526 research outputs found
F-term inflation in Superstring Theories
A supersymmetric inflationary stage dominated by an -term has the problem
that the flatness of the potential is spoiled by supergravity corrections, that
is the slow-roll parameter gets contributions of order unity. We show
that in -term inflationary models based on strings there is natural way of
obtaining small values of . This happens in models of hybrid inflation
based on orbifold constructions, in which a modulus field is responsible
for the large value of the potential during inflation, and a second field
with appropriate modular weight is responsible for the roll-over. We
illustrate the mechanism with a model in which the inflaton potential is
provided by gaugino condensation, leading to succesful inflation.Comment: 9 pages, LaTe
The Standard Model instability and the scale of new physics
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 figure
The Minimally Tuned Minimal Supersymmetric Standard Model
The regions in the Minimal Supersymmetric Standard Model with the minimal
amount of fine-tuning of electroweak symmetry breaking are presented for
general messenger scale. No a priori relations among the soft supersymmetry
breaking parameters are assumed and fine-tuning is minimized with respect to
all the important parameters which affect electroweak symmetry breaking. The
superpartner spectra in the minimally tuned region of parameter space are quite
distinctive with large stop mixing at the low scale and negative squark soft
masses at the high scale. The minimal amount of tuning increases enormously for
a Higgs mass beyond roughly 120 GeV.Comment: 38 pages, including 2 appendices, 8 figure
Fitting the Quark and Lepton Masses in String Theories
The capability of string theories to reproduce at low energy the observed
pattern of quark and lepton masses and mixing angles is examined, focusing the
attention on orbifold constructions, where the magnitude of Yukawa couplings
depends on the values of the deformation parameters which describe the size and
shape of the compactified space. A systematic exploration shows that for ,
, --I and possibly orbifolds a correct fit of the physical
fermion masses is feasible. In this way the experimental masses, which are
low--energy quantities, select a particular size and shape of the compactified
space, which turns out to be very reasonable (in particular the modulus
defining the former is ). The rest of the orbifolds are rather
hopeless and should be discarded on the assumption of a minimal scenario. On the other hand, due to stringy selection
rules, there is no possibility of fitting the Kobayashi--Maskawa parameters at
the renormalizable level, although it is remarked that this job might well be
done by non--renormalizable couplings.Comment: 19 page
Higgs Boson Bounds in Three and Four Generation Scenarios
In light of recent experimental results, we present updated bounds on the
lightest Higgs boson mass in the Standard Model (SM) and in the Minimal
Supersymmetric extension of the Standard Model (MSSM). The vacuum stability
lower bound on the pure SM Higgs boson mass when the SM is taken to be valid up
to the Planck scale lies above the MSSM lightest Higgs boson mass upper bound
for a large amount of SUSY parameter space. If the lightest Higgs boson is
detected with a mass M_{H} < 134 GeV (150 GeV) for a top quark mass M_{top} =
172 GeV (179 GeV), it may indicate the existence of a fourth generation of
fermions. The region of inconsistency is removed and the MSSM is salvagable for
such values of M_{H} if one postulates the existence of a fourth generation of
leptons and quarks with isodoublet degenerate masses M_{L} and M_{Q} such that
60 GeV 170 GeV.Comment: 7 pages, 4 figures. To be published in Physical Review
The Problem of the Stabilization of the Dilaton in String Theories
The crucial problem of how the dilaton field is stabilized at a
phenomenologically acceptable value in string theories remains essentially
unsolved. We show that the usual scenario of assuming that the dilaton is fixed
by the (SUSY breaking) dynamics of just the dilaton itself (dilaton dominance
scenario) is {\em inconsistent} unless the K\"ahler potential receives very
important perturbative or non-perturbative contributions. Then, the usual
predictions about soft breaking terms are lost, but still is possible to derive
model-independent predictions for them.Comment: 7 pages, LaTeX. Talk given at SUSY-96, Univ. of Maryland, College
Park, May 29 - June 1, 199
Fermion Masses and Mixing in Intersecting Branes Scenarios
We study the structure of Yukawa couplings in intersecting D6-branes wrapping
a factorizable 6-torus compact space T^6. Models with MSSM-like spectrum are
analyzed and found to fail in predicting the quark mass spectrum because of the
way in which the family structure for the left-handed, right-handed quarks and,
eventually, the Higgses is `factorized' among the different tori. In order to
circumvent this, we present a model with three supersymmetric Higgs doublets
which satisfies the anomaly cancellation condition in a more natural way than
the previous models, where quarks were not treated universally regarding their
branes assignments, or some particular branes were singled out being invariant
under orientifold projection. In our model, the family structures for the left,
right quarks, left leptons and the Higgses arise from one of the tori and can
naturally lead to universal strength Yukawa couplings which accommodate the
quark mass hierarchy and the mixing angles.Comment: 21 pages, latex, matches the Phys. Rev. D versio
Phenomenological viability of string and M-theory scenarios
We analyze the constraints that a correct phenomenology and the absence of
dangerous charge and color breaking (CCB) minima or unbounded from below (UFB)
directions impose on the parameter space of different superstring and M-theory
scenarios. Namely, we analyze in detail the case where supersymmetry (SUSY)
breaking is driven by non-vanishing dilaton and/or moduli F-terms in weakly and
strongly coupled strings, and the specific case where the mechanism of SUSY
breaking is gaugino condensation with or without the participation of
non-perturbative contributions to the K{\"a}hler potential. The results
indicate that, only in some small windows of the parameter space all the
previous tests are succesfully passed. We also discuss the impact of
non-universality of the soft breaking terms on CCB/UFB bounds.Comment: 18 pages + 10 figures, PostScript fil
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