2,863 research outputs found
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
Unification Bounds on the Possible N=2 Supersymmetry Breaking Scale
In this letter, the possible appearance of N=2 supersymmetry at a low energy
scale is investigated in the context of unified theories. Introducing mirror
particles for all the gauge and matter multiplets of the Minimal Supersymmetric
extension of the Standard Model (MSSM), the measured values of sin^2 \theta_W
and \alpha_3(M_Z) indicate that the N=2 threshold scale M_{S_2} cannot be lower
than \sim 10^{14}GeV. If the U(1) normalization coefficient k is treated as a
free parameter, M_{S_2} can be as low as 10^9 GeV. On the other hand, if mirror
quarks and leptons are absent and a non-standard value for k is used, N=2
supersymmetry breaking could in principle occur at the electroweak scale.Comment: 10 pages, LATEX, 2 eps figure
D-branes and the Standard Model
We perform a systematic study of the Standard Model embedding in a D-brane
configuration of type I string theory at the TeV scale. We end up with an
attractive model and we study several phenomenological questions, such as gauge
coupling unification, proton stability, fermion masses and neutrino
oscillations. At the string scale, the gauge group is U(3)_color x U(2)_weak x
U(1)_1 x U(1)_bulk. The corresponding gauge bosons are localized on three
collections of branes; two of them describe the strong and weak interactions,
while the last abelian factor lives on a brane which is extended in two large
extra dimensions with a size of afew microns. The hypercharge is a linear
combination of the first three U(1)s. All remaining U(1)s get masses at the TeV
scale due to anomalies, leaving the baryon and lepton numbers as
(perturbatively) unbroken global symmetries at low energies. The conservation
of baryon number assures proton stability, while lepton number symmetry
guarantees light neutrino masses that involve a right-handed neutrino in the
bulk. The model predicts the value of the weak angle which is compatible with
the experiment when the string scale is in the TeV region. It also contains two
Higgs doublets that provide tree-level masses to all fermions of the heaviest
generation, with calculable Yukawa couplings; one obtains a naturally heavy top
and the correct ratio m_b/m_tau. We also study neutrino masses and mixings in
relation to recent solar and atmospheric neutrino data.Comment: 42 pages, Latex2e, 6 figures, final version to be published in Nucl.
Phys.
Solving the Decompactification Problem in String Theory
We investigate heterotic ground states in four dimensions in which N=4
supersymmetry is spontaneously broken to N=2. N=4 supersymmetry is restored at
a decompactification limit corresponding to . We calculate the
full moduli dependent threshold corrections and confirm that they are supressed
in the decompactification limit as expected from the restoration
of N=4 supersymmetry. This should be contrasted with the behavior of the
standard N=2 groundstates where the coupling blow up linearly with the volume
of the decompactifying manifold. This mechanism provides a solution to the
decompactification problem for the gauge coupling constants.
We also discuss how the mechanism can be implemented in ground states with
lower supersymmetry.Comment: 14pp, LaTeX some typos correcte
On the existence of singularity-free solutions in quartic gravity
We study a general field theory of a scalar field coupled to gravitation
through a quadratic Gauss-Bonnet term . We show that,
under mild assumptions about the function , the classical solutions
in a spatially flat FRW background include singularity - free solutions.Comment: 9 pages, LATEX, uses epsf.tex macro, (1 figure included in
uuencode+compress EPSF form), IOA-29
Universal moduli-dependent thresholds in Z(2)xZ(2) orbifolds
In the context of a recently proposed method for computing exactly string
loop corrections regularized in the infra-red, we determine and calculate the
universal moduli-dependent part of the threshold corrections to the gauge
couplings for the symmetric orbifold model. We show that these
corrections decrease the unification scale of the underlying effective field
theory. We also comment on the relation between this infra-red regularization
scheme and other proposed methods.Comment: 12 pages, Latex, contains two figures, final version, typos correcte
A closer look at string resonances in dijet events at the LHC
The first string excited state can be observed as a resonance in dijet
invariant mass distributions at the LHC, if the scenario of low-scale string
with large extra dimensions is realized. A distinguished property of the dijet
resonance by string excited states from that the other "new physics" is that
many almost degenerate states with various spin compose a single resonance
structure. It is examined that how we can obtain evidences of low-scale string
models through the analysis of angular distributions of dijet events at the
LHC. Some string resonance states of color singlet can obtain large mass shifts
through the open string one-loop effect, or through the mixing with closed
string states, and the shape of resonance structure can be distorted. Although
the distortion is not very large (10% for the mass squared), it might be able
to observe the effect at the LHC, if gluon jets and quark jets could be
distinguished in a certain level of efficiency.Comment: 12 pages, 8 figure
String theory predictions for future accelerators
We consider, in a string theory framework, physical processes of
phenomenological interest in models with a low string scale. The amplitudes we
study involve tree-level virtual gravitational exchange, divergent in a
field-theoretical treatment, and massive gravitons emission, which are the main
signatures of this class of models. First, we discuss the regularization of
summations appearing in virtual gravitational (closed string) Kaluza-Klein
exchanges in Type I strings. We argue that a convenient manifestly ultraviolet
convergent low energy limit of type I string theory is given by an effective
field theory with an arbitrary cutoff in the closed (gravitational)
channel and a related cutoff in the open (Yang-Mills) channel.
We find the leading string corrections to the field theory results. Second, we
calculate exactly string tree-level three and four-point amplitudes with gauge
bosons and one massive graviton and examine string deviations from the
field-theory result.Comment: 39 pages, 8 figures, references adde
N=1 supersymmetric SU(4)xSU(2)LxSU(2)R effective theory from the weakly coupled heterotic superstring
In the context of the free-fermionic formulation of the heterotic
superstring, we construct a three generation N=1 supersymmetric
SU(4)xSU(2)LxSU(2)R model supplemented by an SU(8) hidden gauge symmetry and
five Abelian factors. The symmetry breaking to the standard model is achieved
using vacuum expectation values of a Higgs pair in (4bar,2R)+(4,2R) at a high
scale. One linear combination of the Abelian symmetries is anomalous and is
broken by vacuum expectation values of singlet fields along the flat directions
of the superpotential. All consistent string vacua of the model are completely
classified by solving the corresponding system of F- and D-flatness equations
including non-renormalizable terms up to sixth order. The requirement of
existence of electroweak massless doublets further restricts the
phenomenologically viable vacua. The third generation fermions receive masses
from the tree-level superpotential. Further, a complete calculation of all
non-renormalizable fermion mass terms up to fifth order shows that in certain
string vacua the hierarchy of the fermion families is naturally obtained in the
model as the second and third generation fermions earn their mass from fourth
and fifth order terms. Along certain flat directions it is shown that the ratio
of the SU(4) breaking scale and the reduced Planck mass is equal to the up
quark ratio m_c/m_t at the string scale. An additional prediction of the model,
is the existence of a U(1) symmetry carried by the fields of the hidden sector,
ensuring thus the stability of the lightest hidden state. It is proposed that
the hidden states may account for the invisible matter of the universe.Comment: Latex2e file, 50 pages, uses rotating.st
Gauge Unification and Quark Masses in a Pati-Salam Model from Branes
We investigate the phase space of parameters in the Pati-Salam model derived
in the context of D-branes scenarios, requiring low energy string scale. We
find that a non-supersymmetric version complies with a string scale as low as
10 TeV, while in the supersymmetric version the string scale raises up to ~2 x
10^7 TeV. The limited energy region for RGE running demands a large tan(beta)
in order to have experimentally acceptable masses for the top and bottom
quarks.Comment: 11 pages, LaTeX, 7 figures include
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