Search

We would like to thank A. Carmona, M. Perez-Victoria and L.L. Salcedo for fruitful discussions. The authors thank the ICTP South American Institute for Fundamental Research (SAIFR), Sao Paulo, Brazil, and its Program on Particle Physics, September 30-November 30, 2019, where part of this work was done, for hospitality. The work of EM is supported by the Spanish MINEICO under Grants FIS2017-85053-C2-1-P and PID2020-114767GB-I00, by the FEDER/Junta de Andalucia-Consejeria de Economia y Conocimiento 2014-2020 Operational Programme under Grant A-FQM-178-UGR18, by Junta de Andalucia under Grant FQM-225, and by the Consejeria de Conocimiento, Investigacion y Universidad of the Junta de Andalucia and European Regional Development Fund (ERDF) under Grant SOMM17/6105/UGR. The research of EM is also supported by the Ramon y Cajal Program of the Spanish MINEICO under Grant RYC-2016-20678. The work of MQ is partly supported by Spanish MINEICO under Grant FPA2017-88915-P, by the Catalan Government under Grant 2017SGR1069, and by Severo Ochoa Excellence Program of MINEICO under Grant SEV-2016-0588. IFAE is partially funded by the CERCA program of the Generalitat de Catalunya., Green's functions with continuum spectra are a way of avoiding the strong bounds on new physics from the absence of new narrow resonances in experimental data. We model such a situation with a five-dimensional model with two branes along the extra dimension z, the ultraviolet (UV) and the infrared (IR) one, such that the metric between the UV and the IR brane is AdS(5), thus solving the hierarchy problem, and beyond the IR brane the metric is that of a linear dilaton model, which extends to z -> infinity. This simplified metric, which can be considered as an approximation of a more complicated (and smooth) one, leads to analytical Green's functions (with a mass gap m(g) similar to TeV and a continuum for s > m(g)(2)) which could then be easily incorporated in the experimental codes. The theory contains Standard Model gauge bosons in the bulk with Neumann boundary conditions in the UV brane. To cope with electroweak observables the theory is also endowed with an extra custodial gauge symmetry in the bulk, with gauge bosons with Dirichlet boundary conditions in the UV brane, and without zero (massless) modes. All Green's functions have analytical expressions and exhibit poles in the second Riemann sheet of the complex plane at s = M-n(2) - iM(n)Gamma(n), denoting a discrete (infinite) set of broad resonances with masses (M-n) and widths (Gamma(n)). For gauge bosons with Neumann or Dirichlet boundary conditions, the masses and widths of resonances satisfy the (approximate) equation s = -4m(g)(2) W-n(2)[+/-(1 + i)/4], where Wn is the n-th branch of the Lambert function., Spanish MINEICO FIS2017-85053-C2-1-P PID2020-114767GB-I00 FPA2017-88915-P, FEDER/Junta de Andalucia-Consejeria de Economia y Conocimiento 2014-2020 Operational Programm A-FQM-178-UGR18, Junta de Andalucia FQM-225, Consejeria de Conocimiento, Investigacion y Universidad of the Junta de Andalucia and European Regional Development Fund (ERDF) SOMM17/6105/UGR, German Research Foundation (DFG) RYC-2016-20678, Catalan Government 2017SGR1069, Severo Ochoa Excellence Program of MINEICO SEV-2016-0588, CERCA program of the Generalitat de Catalunya

We show that the baryon asymmetry of the universe can be explained in models where the Higgs couples to the Chern-Simons term of the hypercharge group and is away from the late-time minimum of its potential during inflation. The Higgs then relaxes toward this minimum once inflation ends which leads to the production of (hyper)magnetic helicity. We discuss the conditions under which this helicity can be approximately conserved during its joint evolution with the thermal plasma. At the electroweak phase transition the helicity is then converted into a baryon asymmetry by virtue of the chiral anomaly in the standard model. We propose a simple model which realizes this mechanism and show that the observed baryon asymmetry of the universe can be reproduced., 35 pages, 10 figures, 2 tables; v2: small improvements, added comments and references, matches published version

We would like to thank O.J.P. Eboli, M. Perez-Victoria and L.L. Salcedo for fruitful discussions. The work of E.M. is supported by the Spanish MINEICO under grant FIS2017-85053-C2-1-P, by the FEDER/Junta de Andalucia-Consejeria de Economia y Conocimiento 2014-2020 Operational Programme under grant A-FQM-178-UGR18, by Junta de Andalucia under grant FQM-225, and by the Consejeria de Conocimiento, Investigacion y Universidad of the Junta de Andalucia and European Regional Development Fund (ERDF) under grant SOMM17/6105/UGR. The research of E.M. is also supported by the Ramon y Cajal Program of the Spanish MINEICO under grant RYC-2016-20678. The work of M.Q. is partly supported by the Spanish MINEICO under grant FPA2017-88915-P, by the Catalan Government under grant 2017SGR1069, and by the Severo Ochoa Excellence Program of MINEICO under grant SEV-2016-0588. IFAE is partially funded by the CERCA program of the Generalitat de Catalunya., Continuum spectra can be a way out to alleviate the tension generated by the elusiveness of narrow resonances of new physics in direct experimental searches. Motivated by the latter, we consider the linear dilaton model with a continuum spectrum of KK modes. It is provided by a critical exponential bulk potential for the scalar field stabilizing the distance, between the UV boundary at y = 0 and a naked (good) singularity at y = y(s), in proper coordinates, which corresponds in conformal coordinates to z(s) -> infinity. The cutoff M s in this theory is an intermediate scale M-s similar or equal to 10(-5) M-P(1) and the warped factor solves the hierarchy between M-s and the TeV, while the hierarchy between M-P1 and M-s has to be solved by a (Little) String The- ory with coupling g(s) similar or equal to 10(-5). The Standard Model is localized on a 4D IR brane. The graviton and radion Green's and spectral functions have a continuum of states with a TeV mass gap, and isolated poles consisting of the 4D graviton and the light radion/dilaton. We construct the effective field theory below the mass gap where the continua of KK modes are integrated out, generating a set of dimension eight operators which contribute to low-energy electroweak precision observables, and high-energy violation of unitarity in vector boson scattering processes. The radion mass depends on the stabilizing UV brane potential and its wave function is localized toward the IR which enhances its coupling with the SM fields., Spanish MINEICO FPA2017-88915-P FIS2017-85053-C2-1-P, FEDER/Junta de Andalucia-Consejeria de Economia y Conocimiento 2014-2020 Operational Programme A-FQM-178-UGR18, Junta de Andalucia FQM-225, European Commission SOMM17/6105/UGR, German Research Foundation (DFG) RYC-2016-20678, Catalan Government 2017SGR1069, Severo Ochoa Excellence Program of MINEICO SEV-2016-0588, CERCA program of the Generalitat de Catalunya

We systematically study the holographic phase transition of the radion field in a five-dimensional warped model which includes a scalar potential with a powerlike behavior. We consider Kaluza-Klein (KK) resonances with masses mKK at the TeV scale or beyond. The backreaction of the radion field on the gravitational metric is taken into account by using the superpotential formalism. The confinement/deconfinement first order phase transition leads to a gravitational wave stochastic background which mainly depends on the scale mKK and the number of colors, N, in the dual theory. Its power spectrum peaks at a frequency that depends on the amount of tuning required in the electroweak sector. It turns out that the present and forthcoming gravitational wave observatories can probe scenarios where the KK resonances are very heavy. Current aLIGO data already rule out vector boson KK resonances with masses in the interval mKK ∼ ð1–10Þ × 105 TeV. Future gravitational experiments will be sensitive to resonances with masses mKK ≲ 105 TeV (LISA), 108 TeV (aLIGO Design) and 109 TeV (ET). Finally, we also find that the big bang nucleosynthesis bound in the frequency spectrum turns into a lower bound for the nucleation temperature as Tn ≳ 10−4n, Spanish MINEICO FIS2017-85053-C2-1-P FPA2017-88915-P, FEDER/Junta de Andalucia-Consejeria de Economia y Conocimiento 2014-2020 Operational Programme A-FQM-178-UGR18, Junta de Andalucia FQM-225, European Union (EU) SOMM17/6105/UGR, German Research Foundation (DFG) RYC-2016-20678, Catalan Government 2017SGR1069, evero Ochoa Excellence Program of MINEICO SEV2016-0588

We find new limits on the $\lambda'_{3jk}$ R-parity violating (RPV) couplings of the minimal supersymmetric standard model, using Drell-Yan differential cross sections at the LHC and electroweak precision measurements from LEP and SLC. Specifically, limits on six out of the nine $\lambda'_{3jk}$-couplings, with $j=1\text{ or }2$, are obtained using Drell-Yan data, with the remaining three (for $j=3$) bounded by precision electroweak data. We also update the limits on all $\lambda'_{ijk}$-couplings using electroweak data and find new bounds on $\lambda'_{132}$ and $\lambda'_{232}$ that are stronger than obtained elsewhere. A table of all current bounds on $\lambda'_{ijk}$ is given in an appendix.

We forecast the prospective of detection for a stochastic gravitational wave background sourced by cosmological first-order phase transitions. We focus on first-order phase transitions with negligible plasma effects, and consider the experimental infrastructures built by the end of the LISA mission. We make manifest the synergy among LISA, pulsar time array experiments, and ground-based interferometers. For phase transitions above the TeV scale or below the electroweak scale, LISA can detect the corresponding gravitational wave signal together with Einstein Telescope, SKA or even aLIGO-aVIRGO-KAGRA. For phase transitions at the electroweak scale, instead, LISA can be the only experiment observing the gravitational wave signal. In case of detection, by using a parameter reconstruction method that we anticipate in this work, we show that LISA on its own has the potential to determine when the phase transition occurs and, consequently, the energy scale above which the standard model of particle physics needs to be modified. The result may likely guide the collider community in the post-LHC era., Comment: 7 pages, 6 figures; v3: some clarifications, extra references, matches published version

Stop searches in supersymmetric frameworks with $R$-parity conservation usually assume the lightest neutralino to be the lightest supersymmetric particle. In this paper we consider an alternative scenario in which the left-handed tau sneutrino is lighter than neutralinos and stable at collider scales, but possibly unstable at cosmological scales. Moreover the (mostly right-handed) stop $\widetilde t$ is lighter than all electroweakinos, and heavier than the scalars of the third generation doublet, whose charged component, $\widetilde\tau$, is heavier than the neutral one, $\widetilde\nu$. The remaining supersymmetric particles are decoupled from the stop phenomenology. In most of the parameter space, the relevant stop decays are only into $t \widetilde\tau \tau$, $t\widetilde\nu\nu$ and $b \widetilde\nu \tau$ via off-shell electroweakinos. We constrain the branching ratios of these decays by recasting the most sensitive stop searches. Due to the "double invisible" kinematics of the $\widetilde t\to t\widetilde\nu\nu$ process, and the low efficiency in tagging the $t\widetilde\tau\tau$ decay products, light stops are generically allowed. In the minimal supersymmetric standard model with $\sim$ 100 GeV sneutrinos, stops with masses as small as $\sim$ 350 GeV turn out to be allowed at 95% CL., Comment: 25 pages

We show that natural supersymmetry can be embedded in a five-dimensional theory with supersymmetry breaking \`a la Scherk-Schwarz (SS). There is no 'gluino-sucks' problem for stops localized in the four-dimensional brane and gluinos propagating in the full five-dimensional bulk, and sub-TeV stops are easily accommodated. The $\mu / B_\mu$ problem is absent as well; the SS breaking generates a Higgsino Dirac mass and no bilinear Higgs mass parameter in the superpotential is required. Moreover, for non-maximal SS twists leading to $\tan \beta \simeq 1$, the Higgs spectrum is naturally split, in agreement with LHC data. The 125-GeV Higgs mass and radiative electroweak symmetry breaking can be accommodated by minimally extending the Higgs sector with $Y=0$ $SU(2)_L$ triplets., Comment: 34 pages, 12 figures; v2, matches PRD published version

The Supersymmetric Custodial Triplet Model, a supersymmetric generalization of the Georgi-Machacek model, has proven to be an interesting modification of the MSSM. It extends the MSSM Higgs sector by three extra SU(2)L triplets in such a way that approximate custodial invariance is preserved and rho-parameter deviations are kept under control. By means of a sizeable triplet contribution to electroweak breaking the model is able to generate a barrier at tree level between the false vacuum and the electroweak one. This will result in a strong first order phase transition for an important region of the parameter space. We also look at the gravitational waves that could be generated as a result of the phase transition and show how future interferometers could be used as a probe of the model., Comment: 18 pages, 8 figures

For a 5D Standard Model propagating in an AdS background with an IR localized Higgs, compatibility of bulk KK gauge modes with EWPT yields a phenomenologically unappealing KK spectrum (m > 12.5 TeV) and leads to a "little hierarchy problem". For a bulk Higgs the solution to the hierarchy problem reduces the previous bound only by sqrt(3). As a way out, models with an enhanced bulk gauge symmetry SU(2)_R x U(1)_(B-L) were proposed. In this note we describe a much simpler (5D Standard) Model, where introduction of an enlarged gauge symmetry is no longer required. It is based on a warped gravitational background which departs from AdS at the IR brane and a bulk propagating Higgs. The model is consistent with EWPT for a range of KK masses within the LHC reach.

We analyze a scenario where the right-handed neutrinos make part of a strongly coupled conformal field theory and acquire an anomalous dimension \gamma1/2 giving rise to an inverse see-saw mechanism. In this case light sterile neutrinos do appear and neutrino oscillation experiments are able to probe our model., Comment: 11 pages, 4 figures. v2: Discussion of the lepton number violating case corrected; References added

Electroweak baryogenesis provides an attractive explanation of the origin of the matter-antimatter asymmetry that relies on physics at the weak scale and thus it is testable at present and near future high-energy physics experiments. Although this scenario may not be realized within the Standard Model, it can be accommodated within the MSSM provided there are new CP-violating phases and the lightest stop mass is smaller than the top-quark mass. In this work we provide an evaluation of the values of the stop (m_{\tilde t}) and Higgs (m_H) masses consistent with the requirements of electroweak baryogenesis based on an analysis that makes use of the renormalization group improved Higgs and stop potentials, and including the dominant two-loop effects at high temperature. We find an allowed window in the (m_{\tilde t},m_H)-plane, consistent with all present experimental data, where there is a strongly first-order electroweak phase transition and where the electroweak vacuum is metastable but sufficiently long-lived. In particular we obtain absolute upper bounds on the Higgs and stop masses, m_H\lesssim 127 GeV and m_{\tilde t}\lesssim 120 GeV, implying that this scenario will be probed at the LHC., 26 pages, 6 figures; comments added

We investigate the nature (Dirac vs. Majorana) and size of left-handed neutrino masses in a supersymmetric five-dimensional model compactified in the interval [0,\pi R], where quarks and leptons are localized on the boundaries while the gauge and Higgs sectors propagate in the bulk of the fifth dimension. Supersymmetry is broken by Scherk-Schwarz boundary conditions and electroweak breaking proceeds through radiative corrections. Right-handed neutrinos propagate in the bulk and have a general five-dimensional mass M, which localizes the zero modes towards one of the boundaries, and arbitrary boundary terms. We have found that for generic boundary terms left-handed neutrinos have Majorana masses. However for specific boundary configurations left-handed neutrinos are Dirac fermions as the theory possesses a conserved global U(1) symmetry which prevents violation of lepton number. The size of neutrino masses depends on the localization of the zero-modes of right-handed neutrinos and/or the size of the five-dimensional neutrino Yukawa couplings. Left-handed neutrinos in the sub-eV range require either MR~10 or Yukawa couplings ~10^{-3}R, which make the five-dimensional theory perturbative up to its natural cutoff., Comment: 25 pages, 5 figures. v2: Added references

Extensions of the Standard Model Higgs sector with electroweak charged scalars can possess exotic `Higgs' bosons with vanishing or suppressed couplings to Standard Model fermions. These `fermiophobic' scalars, which cannot be produced via gluon fusion, are constrained by LHC measurements of the 125 GeV Higgs boson to have a small vacuum expectation value. This implies that vector boson fusion and associated vector boson production are in general suppressed rendering conventional Higgs searches insensitive. However, Drell-Yan Higgs pair production, which is not present in the SM, can be sizeable even in the limit of vanishing exotic Higgs vacuum expectation value. We utilize this to show that diphoton searches at 8 TeV LHC already rule out a large class of neutral fermiophobic Higgs bosons below around 110 GeV. This includes fermiophobic scalars found in two Higgs doublet as well as Higgs triplet and Georgi-Machacek type models. Our results extend the only relevant limit on fermiophobic Higgs bosons obtained by a recent CDF analysis of $4\gamma + X$ Tevatron data. Furthermore, diphoton limits are independent of the decay of the second Higgs boson and thus apply even for degenerate masses in contrast to the CDF search. We also find that if the fermiophobic Higgs has very enhanced couplings to photons, masses as large as $\sim 150$ GeV can be ruled out while if these couplings are somehow highly suppressed, masses below $\sim 90$ GeV can still be ruled out. Finally, we show that $WW$ and $ZZ$ diboson searches may serve as complementary probes for masses above the diphoton limit up to $\sim 250$ GeV and discuss prospects at 13 TeV LHC., Comment: references and small comments added

We present a novel mechanism of supersymmetry breaking embeddable in string theory and simultaneously sharing the main advantages of (sequestered) gravity and gauge mediation. It is driven by a Scherk-Schwarz deformation along a compact extra dimension, transverse to a D-brane stack supporting the supersymmetric extension of the Standard Model. This fixes the magnitude of the gravitino mass, together with that of the gauginos of a bulk gauge group, at a scale as high as $10^{10}$ GeV. Supersymmetry breaking is mediated to the observable sector dominantly by gauge interactions using massive messengers transforming non-trivially under the bulk and Standard Model gauge groups and leading to a neutralino LSP as dark matter candidate. The Higgsino mass $\mu$ and soft Higgs-bilinear B$\mu$-term could be generated at the same order of magnitude as the other soft terms by effective supergravity couplings as in the Giudice-Masiero mechanism., Comment: 16 pages, 13 figures

We show that the recently reported excess in resonant diboson production can be explained in the context of non-custodial composite Higgs models. Dibosons are generated via the s-channel exchange of massive vector bosons present in these models. We discuss the compatibility of the signal excess with other diboson experimental searches. We also discuss the tension between diboson production and other experimental tests of the model that include electroweak precision data, dilepton, dijet and top pair production and show that there is a region of parameter space in which they are all compatible with the excess., Journal of High Energy Physics, 2015 (9), ISSN:1126-6708, ISSN:1029-8479

If the Standard Model (SM) is an effective theory, as currently believed, it is valid up to some energy scale $\Lambda$ to which the Higgs vacuum expectation value is sensitive throughout radiative quadratic terms. The latter ones destabilize the electroweak vacuum and generate the SM hierarchy problem. For a given perturbative Ultraviolet (UV) completion, the SM cutoff can be computed in terms of fundamental parameters. If the UV mass spectrum involves several scales the cutoff is not unique and each SM sector has its own UV cutoff $\Lambda_i$. We have performed this calculation assuming the Minimal Supersymmetric Standard Model (MSSM) is the SM UV completion. As a result, from the SM point of view, the quadratic corrections to the Higgs mass are equivalent to finite threshold contributions. For the measured values of the top quark and Higgs masses, and depending on the values of the different cutoffs $\Lambda_i$, these contributions can cancel even at renormalization scales as low as multi-TeV, unlike the case of a single cutoff where the cancellation only occurs at Planckian energies, a result originally obtained by Veltman. From the MSSM point of view, the requirement of stability of the electroweak minimum under radiative corrections is incorporated into the matching conditions and provides an extra constraint on the Focus Point solution to the little hierarchy problem in the MSSM. These matching conditions can be employed for precise calculations of the Higgs sector in scenarios with heavy supersymmetric fields., Comment: 36 pages, 5 figures; v2: logarithm corrections included, figures improved, references added

The Supersymmetric Custodial Triplet Model (SCTM) adds to the particle content of the MSSM three $SU(2)_L$ triplet chiral superfields with hypercharge $Y=(0,\pm1)$. At the superpotential level the model respects a global $SU(2)_L \otimes SU(2)_R$ symmetry only broken by the Yukawa interactions. The pattern of vacuum expectation values of the neutral doublet and triplet scalar fields depends on the symmetry pattern of the Higgs soft breaking masses. We study the cases where this symmetry is maintained in the Higgs sector, and when it is broken only by the two doublets attaining different vacuum expectation values. In the former case, the symmetry is spontaneously broken down to the vectorial subgroup $SU(2)_V$ and the $\rho$ parameter is protected by the custodial symmetry. However in both situations the $\rho$ parameter is protected at tree level, allowing for light triplet scalars with large vacuum expectation values. We find that over a large range of parameter space, a light neutralino can supply the correct relic abundance of dark matter either through resonant s-channel triplet scalar funnels or well tempering of the Bino with the triplet fermions. Direct detection experiments have trouble probing these model points because the custodial symmetry suppresses the coupling of the neutralino and the $Z$ and a small Higgsino component of the neutralino suppresses the coupling with the Higgs. Likewise the annihilation cross sections for indirect detection lie below the Fermi-LAT upper bounds for the different channels., Comment: 26 pages, 8 figures; v2 revised comments on classification method and indirect detection section. Results unchanged, matches PRD published version

Gauge mediated supersymmetry breaking (GMSB) is an elegant mechanism to transmit supersymmetry breaking from the hidden to the MSSM observable sector, which solves the supersymmetric flavor problem. However the smallness of the generated stop mixing requires superheavy stops to reproduce the experimental value of the Higgs mass. Two possible ways out are: i) To extend GMSB by direct superpotential messenger-MSSM Yukawa couplings to generate sizeable mixing, thus reintroducing the flavor problem; ii) To extend the MSSM Higgs sector with singlets and/or triplets providing extra tree-level corrections to the Higgs mass. Singlets will not get any soft mass from GMSB and triplets will contribute to the $\rho$ parameter which could be an issue. In this paper we explore the second way by introducing extra supersymmetric triplets with hypercharges $Y=(0,\pm 1)$, with a tree-level custodial $SU(2)_L\otimes SU(2)_R$ global symmetry in the Higgs sector protecting the $\rho$ parameter: a supersymmetric generalization of the Georgi-Machacek model, dubbed as supersymmetric custodial triplet model (SCTM). The renormalization group running from the messenger to the electroweak scale mildly breaks the custodial symmetry. We will present realistic low-scale scenarios (with the NLSP being a Bino-like neutralino or the right-handed stau) based on general (non-minimal) gauge mediation and consistent with all present experimental data. Their main features are: i) Light ($\sim 1$ TeV) stops; ii) Exotic couplings ($H^\pm W^\mp Z$ and $H^{\pm\pm} W^\mp W^\mp$) absent in the MSSM and proportional to the triplets VEV, $v_\Delta$; and, iii) A possible (measurable) universality breaking of the Higgs couplings $\lambda_{WZ}=r_{WW}/r_{ZZ}\neq 1$., Comment: 21 pages, 9 figures; v2: references added

We study breaking and restoration of supersymmetry in five-dimensional theories by determining the mass spectrum of fermions from their equations of motion. Boundary conditions can be obtained from either the action principle by extremizing an appropriate boundary action (interval approach) or by assigning parities to the fields (orbifold approach). In the former, fields extend continuously from the bulk to the boundaries, while in the latter the presence of brane mass-terms cause fields to jump when one moves across the branes. We compare the two approaches and in particular we carefully compute the non-trivial jump profiles of the wavefunctions in the orbifold picture for very general brane mass terms. We also include the effect of the Scherk-Schwarz mechanism in either approach and point out that for a suitable tuning of the boundary actions supersymmetry is present for arbitrary values of the Scherk-Schwarz parameter. As an application of the interval formalism we construct bulk and boundary actions for super Yang-Mills theory. Finally we extend our results to the warped Randall-Sundrum background., Comment: 17 pages, v2: affiliations corrected

Transverse (submillimeter) and longitudinal (TeV) extra dimensions can help in dealing with the Higgs hierarchy problem. On the one hand large transverse dimensions can lower the fundamental scale of quantum gravity from the Planck scale to the TeV range. On the other hand longitudinal dimensions can provide genuine extra-dimensional symmetries (higher dimensional gauge symmetry and/or supersymmetry) to protect the Higgs mass against ultraviolet sensitivity. In this article we review recent developments along these directions. To cite this article: K. Benakli, M. Quiros, C. R. Physique 4 (2003).

We study an extension of the minimal supersymmetric standard model with a zero hypercharge triplet, and the effect that such a particle has on stop decays. This model has the capability of predicting a 125.5 GeV Higgs even in the presence of light stops and it can modify the diphoton rate by means of the extra charged fermion triplet coupled to the Higgs. Working in the limit where the scalar triplet decouples, and with small values of mA, we find that the fermion triplet can greatly affect the branching ratios of the stops, even in the absence of a direct stop-triplet coupling. We compare the triplet extension with the MSSM and discuss how the additional fields affect the search for stop pair production., Comment: pdfLateX, 16 pages, 7 figures, 2 tables, Typos, minor changes. Version published in JHEP

In this talk we review the actual situation concerning electroweak phase transition and baryogenesis in the minimal supersymmetric extension of the Standard Model. A strong enough phase transition requires light Higgs and stop eigenstates. For a Higgs mass in the range 110--115 GeV, there is a stop window in the range 105--165 GeV. If the Higgs is heavier than 115 GeV, stronger constrains are imposed on the space of supersymmetric parameters. A baryon-to-entropy ratio is generated by the chargino sector provided that the $\mu$ parameter has a CP-violating phase larger than $\sim$ 0.04., Comment: 10 pages, 6 figures, uses espcrc2.sty. Contribution to the conferences: Strong Electroweak Matter, Centre de Physique Theorique, Universite de Marseille, Marseilles (France), June 14-17, 2000; and, Thirty years of supersymmetry, Theoretical Physics Institute, School of Physics and Astronomy, Minneapolis, Minnesota (USA), October 16-27, 2000

We apply a general formalism for the improved effective potential with several mass scales to compute the scale M of new physics which is needed to stabilize the Standard Model potential in the presence of a light Higgs. We find, by imposing perturbativity of the new physics, that M can be as large as one order of magnitude higher than the instability scale of the Standard Model. This implies that, with the present lower bounds on the Higgs mass, the new physics could easily (but not necessarily) escape detection in the present and future accelerators., Comment: latex2e, 12 pages, 3 figures

In the four-dimensional supersymmetric standard model extended with gauge singlets the lightest Higgs boson mass, $M_H$, has an important contribution proportional to the squared of the superpotential coupling $\lambda$ of singlets to Higgs fields, $\lambda SH_1\cdot H_2$. The requirement of perturbativity up to the unification scale yields an upper bound on $M_H\sim 140$ GeV. In extensions to theories with (longitudinal) extra dimensions at the TeV where such coupling exists and massive Kaluza-Klein states fall into N=2 representations, if either of the Higgs or singlet fields live in the bulk of the extra dimensions, the $\beta$-function of $\lambda$ is suppressed due to the absence of anomalous dimension of hypermultiplets to leading order. This implies a slower running of $\lambda$ and an enhancement of its low energy value. The $M_H$ upper bound increases to values $M_H\lesssim 165$ GeV., Comment: Latex2e, 11 pages, 1 figure

We review the baryogenesis scenario in the MSSM at the perturbative level and, in particular, the impact of two-loop corrections on the strength of the phase transition and the amount of generated baryon asymmetry. We confirm the baryogenesis window, where m_H is less than 115GeV, for m_Q less than a few GeV, and the right-handed stop mass is constrained in the region between 100GeV and the top mass. This scenario will be tested at LEP and Tevatron colliders.

We present a simple N=1 five-dimensional model where the fifth dimension is compactified on the orbifold $S^1/Z_2$. Non-chiral matter lives in the bulk of the fifth dimension (five dimensions) while chiral matter lives on the fixed points of the orbifold (four-dimensional boundaries). The massless sector constitutes the Minimal Supersymmetric Standard Model while the massive modes rearrange in N=2 supermultiplets. After supersymmetry breaking by the Scherk-Schwarz mechanism the zero modes can be reduced to the non-supersymmetric Standard Model., Comment: 8 pages, LaTex

We describe the breaking of supersymmetry in M-theory by coordinate dependent (Scherk-Schwarz) compactification of the eleventh dimension. Supersymmetry is spontaneously broken in the gravitational and moduli sector and communicated to the observable sector, living at the end-point of the semicircle, by radiative gravitational interactions. This mechanism shares the generic features of non-perturbative supersymmetry breaking by gaugino condensation, in the presence of a constant antisymmetric field strength, in the weakly coupled regime of the heterotic string, which suggests that both mechanisms could be related by duality. In particular an analysis of supersymmetric transformations in the infinite-radius limit reveals the presence of a discontinuity in the spinorial parameter, which coincides with the result found in the presence of gaugino condensation, while the condensate is identified with the quantized parameter entering the boundary conditions, Comment: 10 pages, latex + espcrc2.sty, no figures. Based on talks given at the 5th International Conference on Supersymmetries in Physics, SUSY 97, May 27-31, 1997, University of Pennsylvania, Philadelphia, PA; and, International Europhysics Conference on High Energy Physics, HEP 97, 19-26 August, Jerusalem, Israel

The minimal supersymmetric extension of the Standard Model (SM) is a well motivated scenario for physics beyond the SM, which allows a perturbative description of the theory up to scales of the order of the Grand Unification scale, where gauge couplings unify. The Higgs mass parameter is insensitive to the ultraviolet physics and is only sensitive to the scale of soft supersymmetry breaking parameters. Present collider bounds suggest that the characteristic values of these parameters may be significantly larger than the weak scale. Large values of the soft breaking parameters, however, induce large radiative corrections to the Higgs mass parameter and therefore the proper electroweak scale may only be obtained by a fine tuned cancellation between the square of the holomorphic \mu-parameter and the Higgs supersymmetry breaking square mass parameter. This can only be avoided if there is a correlation between the scalar and gaugino mass parameters, such that the Higgs supersymmetry breaking parameter remains of the order of the weak scale. The scale at which this happens is dubbed as focus point. In this article, we define the general conditions required for this to happen, for different values of the messenger scale at which supersymmetry breaking is transmitted to the observable sector, and for arbitrary boundary conditions of the sfermion, gaugino, and Higgs mass parameters. Specific supersymmetry breaking scenarios in which these correlations may occur are also discussed., Comment: 19 pages, 9 figures, new refs. added

Higgs triplet models are known to have difficulties obtaining agreement with electroweak precision data and in particular constraints on the $\rho$ parameter. Either a global $SU(2)_L \otimes SU(2)_R$ symmetry has to be imposed on the scalar potential at the electroweak scale, as done in the well-known Georgi-Machacek (GM) model, or the triplet vacuum expectation values must be very small. We construct a supersymmetric model that can satisfy constraints on the $\rho$ parameter, even if these two conditions are not fulfilled. We supersymmetrize the GM model by imposing the $SU(2)_L \otimes SU(2)_R$ symmetry at a scale $\mathcal M$, which we argue should be at or above the messenger scale, where supersymmetry breaking is transmitted to the observable sector. We show that scales $\mathcal M$ well above 100 TeV and sizable contributions from the triplets to electroweak symmetry breaking can be comfortably accommodated. We discuss the main phenomenological properties of the model and demonstrate that the departure from custodial symmetry at the electroweak scale, due to radiative breaking, can show up at the LHC as a deviation in the `universal' relation for the Higgs couplings to $WW$ and $ZZ$. As a by-product of supersymmetry, we also show that one can easily obtain both large tree-level \emph{and} one loop corrections to the Higgs mass. This allows for stops that can be significantly lighter and with smaller mixing than those needed in the MSSM., Comment: v3 small comments added. version to be published in journal

We compute the graviton Kaluza-Klein spectrum on a gravity-dilaton background with a naked singularity for all possible boundary conditions at the singularity which are consistent with unitary evolution. We apply methods from non-relativistic quantum mechanics with singular Schr\"{o}dinger potentials. In general the spectrum contains a tachyon, a sign of instability. Only for a particular boundary condition at the singularity is the spectrum free of tachyons. In this case the lowest-lying graviton mode is massless. We argue that this result will also hold for other backgrounds with similar geometry near the curvature singularity. We complete our study with a brief discussion on radion perturbations and the Higgs mechanism on this singular background., Comment: 23 pages, 10 figures. Presentation improved. Unchanged in substance. To appear in Phys Rev D

We analyze the extension of the Minimal Supersymmetric Standard Model which includes extra Y=(0,\pm 1) supersymmetric triplets with a global SU(2)_L \otimes SU(2)_R symmetry spontaneousy broken to the custodial SU(2)_V by the vacuum expectation value of the neutral scalar components of doublets and triplets. The model is the supersymmetrization of the non-supersymmetric model introduced long ago by Georgi and Machacek where the \rho-parameter is kept to unity at the tree-level by the custodial symmetry. Accordingly the scalar sector is classified into degenerate SU(2)_V multiplets: singlets, triplets (including the one containing the Godstone bosons) and fiveplets. The singly and doubly charged chiral superfields play a key role in the unitarization of the theory. The couplings of the Standard Model-like Higgs to vector bosons (including \gamma\gamma) and fermions, and the corresponding Higgs signal strengths, are in agreement with LHC experimental data for a large region of the parameter space. Breaking of custodial invariance by radiative corrections suggests a low-scale mechanism of supersymmetry breaking., Comment: 33 pages, 9 figures. v2: Version to appear in PRD. Typos corrected and references added. Figs. 9 and 10 improved

We perform an analysis of the behaviour of the electroweak phase transition in the Minimal Supersymmetric Standard Model, in the presence of light stops. We show that, in previously unexplored regions of parameter space, the order parameter $v(T_c)/T_c$ can become significantly larger than one, for values of the Higgs and supersymmetric particle masses consistent with the present experimental bounds. This implies that baryon number can be efficiently generated at the electroweak phase transition. As a by-product of this study, we present an analysis of the problem of colour breaking minima at zero and finite temperature, and we use it to investigate the region of parameter space preferred by the best fit to the present precision electroweak measurement data, in which the left-handed stops are much heavier than the right-handed ones., 16 pages, LaTeX + 4 figures, psfig. (uuencoded)

We generalize the analytical expressions for the two-loop leading-log neutral Higgs boson masses and mixing angles to the case of general left- and right-handed soft supersymmetry breaking stop and sbottom masses and left--right mixing mass parameters ($m_Q, m_U, m_D, A_t, A_b$). This generalization is essential for the computation of Higgs masses and couplings in the presence of light stops. At high scales we use the minimal supersymmetric standard model effective potential, while at low scales we consider the two-Higgs doublet model (renormalization group improved) effective potential, with general matching conditions at the thresholds where the squarks decouple. We define physical (pole) masses for the top-quark, by including QCD self-energies, and for the neutral Higgs bosons, by including the leading one-loop electroweak self-energies where the top/stop and bottom/sbottom sectors propagate. For $m_Q=m_U=m_D$ and moderate left--right mixing mass parameters, for which the mass expansion in terms of renormalizable Higgs quartic couplings is reliable, we find excellent agreement with previously obtained results., 35 pages, Latex, 9 uuencoded figures

We will explore the consequences on the electroweak breaking condition, the mass of supersymmetric partners and the scale at which supersymmetry is broken, for arbitrary values of the supersymmetric parameters tan(beta) and the stop mixing X_t, which follow from the Higgs discovery with a mass m_H\simeq 126 GeV at the LHC. Within the present uncertainty on the top quark mass we deduce that radiative breaking requires tan(beta) \gtrsim 7 for maximal mixing X_t\simeq \sqrt{6}, and tan(beta) \gtrsim 20 for small mixing X_t\lesssim 1. The scale at which supersymmetry is broken \mathcal M can be of order the unification or Planck scale only for large values of tan(beta) and negligible mixing X_t\simeq 0. On the other hand for maximal mixing and large values of tan(beta) supersymmetry should break at scales as low as \mathcal M\simeq 10^5 GeV. The uncertainty in those predictions stemming from the uncertainty in the top quark mass, i.e. the top Yukawa coupling, is small (large) for large (small) values of tan(beta). In fact for tan(beta)=1 the uncertainty on the value of \mathcal M is of several orders of magnitude., Comment: 16 pages, 7 figures; v2: numerical typo corrected in codes, and 2 loop radiative corrections added. Some conclusions slightly changed

Extending the Higgs sector of the MSSM by triplets alleviates the little hierarchy problem and naturally allows for enhancements in the diphoton decay rate of the lightest CP-even Higgs h. In the present paper we analyze in detail the Higgs phenomenology of this theory with m_h~126 GeV. We mostly focus on a light Higgs sector where the pseudoscalar A, the next-to-lightest CP-even scalar H and the charged H^\pm Higgses are naturally at the electroweak scale. It turns out that for any value m_A > m_h there is a parameter region at small tan(beta) where the CP-even Higgs sector appears at colliders as the SM one, except for loop-induced corrections. Notably the existence of this SM-like point, which is absent in the MSSM, is shared with supersymmetric theories where there are extra singlets. We also highlight a second parameter region at small m_A and small tan(beta) where the h signal strengths, diphoton channel included, are SM-like except those of bottoms and taus which can have at most a 10-15 % splitting. Improvements in the A and H^\pm searches are worthwhile in order to discriminate this scenario from the SM., 24 pages, 6 figures; v2: added references; v3: some comments clarified, Fig.6 improved, version published in JHEP

In this paper we have considered the possibility that the Standard Model, and its minimal extension with the addition of singlets, merges with a high-scale supersymmetric theory at a scale satisfying the Veltman condition and therefore with no sensitivity to the cutoff. The matching of the Standard Model is achieved at Planckian scales. In its complex singlet extension the matching scale depends on the strength of the coupling between the singlet and Higgs fields. For order one values of the coupling, still in the perturbative region, the matching scale can be located in the TeV ballpark. Even in the absence of quadratic divergences there remains a finite adjustment of the parameters in the high-energy theory which should guarantee that the Higgs and the singlets in the low-energy theory are kept light. This fine-tuning (unrelated to quadratic divergences) is the entire responsibility of the ultraviolet theory and remains as the missing ingredient to provide a full solution to the hierarchy problem., Comment: v1: 24 pages, 7 figures; v2: 24 pages, 7 figures, added references; v3: 26 pages, 8 figures, text improved, added section on dark matter, matches version published in PRD

We consider the top-bottom doublet in the background of the sphaleron for the realistic case of large non-degeneracy of fermion masses, in particular $m_b=5$ GeV and $m_t=175$ GeV. We propose an axially symmetric $(r,\theta)$-dependent ansatz for fermion fields and investigate the effects of the non-degeneracy on them. The exact solution is described, with an error less than 0.01\%, by a set of ten radial functions. We also propose an approximate solution, in the $m_b/m_t\rightarrow 0$ limit, with an error ${\cal O}(m_b/m_t)$. We have found that the effects of non-degeneracy provide a $\theta$-dependence typically $\sim 10\%$., Comment: 12 pages, latex + psfig, 4 uuencoded figures

We consider the generation of the hierarchical charged lepton spectrum and anarchic neutrino masses and mixing angles in warped extra dimensional models with Randall-Sundrum metric. We have classified all possible cases giving rise to realistic spectra for both Dirac and Majorana neutrinos. An anarchic neutrino spectrum requires a convenient bulk symmetry broken by boundary conditions on both UV and IR branes. We have in particular considered the case of Majorana neutrinos with a continuous bulk symmetry. To avoid unwanted massless extra gauge bosons the 4D group should be empty. If the 4D coset is not vanishing it can provide a Wilson Line description of the neutrino Majorana mass matrix. We have studied an example based on the bulk gauge group U(3)_{L} \otimes U(3)_{N} \otimes_{i} U(1)_{E^i} with the Wilson Line in SO(3)_{N} satisfying all required conditions. A \chi^2-fit to experimental data exhibits the 95% CL region in the parameter space with no fine-tuning. As a consequence of the symmetries of the theory there is no tree-level induced lepton flavor violation and so one-loop processes are consistent with experimental data for KK-modes about a few TeV. The model is easily generalizable to models with IR deformed metrics with similar conclusions., Comment: 28 pages, 9 eps plots, uses axodraw; v2 Title changed, comments on phenomenology added, version to be published in JHEP