Carlos Munoz

Towards a theory of soft terms for the supersymmetric Standard Model

Abstract We perform a systematic analysis of the soft supersymmetry-breaking terms arising in some large classes of four-dimensional strings. The analysis does not assume any specific supersymmetry-breaking mechanism but provides a means of parametrizing our ignorance in a way consistent with some known properties of these four-dimensional strings. We introduce a goldstino angle parameter θ which says where the source of supersymmetry-breaking resides, either predominantly in the dilation sector (sin θ = 1 limit) or in the rest of the chiral fields, notably the moduli (sin θ =0 limit). All formulae for soft parameters take particularly simple forms when written in terms of this angle. The sin θ = 1 limit is (up to small corrections) universal. As sin θ decreases, the model dependence increases and the resulting soft terms may or may not be universal, depending on the model. General expressions for the soft terms as functions of θ for generic four-dimensional strings are provided. For each given string model, one trades the four soft parameters ( M , m , A , B ) of the minimal supersymmetric standard model by the two parameters m 3 2 (gravitino mass) and sin θ. The role of complex phases and the associated constraints from limits on the electric dipole moment of the neutron are discussed. Also emphasized is the importance of treating the problem of the cosmological constant in a self-consistent manner. Three prototype string scenarios are discussed and their low-energy implications are studied by imposing appropriate radiative SU(2) L × U(1) breaking. The supersymmetric particle spectra present definite patterns which may be experimentally tested at future colliders.

Strong constraints on the parameter space of the MSSM from charge and color breaking minima

Abstract A complete analysis of all the potentially dangerous directions in the field space of the minimal supersymmetric standard model is carried out. They are of two types, the ones associated with the existence of charge and color breaking minima in the potential deeper than the realistic minimum, and the directions in the field space along which the potential becomes unbounded from below. The corresponding new constraints on the parameter space are given in an analytic form, representing a set of necessary and sufficient conditions to avoid dangerous directions. They are very strong and, in fact, there are extensive regions in the parameter space that become forbidden. This produces important bounds, not only on the value of A, but also on the values of B, M 1 2 and m. Finally, the crucial issue of the one-loop corrections to the scalar potential has been taken into account in a proper way.

Dark matter detection in the light of recent experimental results

The existence of dark matter was suggested, using simple gravitational arguments, seventy years ago. Although we are now convinced that most of the mass in the Universe is indeed some nonluminous matter, we still do not know its composition. The problem of the dark matter in the Universe is reviewed here. Particle candidates for dark matter are discussed with particular emphasis on Weakly Interacting Massive Particles (WIMPs). Experiments searching for these relic particles, carried out by many groups around the world, are also reviewed, paying special attention to their direct detection by observing the elastic scattering on target nuclei through nuclear recoils. Finally, we concentrate on the theoretical models predicting WIMPs, and in particular on supersymmetric extensions of the standard model, where the leading candidate for WIMP, the neutralino, is present. There, we compute the cross-section for the direct detection of neutralinos, and compare it with the sensitivity of detectors. We mainly discuss supergravity, superstring and M theory scenarios.

Soft supersymmetry breaking terms from supergravity and superstring models

We review the origin of soft supersymmetry-breaking terms in N=1 supergravity models of particle physics. We first consider general formulae for those terms in general models with a hidden sector breaking supersymmetry at an intermediate energy scale. The results for some simple models are given. We then consider the results obtained in some simple superstring models in which particular assumptions about the origin of supersymmetry breaking are made. These are models in which the seed of supersymmetry breaking is assumed to be originated in the dilaton/moduli sector of the theory.

Dynamical supersymmetry breaking with a large internal dimension

Abstract 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 μ-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.

The low-energy supersymmetric spectrum according to N = 1 supergravity guts

Abstract We reexamine the radiative SU(2) × U(1) breaking induced by broken N = 1 supergravity in the light of recent experimental results. Explicit formulae are given for the supersymmetric particle spectrum and the SU(2) × U(1) breaking condition. The experimental results (t-quark mass) constrain the supergravity parameters, particularly the bare higgsino mass μ 0 and the “mixing” Higgs mass term μ 3 2 . Models with globally supersymmetric “observable” sectors (except for gaugino masses) are excluded in their simplest version. If the identity B = A − 1 is assumed, further restrictions on the range of the supergravity parameters are found. The particle spectra leading to possible supersymmetric interpretations of the CERN missing p T events are analyzed. We study in detail the following three scenarios: (i) m q ⋍ 80–140 GeV , M g = 3 GeV ; (ii) q ⋍ M g ⋍ 40 GeV ; (iii) M g = 40 GeV , m q ⪢ M g and give the corresponding spectra of sparticles as a function of the A -parameter.

Aspects of type I string phenomenology

Abstract We study different phenomenological aspects of compact, D = 4, N = 1 Type IIB orientifolds considered as models for unification of the standard model and gravity. We discuss the structure of the compactification, string and unification scales depending on the different possible D-brane configurations. It is emphasized that in the context of Type I models the M W / M Planck hierarchy problem is substantially alleviated and may be generated by geometrical factors. We obtain the effective low-energy supergravity Lagrangian and derive the form of soft SUSY-breaking terms under the assumption of dilaton/moduli dominance. We also discuss the role of anomalous U (1)s and of twisted moduli in this class of theories. A novel mechanism based on the role of singularities is suggested to achieve consistency with gauge coupling unification in low string scale models.

A natural solution to the μ problem

Abstract We propose a simple mechanism for solving the μ-problem in the context of minimal low-energy supergravity models. This is based on the appearance of non-renormalizable couplings in the superpotential. In particular, if H1H2 is an allowed operator by all the symmetries of the theory, it is natural to promote the usual renormalizable superpotential W0 to W0+λW0H1H2, yielding an effective μ parameter whose size is directly related to the gravitino mass once supersymmetry is broken (this result is maintained if H1H2 couples with different strengths to the various terms present in W0). On the other hand, the μ-term must be absent in W0, otherwise the natural scale for μ would be M>Pl. Remarkably enough, this is entirely justified in the SUGRA theories coming from superstrings, where mass terms for light fields are forbidden in the superpotential. We also analyze the SU(2)×U(1) breaking, finding that it takes place satisfactorily. Finally, we give a realistic example in which SUSY is broken by gaugino condensation where the mechanism proposed for solving the μ-problem can be gracefully implemented.

Proposal for a Supersymmetric Standard Model

This paper has been withdrawn by the author.The formula which was proposed gives only a second order deformation.The fact that neutrinos are massive suggests that the minimal supersymmetric standard model (MSSM) might be extended in order to include three gauge-singlet neutrino superfields with Yukawa couplings of the type H2Lnuc. We propose to use these superfields to solve the mu problem of the MSSM without having to introduce an extra singlet superfield as in the case of the next-to-MSSM (NMSSM). In particular, terms of the type nuc H1H2 in the superpotential may carry out this task spontaneously through neutrino vacuum expectation values. In addition, terms of the type (nuc)3 avoid the presence of axions and generate effective Majorana masses for neutrinos at the electroweak scale. On the other hand, these terms break lepton number and R parity explicitly. For Dirac masses of the neutrinos of order 10(-4) GeV, eigenvalues reproducing the correct scale of neutrino masses are obtained.

Some issues in soft SUSY-breaking terms from dilaton¦moduli sectors

We study the structure of the soft SUSY-breaking terms obtained from some classes of 4-D strings under the assumption of dilaton/moduli dominance in the process of SUSY-breaking. We generalize previous analyses in several ways and in particular consider the new features appearing when several moduli fields contribute to SUSY breaking (instead of an overall modulus T ). Some qualitative features indeed change in the multimoduli case. A general discussion for symmetric Abelian orbifolds as well as explicit examples are given. Certain general sumrules involving soft terms of different particles are shown to apply to large classes of models. Unlike in the overall modulus T case, gauginos may be lighter than scalars even at the tree-level. However, if one insists in getting that pattern of soft terms, these sum rules force some of the scalars to get negative mass2. These tachyonic masses could be a problem for standard model 4-D strings but an advantage in the case of string-GUTs. We also discuss the possible effects of off-diagonal metrics for the matter fields which may give rise to flavour-changing neutral currents. Different sources for the bilinear B soft term are studied. It is found that the Giudice-Masiero mechanism for generating a “μ-term”, as naturally implemented in orbifolds, leads to the prediction ¦tgβ¦=1 at the string scale, independently of the Goldstino direction.