we perform an analysis to determine the existence of Q-ball states, and any constraints required by the theories that possess them. In particular, in Part I, the thick-wall analysis presents us with necessary conditions, whereas in Part II, the thick-wall analysis presents us with sufficient conditions. Fi- nally, Part III analyses theories when the underlying scalars of the theory are pseudo- Nambu-Goldstone bosons arising from the breaking of an approximate chiral sym- metry, as in the Standard Model - this final part comprises Chapters 7 and 8. We show that stable Q-balls are not part of the spectrum of the leading-order Chiral lagrangian, but that they can arise in the class of BSM models we study. The bulk of the original work of this thesis can be found in Refs. [18,19,100-102], with the remaining work unpublished.

Despite their name, there is clear theoretical evidence that black holes emit radiation. The nature and spectrum of this radiation depends on many features of the black hole and the emitted particles. With the profound implications that black hole evaporation has on our understanding of the marriage of quantum mechanics and general relativity, it is essential to study the details of this radiation in new contexts. In this thesis we examine certain important modifications to the spectrum of radiation from black holes that can occur. Firstly, we study the case that the particles emitted by the black hole are spatially extended, rather than pointlike, and find that the rate of emission is suppressed by a factor which depends sensitively on the size of the particle. We then study the emission of electrically charged particles from black holes from a perspective that elucidates the physical picture of emission; namely, in terms of tunnelling of particles both through the horizon and the external electric field. We finally consider the cosmological consequences of modifying the mass-temperature relation of black holes by supposing that they can radiate into higher dimensions. In particular, we argue that current constraints on the density of black holes in the universe are evaded in certain extra-dimensional theories, in which black holes are substantially colder than their four-dimensional counterparts.

New dynamics beyond the Standard Model is expected at scales probed by the Large Hadron Collider. In particular, weak scale supersymmetry can provide a dark matter candidate and stabilise the electroweak hierarchy. This thesis presents two studies investigating new strategies to discover such phenomena. First, a theoretical assessment uses six search strategies based on 3.2fb-1 of 13 TeV data to constrain a 19-parameter framework of supersymmetry. This identified the fermionic partners of Higgs bosons, called Higgsinos, as a key target for dark matter searches. Such scenarios are very difficult to discover at colliders in electroweak production, as produced states are one to tens of GeV heavier than the dark matter and leave little detector activity. The second part of this thesis presents a new experimental strategy targeting these so-called compressed scenarios, exploiting electrons and muons down to the lowest transverse momenta detectable by the ATLAS Experiment. Using 36.1fb-1 of 13 TeV data, this strategy opens hadron collider sensitivity to Higgsino dark matter down to mass splittings of 3 GeV, and coannihilation scenarios involving winos and sleptons down to mass splittings of 2.5 and 1 GeV respectively. While no statistically significant excesses are observed, this landmark LHC sensitivity surpasses lepton collider limits for the first time in nearly two decades.

General relativity describes gravity in terms of an interacting massless spin-2 field - the graviton. This 100-year-old theory has been spectacularly successful in explaining observations. However, theoretical exploration and the cosmological constant problem motivate the study of alternative theories of gravity. Recently, there has been great progress in understanding theories that give the graviton a mass. This thesis considers several aspects of these massive spin-2 effective field theories and related theories. These theories are first studied from the perspective of scattering amplitudes. The most general 2 → 2 scattering amplitude is constructed for theories containing a single massive graviton or vector. These amplitudes are then used to find the highest strong coupling scales in such theories, assuming a particular scaling of fields and momenta. Generalisations to include additional fields and self-interactions for massive higher-spin fields are also discussed. Constraints that arise from the existence of an ultraviolet completion are then studied. It is shown using dispersion relation arguments that the pseudo-linear massive spin-2 theory cannot admit an analytic, Lorentz-invariant, and unitary ultraviolet completion, but that such completions are not ruled out for massive vector theories. The behaviour of massive spin-2 theories under dimensional reduction is also explored. Stability conditions and the lower-dimensional spectrum are derived for the Kaluza-Klein dimensional reduction of a partially massless graviton and a massive graviton on an Einstein product manifold. Additionally, the nonlinear dimensional reduction of the zero modes in dRGT massive gravity is shown to produce a mass-varying massive gravity theory. Lastly, attempts to construct a version of unimodular gravity containing a massive graviton are discussed. A candidate theory is proposed and is shown to have pathologies. Dimensional reduction is then used to generate massive spin-2 theories with noncanonical kinetic terms and auxiliary fields. These theories are shown to be equivalent to the Fierz-Pauli theory, which provides further evidence for the uniqueness of the kinetic term used in dRGT massive gravity.

The ongoing quest to map the space of possible, consistent field theories, and to determine their properties, is extremely interesting from a purely theoretical perspective, as well as being a prerequisite for the phenomenological application of any such theories. To that end, this thesis is concerned with various theoretical aspects of multi-gravity theories - that is, theories which contain multiple, interacting spin-2 fields. The analysis presented here will proceed by using the Stueckelberg trick to reintroduce the multiple copies of diffeomorphism invariance which the theory would possess if none of the spin-2 fields interacted, and it is discussed in detail how to apply this to multi-gravity, and in particular how this differs from massive and bi-gravity. The structure of the interactions which arise from the helicity-0 modes of the massive gravitons is examined in the so-called decoupling limit. It is found that the structure these take in bi-gravity is generalised to that of multi-Galileons, although this is not always manifest; an extension of the so-called Galileon duality is used to probe this. Multi-gravity theories can be elegantly described using graphs to encode the network of interactions between the different fields, and this forms the basis for the other two questions which will be covered. When the theory contains a cycle of interactions, it is explicitly shown how this leads to the introduction of a ghost-like instability which is not present otherwise, and how this lowers the cutoff of the effective theory. In the absence of cycles, the structural properties of the graph which represents a given theory continue to play a large role in determining the scale at which the effective theory breaks down. This is the final topic, which is studied in detail, and upper and lower bounds on this scale are derived which depend on various properties of the graph.

The hierarchy problem of the electroweak scale is an intriguing puzzle which can conceivably be solved during upcoming runs of the Large Hadron Collider (LHC). Supersymmetry (SUSY) is an attractive potential solution to this problem, though the fact that no supersymmetric particles have been discovered thus far raises fine tuning of most models to &order;(1&percnt;). Extradimensional SUSY is especially interesting in light of this fact since certain models are easier to reconcile with LHC data. This thesis discusses two different extradimensional SUSY scenarios: auto-concealment and Maximally Natural SUSY (MNSUSY). The auto-concealment mechanism applies when the lightest ordinary superymmetric particle (LOSP), a brane localized state, promptly decays to the Kaluza-Klein (KK) tower of a bulk lightest supersymmetric particle (LSP). This dynamically realizes the compression mechanism for hiding SUSY as decays into the more numerous heavier KK LSP states are favored. LHC limits on LOSP squarks weaken to ≲ 450 GeV while limits on LOSP right-handed sleptons evaporate. Slepton searches perform poorly in this case because LHC analyses are blind to compressed slepton spectra. To help fill this gap, this thesis presents a monojet-like search sensitive to the very compressed range 3 GeV < m &itilde; − m&Tilde;_x0₁ < 24 GeV. The analysis should allow LHC14 with 100 fb−1 to search for degenerate left-handed selectrons and smuons in the compressed region up to m &itilde;L ≲ 150 GeV. In addition, it should be sensitive to m &itilde;L ≲ 110 GeV for auto-concealed SUSY. To expand the class of extradimensional SUSY models the LHC is able to test this thesis also describes the Lagrangian and mass matrices of MNSUSY, which are needed to implement this model in the Feynman rules generator program FeynRules for use in Monte Carlo programs for collider simulations. It also describes benchmark scenarios useful for the first collider phenomenology studies.

The properties of dark matter - its microphysical form, and its cosmological origin and history - are one of the most important mysteries in fundamental physics. So far, evidence for matter beyond the Standard Model comes entirely from gravitational effects. However, other experiments are reaching the point where the 'simplest' models of dark matter are coming into tension with data, and may be strongly constrained by medium-term observations. This, along with theoretical considerations, motivates the exploration of other possibilities for the history and composition of dark matter, especially those with the possibility of new, generic observational signatures. In this thesis, we explore some different classes of new dark matter models, focussing on regimes in which they may display approximately model-independent phenomenology. Firstly, we look at a class of dark matter models featuring large-number, stable composite states, and investigate how these may be synthesised in the early universe. As the example of Standard Model nuclear physics and Big Bang Nucleosynthesis demonstrates, the properties of small-number composite states in strongly-coupled theories may be complicated, and sensitive to the precise details of the theory. However, it may reasonably be expected that the properties of large enough composite states will obey simple geometrical scaling laws. In this case, if large enough states are synthesised in the early universe, the overall results of the synthesis process may become broadly independent of the detailed parameters of the model, and of initial conditions. We model 'dark nucleosynthesis' in such a regime, and find that the late-time number distribution takes on one of two characteristic forms, in both cases with weak dependence on small-number initial conditions and behaviour. Following on from this, we consider the scattering phenomenology that would result from dark matter being made up of such large composite states. This includes the coherent enhancement of scattering rates - for example, at direct detection experiments - compared to e.g. collider production processes. The spatially extended nature of composite dark matter states could also lead to characteristic momentum-dependent form factors in scattering processes, which may be identifiable in direct detection experiments. In addition, inelastic interactions between dark matter states may be important in astrophysical settings. Illustrating the effects of dark-sector energy injections, we present calculations for dark matter halo modifications through velocity kicks. As an example application, we discuss a different class of asymmetric dark matter models, in which late-time decays of part of the dark matter can re-populate a symmetric component, giving annihilation signals in galactic halos. The velocity kicks arising from the decay process may modify the spatial profile of such signals, to the extent to eliminating them almost completely from low-escape-velocity systems such as dwarf galaxies.

Low scale supersymmetry (SUSY) is a compelling solution to the electroweak hierarchy problem. However, increasingly strong limits on the masses of superpartners, first from LEP and now the LHC, mean that the simplest models require significant fine tuning. This thesis is dedicated to the study of a possible alternative low energy superpartner spectrum, natural SUSY, in which only superparticles directly involved in stabilising the electroweak scale are light, alleviating collider limits and potentially reducing tuning. After reviewing how low scale SUSY is motivated by the hierarchy problem, we build a model of SUSY breaking and mediation that successfully generates a natural SUSY spectrum. This also suppresses the first two generation fermion Yukawas, and leads to small parameters in the hidden sector, which are required for successful SUSY breaking. A challenge in models of natural SUSY is raising the physical Higgs mass to 125 GeV, and we study the possibility that this could occur through the addition of a singlet to the theory. If stops are very light, the coupling of the singlet to the Higgs needs to be so large that it becomes nonperturbative before the scale of grand unification, raising the concern that precision gauge coupling unification may be upset. However, we find that this is not necessarily the case. Rather it is possible this could correct for the present &bsim; 3&percnt; discrepancy in the two-loop minimal supersymmetric model's unification prediction. We then turn to the fine tuning in models of natural SUSY, emphasising that this should be measured with respect to the theory's ultraviolet (UV) parameters. We show that the first two generation sfermions can be made relatively heavy, beyond LHC reach, without introducing tuning. However, the gluino generates a significant tuning through the stops during the renormalisation group flow. As a result, there is no fine tuning benefit in reducing the stop masses below (50 - 75)&percnt; of the weak scale gluino mass, and we obtain strong lower bounds on the tuning of theories compatible with collider limits. We also study theories with Dirac gauginos, which have relatively low fine tuning even if the scale of mediation is high. Finally, we consider the effect of relaxing a common assumption and allowing the hidden SUSY breaking sector to modify the running of the visible sector soft masses. This may plausibly occur in realistic models and could dramatically reduce the fine tuning of SUSY theories.

This thesis is dedicated to the study of a prominent class of dark matter (DM) models, in which the DM relic density is linked to the baryon asymmetry, often referred to as Asymmetric Dark Matter (ADM) theories. In ADM the relic density is set by a particle-antiparticle asymmetry, in direct analogue to the baryons. This is partly motivated by the observed proximity of the baryon and DM relic densities Ω_{DM} ≈ 5 Ω_{B}, as this can be explained if the DM and baryon asymmetries are linked. A general requisite of models of ADM is that the vast majority of the symmetric component of the DM number density, the DM-antiDM pairs, must be removed for the asymmetry to set the DM relic density and thus to explain the coincidence of Ω_{DM} and Ω_{B}. However we shall argue that demanding the efficient annihilation of the symmetric component leads to a tension with experimental constraints in a large class of models. In order to satisfy the limits coming from direct detection and colliders searches, it is almost certainly required that the DM be part of a richer hidden sector of interacting states. Subsequently, examples of such extended hidden sectors are constructed and studied, in particular we highlight that the presence of light pseudoscalars can greatly aid in alleviating the experimental bounds and are well motivated from a theoretical stance. Finally, we highlight that self-conjugate DM can be generated from hidden sector particle asymmetries, which can lead to distinct phenomenology. Further, this variant on the ADM scenario can circumvent some of the leading constraints.

Scattering amplitudes are fundamental and remarkably rich observables in quantum field theory. The basic observation that makes scattering amplitudes fascinating is that in theories with massless particles of spin s ≥ 1, they are much simpler than might be expected from traditional Feynman diagram techniques for computing them. This simple observation might ultimately have profound consequences for our view of quantum field theory. The broad aim of this thesis is to understand and exploit the hidden simplicity and structure in scattering amplitudes. The quantum field theory with the simplest scattering amplitudes in four dimensions is planar = 4 supersymmetric Yang-Mills theory. This theory has provided considerable inspiration in developing new computational techniques and has provided many important theoretical insights. In this theory, there is a remarkable correspondence between scattering amplitudes and null polygonal Wilson loops, observables which on first inspection look very different. In this thesis, we will provide new insights into this correspondence using methods from twistor theory and exploit the symmetries of the problem to find new ways of computing scattering amplitudes.

This thesis studies the cosmology of ultra-light scalar fields with masses in the range 10−33 eV ? m ? 10−18 eV and their effects on cosmology. The existence of such fields is motivated by the theoretical framework of the "String Axiverse". All types of string theory contain multiple axion fields associated with antisymmetric tensor fields compactified on closed cycles in the compact space. Since the masses of these fields scale exponentially with the volume of the cycle, it is possible for them to be naturally light. We study the effects of these fields as a component of the dark matter and show analytically and numerically that they cause a suppression of structure formation on cosmological scales set by the inverse mass. We show that it will be possible with future galaxy redshift and weak lensing surveys to detect an ultra- light field comprising of order a percent of the total dark matter. If such a field is allowed to couple to the geometry that provided its mass via a phenomenological scalar potential for the axion and modulus, then the expansion of the universe can be altered significantly. In particular, we find that it is possible to have multiple epochs of accelerated expansion over a large region of parameter space, and to have a flat universe with a big crunch in the distant future. Finally, we address the issue of isocurvature perturbations in axion cosmologies, and demonstrate that in the ultra-light case the power spectrum is effected. This may have implications for the conclusions made about fine tuning in the axiverse in relation to a potential detection of tensor modes in the CMB that are different to the case of a standard axion.

Under the umbrella of Theoretical Physics, progress in ‘Beyond the Standard Model’ (BSM) physics proceeds broadly along two main avenues of investigation. The first is concerned with constructing theories that attempt to explain observations, or address theoretical problems, which cannot be explained within the tremendously successful Standard Model (SM) of particle physics. The second involves looking for new ways to observe or test BSM physics, and such tests are usually developed with current experimental hints, or attractive theoretical models, in mind. This thesis contains material which falls under both approaches. Part I is concerned with Supersymmetry (SUSY). We review the basics of SUSY, and the current state of this field, and then present a novel model for SUSY at the TeV scale. This model has a Higgs sector similar to the SM and possesses a continuous U(1)R symmetry, dramatically suppressing contributions to flavour-changing neutral currents, which can be problematic in SUSY models. After this we demonstrate that if more than one SUSY-breaking sector is present then this could lead to a rich spectrum of states with mass roughly twice the gravitino mass. In particular, if SUSY-breaking in a hidden sector arises dynamically then multiple ‘Goldstini’ and ‘Modulini’ states can arise, which couple to visible sector fields via the ‘Goldstino Portal’. We also demonstrate a new phenomenon which can occur in the context of multiple hidden sectors. If one sector breaks SUSY then this can ‘stimulate’ other sectors into also breaking SUSY, even if they are incapable of doing so on their own. Part II focusses on the matter in our Universe. We review our current understand- ing of how the visible matter in our Universe came into existence, and our current understanding of the nature of dark matter (DM). Following this we describe how DM could potentially be indirectly observed through its effects on cold white dwarf stars. Alternatively, if DM were detected by independent means, then observed cold white dwarfs could be used to place limits on the DM density in globular clusters, giving clues as to how these clusters of stars formed. We then present a new model for the co-generation of both the visible and dark matter in our Universe. This proceeds by generating a particle anti-particle asymmetry in the dark sector, which is then shared with the visible sector. This model predicts the existence of a light, m ≲ 5 eV, scalar particle which derivatively couples to DM, and provides a final state for the symmetric DM component to annihilate away into. Work completed during the period of this D.Phil is contained in [1–8], however only material in [3–6, 8] is presented in this thesis.

Identifying the relic particles that constitute the cold dark matter in our Universe is an outstanding problem in astro-particle physics. Direct detection experiments are among the most promising methods of detecting particle dark matter through non-gravitational interactions. In this thesis, the usual assumptions made when calculating the event rate at direct detection experiments are examined. Varying astrophysical parameters and the dark matter velocity distribution leads to significant changes in acceptance regions and exclusion curves for scenarios in which the tail of the velocity distribution is sampled; this includes 'light dark matter' (mass less than 10 GeV) and 'inelastic dark matter'. The DAMA and CoGeNT collaborations both report an annual modulation in their event rate that they attribute to dark matter. Two analyses of these experiments are performed. In the first, it is shown that these experiments can be compatible with each other and with the constraints from other direct detection experiments. This requires some isospin violation in the couplings of dark matter to protons and neutrons and a small inelastic splitting to boost the modulation fraction. The second analysis provides a comparison of the modulation signals free from all astrophysical parameters, under the assumption that dark matter scatters elastically. Again it is found that some isospin violation and a boosted modulation fraction is required in order that DAMA and CoGeNT are consistent with all experiments. A boosted modulation fraction may arise from a velocity distribution different from the Maxwell-Boltzmann distribution, which is usually assumed. Finally, a supersymmetric theory in which the dark matter candidate is a mixture of left- and right-handed sneutrino is considered. This theory has many novel signatures at colliders, indirect detection and direct detection experiments.

String theory is currently the best-known candidate for a theory of quantum gravity, having the necessary ingredients to describe all known elementary particles and interactions. It also includes several novel features, arising, for instance, from the additional six compact dimensions required for its internal consistency, making it the natural arena to construct extensions of the Standard Model. In this thesis, we analyze some of the new phenomenological aspects introduced by string theory within the framework of low energy effective theories, focusing on their applications to cosmology, astrophysics and collider experiments. We first consider a particular realization of the brane-world scenario in branonium bound states, showing that the orbital motion of a probe antibrane about a central brane stack leads to a resonant amplification of its world-volume scalar modes. We analyze the cosmological development of this process and also its potential relevance for either dark or baryonic matter generation in the early universe. We then focus on the spectrum of quark and lepton string excitations in warped compactifications, modeled by an effective 5-dimensional Randall- Sundrum throat. Motivated by the observed fermion mass hierarchy, we show that the spin-3/2 Regge excitation of the right-handed top quark is the lightest of such resonances in a significant region of parameter space, possibly lying below the TeV scale, and discuss its potential signatures at the Tevatron and at the LHC. Finally, we study the emission of sub-eV scalar particles by maximally rotating Kerr black holes, motivated by the recent string axiverse proposal. We focus on the spectrum of unstable scalar bound states in the superradiant regime, leading to an exponentially large axion cloud around astrophysical black holes, and analyze two semi-analytical methods for computing the growth rate of this instability, comparing the obtained results with previous analytical and numerical analyses.

In this work we analyze various implications of the presence of large field vacum expectation values (VEVs) along supersymmetric flat directions during the early universe. First, we discuss supersymmetric leptogenesis and the gravitino bound. Supersym- metric thermal leptogenesis with a hierarchical right-handed neutrino mass spectrum normally requires the mass of the lightest right-handed neutrino to be heavier than about 109 GeV. This is in conflict with the upper bound on the reheating temperature which is found by imposing that the gravitinos generated during the reheating stage after inflation do not jeopardize successful nucleosynthesis. We show that a solution to this tension is actually already incorporated in the framework, because of the presence of flat directions in the supersymmetric scalar potential. Massive right- handed neutrinos are efficiently produced non-thermally and the observed baryon asymmetry can be explained even for a reheating temperature respecting the gravitino bound if two conditions are satisfied: the initial value of the flat direction must be close to Planckian values and the phase-dependent terms in the flat direction potential are either vanishing or sufficiently small. We then show that flat directions also contribute to the total curvature perturbation. Such perturbation is generated at the first oscillation of the flat direction condensate when the latter relaxes to the minimum of its potential after the end of inflation. If the contribution to the total curvature perturbation from supersymmetric flat direction is the dominant one, then a significant level of non-Gaussianity in the cosmological perturbation is also naturally expected. Finally, we argue that supersymmetric flat direction VEVs can decay non perturbatively via preheating even in the case where they undergo elliptic motion in the complex plane instead of radial motion through the origin. It has been generally argued that in this case adiabaticity is never violated and preheating is inefficient. Considering a toy U(1) gauge theory, we explicitly calculate the scalar potential, in the unitary gauge, for excitations around several flat directions. We show that the mass matrix for the excitations has non-diagonal entries which vary with the phase of the flat direction vacuum expectation value. Furthermore, this mass matrix has zero eigenvalues whose eigenstates change with time. We show that these light degrees of freedom are produced copiously in the non-perturbative decay of the flat direction VEV.