832 research outputs found
Orbifold-induced term and electroweak symmetry breaking
It is known that a Higgs 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 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 and all the moduli
. 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 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 ). 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 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. 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 soft term are studied. It is found that the Giudice-Masiero
mechanism for generating a ``-term'', as naturally implemented in
orbifolds, leads to the prediction 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 -- pb, i.e. where current dark matter
experiments are sensitive. For instance, this can be obtained for .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 , but also on the values of
and . 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
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