Orbifold-induced μ\mu term and electroweak symmetry breaking

It is known that a Higgs $\mu$ term can be naturally generated through the K\"ahler potential in orbifold string models in which one of the three compactified complex planes has order two. In this class of models explicit expressions for both the $\mu$ parameter and the soft SUSY-breaking parameters can be obtained under the assumption that the goldstino is an arbitrary linear combination of the fermionic partners of the dilaton $S$ and all the moduli $T_i,U_i$. We apply this picture to the MSSM and explore the consistency of the obtained boundary conditions with radiative gauge symmetry breaking. We find that consistency with the measured value of the top-quark mass can only be achieved if the goldstino has a negligible dilatino component and relevant components along the $T_3,U_3$ moduli associated to the order-two complex plane.Comment: 9 pages, LaTeX fil

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.Comment: 24 pages, to appear in the book `Perspectives on Supersymmetry', World Scientific, Editor G. Kane; some comments and references adde

Challenges for MSSM Higgs searches at Hadron Colliders

In this article we analyze the impact of B-physics and Higgs physics at LEP on standard and non-standard Higgs bosons searches at the Tevatron and the LHC, within the framework of minimal flavor violating supersymmetric models. The B-physics constraints we consider come from the experimental measurements of the rare B-decays b -> s gamma and B_u -> tau nu and the experimental limit on the B_s -> mu+ mu- branching ratio. We show that these constraints are severe for large values of the trilinear soft breaking parameter A_t, rendering the non-standard Higgs searches at hadron colliders less promising. On the contrary these bounds are relaxed for small values of A_t and large values of the Higgsino mass parameter mu, enhancing the prospects for the direct detection of non-standard Higgs bosons at both colliders. We also consider the available ATLAS and CMS projected sensitivities in the standard model Higgs search channels, and we discuss the LHC's ability in probing the whole MSSM parameter space. In addition we also consider the expected Tevatron collider sensitivities in the standard model Higgs h -> b bbar channel to show that it may be able to find 3 sigma evidence in the B-physics allowed regions for small or moderate values of the stop mixing parameter.Comment: 22 pages, 5 figures. Added references and acknowledgements. Submitted to PR

FCNCs in supersymmetric multi-Higgs doublet models

We conduct a general discussion of supersymmetric models with three families in the Higgs sector. We analyse the scalar potential, and investigate the minima conditions, deriving the mass matrices for the scalar, pseudoscalar and charged states. Depending on the Yukawa couplings and the Higgs spectrum, the model might allow the occurrence of potentially dangerous flavour changing neutral currents at the tree-level. We compute model-independent contributions for several observables, and as an example we apply this general analysis to a specific model of quark-Higgs interactions, discussing how compatibility with current experimental data constrains the Higgs sector.Comment: 30 pages, 9 figures. Comments and references added. Final version published in Physical Review

Very Constrained Minimal Supersymmetric Standard Models

We consider very constrained versions of the minimal supersymmetric extension of the Standard Model (VCMSSMs) which, in addition to constraining the scalar masses m_0 and gaugino masses m_{1/2} to be universal at some input scale, impose relations between the trilinear and bilinear soft supersymmetry breaking parameters A_0 and B_0. These relations may be linear, as in simple minimal supergravity models, or nonlinear, as in the Giudice-Masiero mechanism for generating the Higgs-mixing mu term. We discuss the application of the electroweak vacuum conditions in VCMSSMs, which may be used to make a prediction for tan beta as a function of m_0 and m_{1/2} that is usually unique. We baseline the discussion of the parameter spaces allowed in VCMSSMs by updating the parameter space allowed in the CMSSM for fixed values of tan beta with no relation between A_0 and B_0 assumed {\it a priori}, displaying contours of B_0 for a fixed input value of A_0, incorporating the latest CDF/D0 measurement of m_t and the latest BNL measurement of g_mu - 2. We emphasize that phenomenological studies of the CMSSM are frequently not applicable to specific VCMSSMs, notably those based on minimal supergravity, which require m_0 = m_{3/2} as well as A_0 = B_0 + m_0. We then display (m_{1/2}, m_0) planes for selected VCMSSMs, treating in a unified way the parameter regions where either a neutralino or the gravitino is the LSP. In particular, we examine in detail the allowed parameter space for the Giudice-Masiero model.Comment: 26 pages, 32 eps figure

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 analysis 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 sum-rules 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 mass$^2$. 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 ``$\mu$-term'', as naturally implemented in orbifolds, leads to the prediction $|tg\beta |=1$ at the string scale, independently of the Goldstino direction.Comment: 27 pages, no figures, Latex; version to appear in Z. Phys. C (some changes in the discussion on gaugino versus scalar masses in sections 2 and 5, minor corrections elsewhere

Determination of the String Scale in D-Brane Scenarios and Dark Matter Implications

We analyze different phenomenological aspects of D-brane constructions. First, we obtain that scenarios with the gauge group and particle content of the supersymmetric standard model lead naturally to intermediate values for the string scale, in order to reproduce the value of gauge couplings deduced from experiments. Second, the soft terms, which turn out to be generically non universal, and Yukawa couplings of these scenarios are studied in detail. Finally, using these soft terms and the string scale as the initial scale for their running, we compute the neutralino-nucleon cross section. In particular we find regions in the parameter space of D-brane scenarios with cross sections in the range of $10^{-6}$--$10^{-5}$ pb, i.e. where current dark matter experiments are sensitive. For instance, this can be obtained for $\tan\beta > 5$.Comment: Figures improved, misprints corrected, results basically unchange

Strong Constraints on the Parameter Space of the MSSM from Charge and Color Breaking Minima

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 $B$ and $M_{1/2}$. Finally, the crucial issue of the one-loop corrections to the scalar potential has been taken into account in a proper way.Comment: 48 pages, LaTeX, 12 uuencoded postscript figures in additional file. Only a small comment about the m=0 (no-scale) limit has been included in sect.6 (Results) and sect.7 (Conclusions