Search

Recent results on Higgs searches at the LHC point towards the existence of a Higgs boson with mass of about 126 GeV whose diphoton decay rate tends to be larger than in the Standard Model. These results are in tension with natural MSSM scenarios: such a Higgs mass requires heavy (third-generation) squarks which reintroduce some amount of fine-tuning and in general the Higgs diphoton decay rate tends to follow the Standard Model result. In this paper we prove that these problems can be alleviated by introducing an extra supersymmetric triplet coupled to the Higgs in the superpotential. This superfield generates a sizeable tree-level correction to the Higgs mass so that the third generation is no longer required to be heavy, and its charged component enhances the diphoton Higgs decay rates by as much as 50% with respect to the Standard Model values. We also show that such a scenario is compatible with present electroweak precision observables., Comment: 11 pages, 8 eps figures; v2: Small corrections in numerical calculations and two figures. Conclusions unchanged

In four-dimensional heterotic superstrings, the dilaton and antisymmetric tensor fields belong to a linear N = 1 supersymmetric multiplet L. We study the lagrangian describing the coupling of one linear multiplet to chiral and gauge multiplets in global and local supersymmetry, with particular emphasis on string tree-level and loop-corrected effective actions. This theory is dual to an equivalent one with chiral multiplets only. But the formulation with a linear multiplet appears to have decisive advantages beyond string tree-level since, in particular, 〉L〉 is the string loop-counting parameter and the duality transformation is in general not exactly solvable beyond tree-level. In this context we discuss some non-renormalization theorems in the effective theory and obtain explicitly some loop corrections to the string effective supergravity for simple compactifications. Finally, we discuss the issue of supersymmetry breaking by gaugino condensation using this formalism.

Perturbative breaking of supersymmetry in four-dimensional string theories predict in general the existence of new large dimensions at the TeV scale. Such large dimensions lie in a domain of energies accessible to particle accelerators. Their main signature is the production of Kaluza-Klein excitations which can be detected at future colliders. We study this possibility for hadron colliders (TEVATRON, LHC) and $e^+ e^-$ colliders (LEP-200, NLC-500)., Comment: 13 pages, LATEX, 4 postscript figures appended at the end, CPTH-A293.0294 and IEM-FT-84/94

A Standard-Model-like Higgs boson should be light in order to comply with electroweak precision measurements from LEP. We consider five-dimensional (5D) warped models -- with a deformation of the metric in the IR region -- as UV completions of the Standard Model with a heavy Higgs boson. Provided the Higgs boson propagates in the 5D bulk the Kaluza Klein (KK) modes of the gauge bosons can compensate for the Higgs boson contribution to oblique parameters while their masses lie within the range of the LHC. The little hierarchy between KK scale and Higgs mass essentially disappears and the naturalness of the model greatly improves with respect to the AdS (Randall-Sundrum) model. In fact the fine-tuning is better than 10% for all values of the Higgs boson mass., 5 pages, 5 figures

The generation of the observed baryon asymmetry may have taken place during the electroweak phase transition, thus involving physics testable at LHC, a scenario dubbed electroweak baryogenesis. In this paper we point out that the magnetic field which is produced in the bubbles of a first order phase transition endangers the baryon asymmetry produced in the bubble walls. The reason being that the produced magnetic field couples to the sphaleron magnetic moment and lowers the sphaleron energy; this strengthens the sphaleron transitions inside the bubbles and triggers a more effective wash out of the baryon asymmetry. We apply this scenario to the Minimal Supersymmetric extension of the Standard Model (MSSM) where, in the absence of a magnetic field, successful electroweak baryogenesis requires the lightest CP-even Higgs and the right-handed stop masses to be lighter than about 127 GeV and 120 GeV, respectively. We show that even for moderate values of the magnetic field, the Higgs mass required to preserve the baryon asymmetry is below the present experimental bound. As a consequence electroweak baryogenesis within the MSSM should be confronted on the one hand to future measurements at the LHC on the Higgs and the right-handed stop masses, and on the other hand to more precise calculations of the magnetic field produced at the electroweak phase transition., 16 pages, 4 figures. Minor corrections and references added to match published version

We elaborate on a recently proposed mechanism to suppress large contributions to the electroweak precision observables in five dimensional (5D) warped models, without the need for an extended 5D gauge sector. The main ingredient is a modification of the AdS metric in the vicinity of the infrared (IR) brane corresponding to a strong deviation from conformality in the IR of the 4D holographic dual. We compute the general low energy effective theory of the 5D warped Standard Model, emphasizing additional IR contributions to the wave function renormalization of the light Higgs mode. We also derive expressions for the S and T parameters as a function of a generic 5D metric and zero-mode wave functions. We give an approximate formula for the mass of the radion that works even for strong deviation from the AdS background. We proceed to work out the details of an explicit model and derive bounds for the first KK masses of the various bulk fields. The radion is the lightest new particle although its mass is already at about 1/3 of the mass of the lightest resonances, the KK states of the gauge bosons. We examine carefully various issues that can arise for extreme choices of parameters such as the possible reintroduction of the hierarchy problem, the onset of nonperturbative physics due to strong IR curvature or the creation of new hierarchies near the Planck scale. We conclude that a KK scale of 1 TeV is compatible with all these constraints., Comment: 44 pages, 11 figures, references added

We study the electroweak phase transition in the minimal extension of the Standard Model: an extra complex singlet with zero vacuum expectation value. The first-order phase transition is strengthened by the cubic term triggered in the one-loop effective potential by the extra boson. Plasma effects are considered to leading order: they shield the cubic terms and weaken the first-order phase transition. We find a region in the parameter space where baryon asymmetry washout is avoided for Higgs masses consistent with present experimental bounds. However in that region the theory becomes non-perturbative for scales higher than $10^{10}\ GeV$., Comment: 11 pages (plus 5 figures.ps available upon request), latex, IEM-FT-67/93

Supersymmetry breaking in string perturbation theory predicts the existence of a new dimension at the TeV scale. The simplest realization of the minimal supersymmetric Standard Model in the context of this mechanism has two important consequences: (i) A natural solution to the $\mu$-problem; (ii) The absence of quadratic divergences in the cosmological constant, which leads to a dynamical determination of the supersymmetry breaking and electroweak scale. We present an explicit example in which the whole particle spectrum is given as a function of the top quark mass. A generic prediction of this mechanism is the existence of Kaluza-Klein excitations for gauge bosons and higgses. In particular the first excitation of the photon could be accessible to future accelerators and give a clear signal of the proposed mechanism., Comment: 27 pages, latex, 6 figures available by FAX upon request

In a general supersymmetric standard model there is an upper bound $m_h$ on the tree level mass of the $CP=+1$ lightest Higgs boson which depends on the electroweak scale, $\tan \beta$ and the gauge and Yukawa couplings of the theory. When radiative corrections are included, the allowed region in the $(m_h,m_t)$ plane depends on the scale $\Lambda$, below which the theory remains perturbative, and the supersymmetry breaking scale $\Lambda_s$, that we fix to $1\ TeV$. In the minimal model with $\Lambda=10^{16}\ GeV$: $m_h, Comment: 13 pages, latex, IEM-FT-64/92 (5 postscript figures availables upon request)

We study radion stabilization in the compact Randall-Sundrum model by introducing a bulk scalar field, as in the Goldberger and Wise mechanism, but (partially) taking into account the backreactions from the scalar field on the metric. Our generalization reconciles the radion potential found by Goldberger and Wise with the radion mass obtained with the so-called superpotential method where backreaction is fully considered. Moreover we study the holographic phase transition and its gravitational wave signals in this model. The improved control over backreactions opens up a large region in parameter space and leads, compared to former analysis, to weaker constraints on the rank N of the dual gauge theory. We conclude that, in the regime where the 1/N expansion is justified, the gravitational wave signal is detectable by LISA., Comment: 42 pages, 4 figures; v2: minor changes for the publication

We propose an alternative to the introduction of an extra gauge (custodial) symmetry to suppress the contribution of KK modes to the T parameter in warped theories of electroweak breaking. The mechanism is based on a general class of warped 5D metrics and a Higgs propagating in the bulk. The metrics are nearly AdS in the UV region but depart from AdS in the IR region, towards where KK fluctuations are mainly localized, and have a singularity outside the slice between the UV and IR branes. This gravitational background is generated by a bulk stabilizing scalar field which triggers a natural solution to the hierarchy problem. Depending on the model parameters, gauge-boson KK modes can be consistent with present bounds on EWPT for m > 1 TeV at 95% CL. The model contains a light Higgs mode which unitarizes the four-dimensional theory. The reduction in the precision observables can be traced back to a large wave function renormalization for this mode., 15 pages, 3 figures

A supersymmetric standard model with an enlarged Higgs sector can evade the strongest top-quark mass bounds from electroweak precision measurements as well as the tree-level bounds on the Higgs boson mass of the minimal model, provide new experimental signatures in the charged Higgs sector and still (technically) solve the m Pl / m W gauge-hierarchy problem. We explore in detail the simplest model satisfying all the above conditions. It contains a Higgs triplet with zero hypercharge and the most general (renormalizable) superpotential consistent with gauge invariance. We study the electroweak breaking and stability conditions of the scalar potential and the mass matrices and sum rules in the bosonic Higgs sector. We present approximate analytic formulae for mass eigenvalues and eigenstates for values of the ϱ-parameter very close to unity. We also make a numerical analysis of the allowed regions in the multi-parameter space and of mass spectra. Finally, a distinct feature of the model, unlike in the minimal model, is that γ and Z have tree-level couplings to W ± and C ± (the charged Higgses). We study the cross section for the process e + e - → W ± C ± at present and future e + e - colliders.

We perform a fit to precise electroweak data to determine the Higgs and top masses. Penalty functions taking into account their production limits are included. We find ${\displaystyle m_H=65^{+245}_{-4}\ GeV}$ and ${\displaystyle m_t=122^{+25}_{-20}\ GeV}$. However whereas the top $\chi^2$ distribution behaves properly near the minimum, the Higgs $\chi^2$ distribution does not, indicating a statistical fluctuation or new physics. In fact no significative bound on the Higgs mass can be given at present. However, if the LEP accuracy is improved and the top is discovered in the preferred range of top masses, a meaningful bound on the Higgs mass could be obtained within the standard model framework., Comment: 11 pages

We present a global fit to neutral-current data (including improved parity-violation data), M W and the last year LEP results in the standard model and its minimal gauge extensions. In the standard model, for m H = 100 GeV [ m H = 1 TeV] and α s ( M Z ) = 0.12 m t = 112 −23 +22 GeV [ m t = 140 −21 +19 GeV] (1 σ ). For gauge extensions with a minimal (arbitrary) Higgs doublet content, the M Z′ lower bound improves (with respect to our previous analysis) by about 200 (50) GeV. In the case with an arbitrary Higgs doublet content, the bound on the Z 0 Z′ mixing is reduced by a factor of about 4. No data set seems to favour a new Z′, meaning that present Z′ limits are governed by the data error size only. The relevance of the different data in the fits is discussed, as well as the correlation between M Z′ and m t,H bounds. Present limits on magic observables and mixing parameters are given.

We reexamine the nucleosynthesis predictions for the simplest class of scalar-tensor theories of gravity, the Jordan-Brans-Dicke theory. Comparison with present observational data on light elements’ abundances provides bounds on the ω-parameter which are as strong as the post-Newtonian ones, or even stronger, depending on the value of Ωh2. Future improvements in the determination of the 4 He and D +3He abundances will increase this bound dramatically. We argue that a future (hypothetical) experimental deviation from general relativity prediction could be restored by a Jordan-Brans-Dicke theory.

We study the possibility of extended inflation in the effective theory of gravity from strings compactified to four dimensions and find that it strongly depends on the mechanism of supersymmetry breaking. We consider a general class of string--inspired models which are good candidates for successful extended inflation. In particular, the $\omega$--problem of ordinary extended inflation is automatically solved by the production of only very small bubbles until the end of inflation. We find that the inflaton field could belong either to the untwisted or to the twisted massless sectors of the string spectrum, depending on the supersymmetry breaking superpotential., Comment: 18pp

We have studied the conformal properties and cosmological solutions of the four-dimensional gravitational action for different supersymmetric and non-supersymmetric string scenarios, assuming flat directions in the configuration space of moduli and dilaton fields. We have explicitly shown the equivalence between different conformal frames and chosen a preferred frame in which the relevant masses are constant. The theory of gravity for the radiation dominated era is General Relativity. During the matter dominated era all analyzed examples are in conflict with the bounds on the time variation of gauge couplings and/or with the post-newtonian bounds of General Relativity.

We compute the two-loop corrections to the mass of the lightest Higgs boson ( m h ), in the minimal and non-minimal (including a gauge singlet) supersymmetric standard model, when the scale of supersymmetry breaking ( M SUSY ) is larger than the electroweak scale. We use the effective potential in the leading logarithm approximation and renormalization group techniques. We find that, even in cases where the one-loop correction is larger than the tree-level mass, the two-loop correction stays within a few percent. We present, for minimal and non-minimal models, two-loop upper bounds, which depend on the top-quark mass ( m t ), and M SUSY . These theoretical bounds, along with future experimental bounds on m t and m h , could be used to push M SUSY towards higher scales.

We discuss present neutral current data constraints (combined with M W and M Z measurements) on m t (and m H ) in the minimal gauge extension of the standard model. The extra gauge boson mass M Z , is the only new parameter (for fixed new gauge couplings λ and θ 1 ). Fits performed with M Z , and m t free, translate into allowed regions in the M Z ,− m t plane where the M Z , lower bound is correlated with the m t upper bound. As a consequence, the m t upper bound in extended electroweak models is stronger (lower) than the one obtained in the minimal standard model, and coincides with it in the limits M Z , → ∞. The influence of the different pieces of data on the fits is also discussed. We briefly comment on other constraints, such as those from charged current data and direct production at large hadron colliders.

We discuss the four-dimensional theory of gravity which arises from compactifications of the heterotic string. Its main feature is that the dilaton field is coupled to all non-gravitational matter in the Universe. Its cosmological solutions correspond to General Relativity or departures therefrom given by a JBD-like theory labelled by an ω-parameter. Unlike in ordinary JBD gravity (where the scalar field is decoupled from the non-gravitational sector), bounds on the time variation of the gauge coupling constants provide very strong constraints on ω. In particular data from the Oklo reactor translate into ω > 1015.

In the standard model a right-handed neutrino can appear only by mixing with the left-handed neutrinos. Precise electroweak data constrain this mixing. We consider models in which the mixing is not proportional to neutrino masses. We have analyzed data and constraints on mt,mH with and without mixing (sine) of neutrinos with new inert fermions. For universal v mixing, sine=0.09; if only vτ mixes, sineτ =0.22. Thus vτ data, Γτ and ΓinvZ, if interpreted without caution, perhaps favor a nonzero vτ mixing and the existence of heavy neutral fermions.

We discuss some cosmological implications of extensions of the Standard Model with hidden sector scalars coupled to the Higgs boson. We put special emphasis on the conformal case, in which the electroweak symmetry is broken radiatively with a Higgs mass above the experimental limit. Our refined analysis of the electroweak phase transition in this kind of models strengthens the prediction of a strongly first-order phase transition as required by electroweak baryogenesis. We further study gravitational wave production and the possibility of low-scale inflation as well as a viable dark matter candidate., 23 pages, 8 figures; some comments added, published version

Electroweak baryogenesis in the minimal supersymmetric extension of the Standard Model may be realized within the light stop scenario, where the right-handed stop mass remains close to the top-quark mass to allow for a sufficiently strong first order electroweak phase transition. All other supersymmetric scalars are much heavier to comply with the present bounds on the Higgs mass and the electron and neutron electric dipole moments. Heavy third generation scalars render it necessary to resum large logarithm contributions to perform a trustable Higgs mass calculation. We have studied the one--loop RGE improved effective theory below the heavy scalar mass scale and obtained reliable values of the Higgs mass. Moreover, assuming a common mass $\tilde m$ for all heavy scalar particles, and values of all gaugino masses and the Higgsino mass parameter about the weak scale, and imposing gauge coupling unification, a two-loop calculation yields values of the mass $\tilde m$ in the interval between three TeV and six hundred TeV. Furthermore for a stop mass around the top quark mass, this translates into an upper bound on the Higgs mass of about 150 GeV. The Higgs mass bound becomes even stronger, of about 129 GeV, for the range of stop and gaugino masses consistent with electroweak baryogenesis. The collider phenomenology implications of this scenario are discussed in some detail., 28 pages, 13 figures, uses axodraw.sty; v2: To appear in JHEP

The coupling of an unparticle operator O_U to Standard Model particles opens up the possibility of unparticle decays into standard model fields. We study this issue by analyzing the pole structure (and spectral function) of the unparticle propagator, corrected to account for one-loop polarization effects from virtual SM particles. We find that the propagator of a scalar unparticle (of scaling dimension 1 < d_U < 2) with a mass gap m_g develops an isolated pole, m_p^2-i m_p Gamma_p, with m_p^2 < m_g^2 below the unparticle continuum that extends above m_g (showing that the theory would be unstable without a mass gap). If that pole lies below the threshold for decay into two standard model particles the pole corresponds to a stable unparticle state (and its width Gamma_p is zero). For m_p^2 above threshold the width is non zero and related to the unparticle decay rate into Standard Model particles. This picture is valid for any value of d_U in the considered range., 11 pages, 4 figures

12 LaTeX pages, 2 figures.-- Published in: JHEP04(2008)028.-- Final full-text version available at: http://dx.doi.org/10.1088/1126-6708/2008/04/028., A renormalizable coupling between the Higgs and a scalar unparticle operator O_U of non-integer dimension d_U, Work supported in part by the European Commission under the European Union through the Marie Curie Research and Training Networks “Quest for Unification” (MRTN-CT- 2004-503369) and “UniverseNet” (MRTN-CT-2006-035863); by the Spanish Consolider- Ingenio 2010 Programme CPAN (CSD2007-0042); by a Comunidad de Madrid project (P-ESP-00346) and by CICYT, Spain, under contracts FPA 2007-60252 and FPA 2005-02211.

We have analyzed the closed bosonic string effective action to all orders in string loops. Using the proper-time regularization of the local logarithmic infinities, we propose a renormalization scheme valid for all genera, which leaves only a dilaton potential as higher string-loop corrections to the partition function. We assume the string is propagating in a genus-dependent background in such a way that the physical (renormalized) background fields are genus-independent. After subtraction of modular infinities, the string-loop corrections to the β-functions are only cosmological terms.

The theory of galaxy formation by cosmic string loops is endangered by experimental bounds on the background of gravitational radiation coming from recent measurements of the millisecond pulsar PSR 1937 + 21 timing stability and from nucleosynthesis. Galaxy formation and bounds on gravitational radiation can be reconciled if the phase transition is inflationary and such that the largest scale crossing outside the horizon during the inflationary era lieshe range between M ⊙ and 10 11 M ⊙ . A simple model with local U(1) cosmic strings and chaotic inflation is analyzed, yielding the corresponding number of e-folds in the interval between 50 and 60.

This is Version nr. 2 (13/08/2007) of the original paper (30/07/2007). Includes corrected typos., We show that scalar unparticles coupled to the Standard Model Higgs at the renormalizable level can have a dramatic impact in the breaking of the electroweak symmetry already at tree level. In particular one can get the proper electroweak scale without the need of a Higgs mass term in the Lagrangian. By studying the mixed unparticle-Higgs propagator and spectral function we also show how unparticles can shift the Higgs mass away from its Standard Model value, λ v^2, and influence other Higgs boson properties. Conversely, we study in some detail how electroweak symmetry breaking affects the unparticle sector by breaking its conformal symmetry and generating a mass gap. We also show that, for Higgs masses above that gap, unparticles can increase quite significantly the Higgs width.

We present a five-dimensional model compactified on an interval where supersymmetry is broken by the Scherk-Schwarz mechanism. The gauge sector propagates in the bulk, two Higgs hypermultiplets are quasilocalized, and quark and lepton multiplets localized, in one of the boundaries. The effective four-dimensional theory is the MSSM with very heavy gauginos, heavy squarks and light sleptons and Higgsinos. The soft tree-level squared masses of the Higgs sector can be negative and they can (partially) cancel the positive one-loop contributions from the gauge sector. Electroweak symmetry breaking can then comfortably be triggered by two-loop radiative corrections from the top-stop sector. The fine tuning required to obtain the electroweak scale is found to be much smaller than in the MSSM, with essentially no fine-tuning for few TeV gaugino masses. All bounds from direct Higgs searches at LEP and from electroweak precision observables can be satisfied. The lightest supersymmetric particle is a (Higgsino-like) neutralino that can accomodate the abundance of Dark Matter consistently with recent WMAP observations., Comment: 23 pages, 3 figures

We study string realizations of split extended supersymmetry, recently proposed in hep-ph/0507192. Supersymmetry is broken by small ($\epsilon $) deformations of intersection angles of $D$-branes giving tree-level masses of order $m_0^2\sim \epsilon M_s^2$, where $M_s$ is the string scale, to localized scalars. We show through an explicit one-loop string amplitude computation that gauginos acquire hierarchically smaller Dirac masses $m_{1/2}^D \sim m_0^2/M_s$. We also evaluate the one-loop Higgsino mass, $\mu$, and show that, in the absence of tree-level contributions, it behaves as $\mu\sim m_0^4/M_s^3$. Finally we discuss an alternative suppression of scales using large extra dimensions. The latter is illustrated, for the case where the gauge bosons appear in N=4 representations, by an explicit string model with Standard Model gauge group, three generations of quarks and leptons and gauge coupling unification., Comment: 32 pages, 3 figures

We consider the split extended (N=2) supersymmetry scenario recently proposed by Antoniadis et al. [hep-ph/0507192] as a realistic low energy framework arising from intersecting brane models. While all scalar superpartners and charged gauginos are naturally at a heavy scale, the model low energy spectrum contains a Higgsino-like chargino and a neutralino sector made out of two Higgsino and two Bino states. We show that the lightest neutralino is a viable dark matter candidate, finding regions in the parameter space where its thermal relic abundance matches the latest determination of the density of matter in the Universe by WMAP. We also discuss dark matter detection strategies within this model: we point out that current data on cosmic-ray antimatter already place significant constraints on the model, while direct detection is the most promising technique for the future. Analogies and differences with respect to the standard split SUSY scenario based on the MSSM are illustrated., Comment: 14 pages, references added, typos corrected, matches with the published version

It has been recently shown that the electroweak baryogenesis mechanism is feasible in Standard Model extensions containing extra fermions with large Yukawa couplings. We show here that the lightest of these fermionic fields can naturally be a good candidate for cold dark matter. We find regions in the parameter space where the thermal relic abundance of this particle is compatible with the dark matter density of the Universe as determined by the WMAP experiment. We study direct and indirect dark matter detection for this model and compare with current experimental limits and prospects for upcoming experiments. We find, contrary to the standard lore, that indirect detection searches are more promising than direct detection ones, and they already exclude a portion of the parameter space., 26 pages, 15 figures

We show how splitting supersymmetry reconciles a class of intersecting brane models with unification. The gauge sector in these models arises in multiplets of extended supersymmetry while matter states are in N=1 representations. A deformation of the angles between the branes gives large masses to squarks and sleptons, as well as supersymmetry breaking contributions to other string states. The latter generate at one-loop heavy Dirac masses for Winos and gluinos and can induce a mass term for the Higgsino doublets. We find that this scenario is compatible with gauge coupling unification at high scale for both cases where the gauge sector is N=2 and N=4 supersymmetric. Moreover a neutralino, combination of neutral Higgsinos and Binos, is a natural candidate for dark matter., 10 pages, minor chages, version to be published in PLB

We provide a description of the five-dimensional Higgs mechanism in supersymmetric gauge theories compactified on the orbifold S^1/Z_2 by means of the N=1 superfield formalism. Goldstone bosons absorbed by vector multiplets can come either from hypermultiplets or from gauge multiplets of opposite parity (Hosotani mechanism). Supersymmetry is broken by the Scherk-Schwarz mechanism. In the presence of massive hypermultiplets and gauge multiplets, with different supersymmetric masses, the radion can be stabilized with positive (de Sitter) vacuum energy. The masses of vector and hypermultiplets can be fine-tuned to have zero (Minkowski) vacuum energy., 20 pages, 2 figures. v2: references added

New fermions, strongly coupled to the Standard Model Higgs boson provide a well motivated extension of the Standard Model (SM). In this work we show that, once new physics at heavier scales is added to stabilize the Higgs potential, such an extension of the SM can strengthen the first order electroweak phase transition and make the electroweak baryogenesis mechanism feasible. We propose a SM extension with TeV Higgsinos, Winos and Binos that satisfy the following properties: a) The electroweak phase transition is strong enough to avoid sphaleron erasure in the broken phase for values of the Higgs mass mH < 300 GeV; b) It provides large CP-violating currents that lead to the observed baryon asymmetry of the Universe for natural values of the CP-violating phase; c) It also provides a natural Dark Matter candidate that can reproduce the observed dark matter density; d) It is consistent with electroweak precision measurements; e) It may arise from a softly broken supersymmetric theory with an extra (asymptotically free) gauge sector; e) It may be tested by electron electric dipole moment experiments in the near future., LateX, 40 pages, 12 embedded postscript figures. A discussion of the stability of the Higgs potential and its connection to a possible ultraviolet completion of the model has been added

In this talk I review the issues of supersymmetry breaking and radion stabilization in a five dimensional theory compactified on the Z_2 orbifold. Supersymmetry breaking by Scherk-Schwarz boundary conditions is interpreted as spontaneous breaking of local supersymmetry by the Hosotani mechanism. The auxiliary field responsible for spontaneous supersymmetry breaking is inside the five-dimensional off-shell minimal supergravity multiplet. Different ways of fixing the supersymmetry breaking order parameter are analyzed. In the presence of supersymmetry breaking the one-loop effective potential for the radion has a minimum that fixes its vacuum expectation value. The radion is stabilized in a metastable Minkowski_4 minimum (versus the AdS_4 vacuum) with a mass in the meV range making it interesting for future deviations from the gravitational inverse-square law., 9 pages, 1 figure. Based on plenary talks presented at SUSY 2003: Supersymmetry in the Desert, held at the University of Arizona, Tucson, AZ, June 5-10, 2003; and at the IPPP Workshop on: String Phenomenology 2003, held at the University of Durham, U.K., July 29-August 4, 2003

We study fermions, such as gravitinos and gauginos in supersymmetric theories, propagating in a five-dimensional bulk where the fifth dimensional component is assumed to be an interval. We show that the most general boundary condition at each endpoint of the interval is encoded in a single complex parameter representing a point in the Riemann sphere. Upon introducing a boundary mass term, the variational principle uniquely determines the boundary conditions and the bulk equations of motion. We show the mass spectrum becomes independent from the Scherk-Schwarz parameter for a suitable choice of one of the two boundary conditions. Furthermore, for any value of the Scherk-Schwarz parameter, a zero-mode is present in the mass spectrum and supersymmetry is recovered if the two complex parameters are tuned., 10 pages. v2: Paragraph on off-shell globally supersymmetric Lagrangian added. Version published in PLB

In orbifold gauge theories localized tadpoles can be radiatively generated at the fixed points where U(1) subgroups are conserved. If the Standard Model Higgs fields are identified with internal components of the bulk gauge fields (Higgs-gauge unification) in the presence of these tadpoles the Higgs mass becomes sensitive to the UV cutoff and electroweak symmetry breaking is spoiled. We find the general conditions, based on symmetry arguments, for the absence/presence of localized tadpoles in models with an arbitrary number of dimensions D. We show that in the class of orbifold compactifications based on T^{D-4}/Z_N (D even, N>2) tadpoles are always allowed, while on T^{D-4}/\mathbb Z_2 (arbitrary D) with fermions in arbitrary representations of the bulk gauge group tadpoles can only appear in D=6 dimensions. We explicitly check this with one- and two-loops calculations, 19 pages, 3 figures, axodraw.sty. v2: version to appear in Nucl. Phys. B

We apply the path-integral formalism to compute the anomalies in general orbifold gauge theories (including possible non-trivial Scherk-Schwarz boundary conditions) where a gauge group G is broken down to subgroups H_f at the fixed points y=y_f. Bulk and localized anomalies, proportional to \delta(y-y_f), do generically appear from matter propagating in the bulk. The anomaly zero-mode that survives in the four-dimensional effective theory should be canceled by localized fermions (except possibly for mixed U(1) anomalies). We examine in detail the possibility of canceling localized anomalies by the Green-Schwarz mechanism involving two- and four-forms in the bulk. The four-form can only cancel anomalies which do not survive in the 4D effective theory: they are called globally vanishing anomalies. The two-form may cancel a specific class of mixed U(1) anomalies. Only if these anomalies are present in the 4D theory this mechanism spontaneously breaks the U(1) symmetry. The examples of five and six-dimensional Z_N orbifolds are considered in great detail. In five dimensions the Green-Schwarz four-form has no physical degrees of freedom and is equivalent to canceling anomalies by a Chern-Simons term. In all other cases, the Green-Schwarz forms have some physical degrees of freedom and leave some non-renormalizable interactions in the low energy effective theory. In general, localized anomaly cancellation imposes strong constraints on model building., Comment: 30 pages, 4 figures. v2: reference added

9 pags., 2 figs, 1 tab., A gauge theory with gauge group G defined in D > 4 space-time dimensions can be broken to a subgroup H on four-dimensional fixed point branes, when compactified on an orbifold. Mass terms for extra-dimensional components of gauge fields Ai (brane scalars) might acquire (when allowed by the brane symmetries) quadratically divergent radiative masses and thus jeopardize the stability of the four-dimensional theory. We have analyzed double-struck Z sign2 compactifications and identified the brane symmetries remnants of the higher-dimensional gauge invariance. No mass term is allowed for D = 5 while for D > 5 a tadpole α Fij α can appear when there are Uα(1) factors in H. A detailed calculation is done for the D = 6 case and it is established that the tadpole is related, although does not coincide, with the Uα(1) anomaly induced on the brane by the bulk fermions. In particular, no tadpole is generated from gauge bosons or fermions in real representations. © 2002 Elsevier Science B.V. All rights reserved., The work of G.v.G. was supported by the DAAD.

We have computed one-loop bulk and brane mass renormalization effects in a five-dimensional gauge theory compactified on the M_4 \times S^1/Z_2 orbifold, where an arbitrary gauge group G is broken by the orbifold action to its subgroup H. The space-time components of the gauge boson zero modes along the H generators span the gauge theory on the orbifold fixed point branes while the zero modes of the higher-dimensional components of the gauge bosons along the G/H generators play the role of Higgs fields with respect to the gauge group H. No quadratic divergences in the mass renormalization of the gauge and Higgs fields are found either in the bulk or on the branes. All brane effects for the Higgs field masses vanish (only wave function renormalization effects survive) while bulk effects are finite and can trigger, depending on the fermionic content of the theory, spontaneous Hosotani breaking of the brane gauge group H. For the gauge fields we do find logarithmic divergences corresponding to mass renormalization of their heavy Kaluza-Klein modes. Two-loop brane effects for Higgs field masses are expected from wave function renormalization brane effects inserted into finite bulk mass corrections., 31 pages, uses axodraw.sty and mcite.sty

We analyze the Scherk-Schwarz (SS) supersymmetry breaking in brane-world five dimensional theories compactified on the orbifold $S^1/\mathbb{Z}_2$. The SS breaking parameter is undetermined at the tree-level (no-scale supergravity) and can be interpreted as the Hosotani vacuum expectation value corresponding to the $U(1)_R$ group in five dimensional N=2 (ungauged) supergravity. We show that the SS breaking parameter is fixed at the loop level to either 0 or 1/2 depending on the matter content propagating in the bulk but in a rather model-independent way. Supersymmetry breaking is therefore fixed through a radiative Scherk-Schwarz mechanism. We also show that the two discrete values of the SS parameter, as well as the supersymmetry breaking shift in the spectrum of the bulk fields, are altered in the presence of a brane-localized supersymmetry breaking arising from some hidden sector dynamics. The interplay between the SS and the brane localized breaking is studied in detail., 16 pages, 2 figures, uses axodraw. References added

We consider five dimensional theories compactified on the orbifold S^1/Z_2 and prove that spontaneous local supersymmetry breaking by Wilson lines and by the Scherk-Schwarz mechanism are equivalent. Wilson breaking is triggered by the SU(2)_R symmetry which is gauged in off-shell N=2 supergravity by auxiliary fields. The super-Higgs mechanism disposes of the would-be Goldstinos which are absorbed by the gravitinos to become massive. The breaking survives in the flat limit, where we decouple all gravitational interactions, and the theory becomes softly broken global supersymmetry., 9 pages, some comments in the discussion of the super-Higgs effect and some references added

19 pags., 2 figs., Electroweak baryogenesis provides a very attractive scenario to explain the origin of the baryon asymmetry. The mechanism of electroweak baryogenesis makes use of the baryon number anomaly and relies on physics that can be tested experimentally. It is today understood that, if the Higgs mass is not larger than 120 GeV, this mechanism may be effective within supersymmetric extensions of the Standard Model. In this work, we reconsider the question of baryon number generation at the electroweak phase transition within the context of the minimal supersymmetric extension of the Standard Model. We derive the relevant diffusion equations, give a consistent definition of the sources, and compare our results with those appearing in the recent literature on this subject., M.C. and C.W. also wish to thank the CERN Theory Division, where part of this work was done, for hospitality. M.S. would like to thank the Theory Group of Instituto de Estructura de la Materia, where this work was partly done, for hospitality. M.Q. would also like to thank the Physics and Astronomy Department of the Johns Hopkins University for the hospitality extended to him as Bearden Visiting Professor. The work of C.W. is supported in part by the US DOE, Div. of HEP, Contract W-31-109-ENG-38. Fermilab is operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH02000 with the DOE. The work of M.S. is supported by the US DOE Contract DE-A505-89ER40518. Work supported in part by CICYT, Spain, under contract FPA2001-1806, and by EU under contracts HPRN-CT-2000-00152 and HPRN-CT-2000-00148.

This is Version nr. 2 (16/04/2001) of the original paper (06/03/2001). Includes corrected typos., A five dimensional N=1 supersymmetric theory compactified on the orbifold $S^1/\mathbb{Z}_2$ is constructed. Gauge fields and $SU(2)_L$ singlets propagate in the bulk ($U$-states) while $SU(2)_L$ doublets are localized at an orbifold fixed point brane ($T$-states). Zero bulk modes and localized states constitute the MSSM and massive modes are arranged into N=2 supermultiplets. Superpotential interactions on the brane are of the type $UTT$. Supersymmetry is broken in the bulk by a Scherk-Schwarz mechanism using the $U(1)_R$ global $R$-symmetry. A radiative finite electroweak breaking is triggered by the top-quark/squark multiplet $\mathbb{T}$ propagating in the bulk. The compactification radius $R$ is fixed by the minimization conditions and constrained to be $1/R \simlt 10-15$ TeV. It is also constrained by precision electroweak measurements to be $1/R \simgt 4$ TeV. The pattern of supersymmetric mass spectrum is well defined. In particular, the lightest supersymmetric particle is the sneutrino and the next to lightest supersymmetric particle the charged slepton, with a squared-mass difference $\sim M_Z^2$. The theory couplings, gauge and Yukawa, remain perturbative up to scales $E$ given, at one-loop, by $ER \simlt 30-40$. Finally, LEP searches on the MSSM Higgs sector imply an absolute lower bound on the SM-like Higgs mass, around 145 GeV in the one-loop approximation, Work supported inpart by CICYT, Spain, under contract AEN98-0816, and by EU under contracts HPRN-CT-2000-00152 and HPRN-CT-2000-00148.

24 pags., 4 figs., We identify a class of chiral models where the one-loop effective potential for Higgs scalar fields is finite without any requirement of supersymmetry. It corresponds to the case where the Higgs fields are identified with the components of a gauge field along compactified extra dimensions. We present a six-dimensional model with gauge group U(3) x U(3) and quarks and leptons accommodated in fundamental and bi-fundamental representations. The model can be embedded in a D-brane configuration of type I string theory and, upon compactification on a T2/Z2 orbifold, it gives rise to the standard model with two Higgs doublets., The work of K B is supported by the EU fourth framework programme TMR contract FMRX-CT98-0194 (DG 12-MIHT). This work is also supported in part by EU under contracts HPRN-CT-2000-00152 and HPRN-CT-2000-00148, in part by IN2P3-CICYT contract Pth 96-3 and in part by CICYT, Spain, under contract AEN98-0816.

Nucleosynthesis bounds on Jordan-Brans-Dicke theories of gravity are updated using improved determinations of 4He, D + 3He and 7Li primordial abundances and neutron life-time. We find, for Nv = 3 and most of the allowed Ω 0 h 0 2 range, stronger bounds than those obtained from post-newtonian experiments, ∥ω∥ > 250 (2σ). In particular at 2σ level, ω > 380 (440) for Ω 0 h 0 2 = 0.25 (1). Furthermore, it is suggested that GR predictions with Nv = 3 are hardly consistent with nucleosynthesis observations at 90% CL, while GR with Nv = 2 seems to accommodate better the observations.

In the standard model, if there is no new physics up to a certain scale, there is a window for the Higgs boson mass, defined by the upper (perturbativity) and the lower (stability) bounds. If the new physics is in the form of right-handed neutrinos, responsible for the small neutrino masses through a seesaw mechanism, then the window narrows, and even disappears for M (the right-handed Majorana mass) ranging from ${10}^{13} \mathrm{GeV}$ for three quasidegenerate neutrinos with ${m}_{\ensuremath{\nu}}\ensuremath{\simeq}2 \mathrm{eV},$ to $5\ifmmode\times\else\texttimes\fi{}{10}^{14} \mathrm{GeV}$ for just one relevant generation with ${m}_{\ensuremath{\nu}}\ensuremath{\simeq}0.1 \mathrm{eV}.$ A slightly weaker upper bound on M, coming from the requirement that the neutrino Yukawa couplings do not develop a Landau pole, is also discussed.