72 research outputs found
Lepton Flavour Violation in Unified Models with U(1)-Family Symmetries
Lepton flavour non-conserving processes are examined in the context of
unified models with U(1)-family symmetries which reproduce successfully the
low-energy hierarchy of the fermion mass spectrum and the Kobayashi - Maskawa
mixing. These models usually imply mixing effects in the supersymmetric scalar
sector. We construct the fermion and scalar mass matrices in two viable models,
and calculate the mixing effects on the , and
rare decays. The relevant constraints on the sparticle
mass spectrum as well as the role of various MSSM parameters are discussed.Comment: 14 pages, 1 figure included in the ps file, uses epsfig.st
A mass hierarchy from recoiling D branes
Using conformal field theory methods we construct a metric that describes the distortion of space-time surrounding a D(irichlet)-brane (solitonic) defect after being struck by another D-brane. By viewing our four-dimensional universe as such a struck brane, embedded in a five-dimensional space-time, we argue on the appearance of a band of massive Kaluza-Klein excitations for the bulk graviton which is localized in a region of the fifth dimension determined by the inverse size of the band. The band incorporates the massless mode (ordinary graviton) and its thickness is determined essentially by the width of the Gaussian distribution describing the (target-space) quantum fluctuations of the intersecting-brane configuration
Low Scale Unification, Newton's Law and Extra Dimensions
Motivated by recent work on low energy unification, in this short note we
derive corrections on Newton's inverse square law due to the existence of extra
decompactified dimensions. In the four-dimensional macroscopic limit we find
that the corrections are of Yukawa type. Inside the compactified space of
n-extra dimensions the sub-leading term is proportional to the (n+1)- power of
the distance over the compactification radius ratio. Some physical implications
of these modifications are briefly discussed.Comment: 12pages, 2 figure
On the GUT scale of F-Theory SU(5)
In F-theory GUTs, threshold corrections from Kaluza-Klein massive modes
arising from gauge and matter multiplets play an important role in the
determination of the weak mixing angle and the strong gauge coupling of the
effective low energy model. In this letter we further explore the induced
modifications on the gauge couplings running and the GUT scale. In particular,
we focus on the KK-contributions from matter curves and analyse the conditions
on the chiral and Higgs matter spectrum which imply a GUT scale consistent with
the minimal unification scenario. As an application, we present an explicit
computation of these thresholds for matter fields residing on specific
non-trivial Riemann surfaces.Comment: 21 page
Higgs Mass Textures in Flipped SU(5)
We analyze the Higgs doublet-triplet mass splitting problem in the version of
flipped SU(5) derived from string theory. Analyzing non-renormalizable terms up
to tenth order in the superpotential, we identify a pattern of field vev's that
keeps one pair of electroweak Higgs doublets light, while all other Higgs
doublets and all Higgs triplets are kept heavy, with the aid of the economical
missing-doublet mechanism found in the field-theoretical version of flipped
SU(5). The solution predicts that second-generation charge -1/3 quarks and
charged leptons are much lighter than those in the third generation.Comment: 15 pages LaTe
A Note on the Supersymmetries of the Self-Dual Supermembrane
In this letter we discuss the supersymmetry issue of the self dual
supermembranes in (8+1) and (4+1)-dimensions. We find that all genuine
solutions of the (8+1)-dimensional supermembrane, based on the exceptional
group G_2, preserve one of the sixteen supersymmetries while all solutions in
(4+1)-dimensions preserve eight of them.Comment: Latex file, 12pages, no figure
Predictive Ansatz for Fermion Masses in SUSY GUTS
We reexamine a succesful fermion mass Ansatz proposed by Giudice for a wide
range of the ratio (where are
the two standard higgs fields), in the context of supersymmetric grand unified
theories. We find that the 7 predictions of the ansatz, and are in good agreement with the experiment when
either {\it i) } or {\it ii)} . A correct
prediction for the bottom mass gives a lower limit on for case
{\it (i)}, in agreement with the previous analysis, while in case {\it (ii)}
.Comment: 8 pages, 3 figures NOT included, available on request, LaTex, IOA
281/92, NTUA 37/92 preprint
Implications of a Heavy Top in Supersymmetric Theories
In the context of the radiative electroweak symmetry breaking scenario, we
investigate the implications of a heavy top quark mass, close to its infrared
fixed point, on the low energy parameters of the minimal supersymmetric
standard model. We use analytic expressions to calculate the Higgs masses as
well as the supersymmetric masses of the third generation. We further assume
bottom-tau unification at the GUT scale and examine the constraints put by this
condition on the parameter space (,), using the
renormalization group procedure at the two-loop level. We find only a small
fraction of the parameter space where the above conditions can be satisfied,
namely , while . We
further analyse the case where all three Yukawa couplings reach the
perturbative limit just after the unification scale. In this latter case, the
situation turns out to be very strict demanding .Comment: 13 pages plus 3 Figures (hard copy) on request, IOA.303/94 and NTUA
44/9
Neutrino Masses in Flipped SU(5)
We analyse the fermion masses and mixings in the flipped SU(5) model. The
fermion mass matrices are evolved from the GUT scale down to by solving
the renormalization group equations for the Yukawa couplings. The constraints
imposed by the charged fermion data are then utilised to make predictions about
the neutrino properties . It is found that the {\it generalized } see-saw
mechanism which occurs naturally in this model can provide {\it i})a solution
to the solar neutrino problem via the MSW mechanism and {\it ii})a sufficiently
large mass to contribute as a hot dark matter component as
indicated by the recent COBE data.Comment: (14 Pages,No figures,TEX,IOA-290/92 preprint
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