263 research outputs found
Flat Directions in Flipped SU(5) I: All-Order Analysis
We present a systematic classification of field directions for the
string-derived flipped SU(5) model that are D- and F-flat to all orders.
Properties of the flipped SU(5) model with field values in these directions are
compared to those associated with other flat directions that have been shown to
be F-flat to specific finite orders in the superpotential. We discuss the
phenomenological Higgs spectrum, and quark and charged-lepton mass textures.Comment: 22 pages. Late
On the Possibility of Optical Unification in Heterotic Strings
Recently J. Giedt discussed a mechanism, entitled optical unification,
whereby string scale unification is facilitated via exotic matter with
intermediate scale mass. This mechanism guarantees that a virtual MSSM
unification below the string scale is extrapolated from the running of gauge
couplings upward from M_Z^o when an intermediate scale desert is assumed. In
this letter we explore the possibility of optical unification within the
context of weakly coupled heterotic strings. In particular, we investigate this
for models of free fermionic construction containing the NAHE set of basis
vectors. This class is of particular interest for optical unification, because
it provides a standard hypercharge embedding within SO(10), giving the standard
k_Y = 5/3 hypercharge level, which was shown necessary for optical unification.
We present a NAHE model for which the set of exotic SU(3)_C
triplet/anti-triplet pairs, SU(2)_L doublets, and non-Abelian singlets with
hypercharge offers the possibility of optical unification. Whether this model
can realize optical unification is conditional upon these exotics not receiving
Fayet-Iliopoulos (FI) scale masses when a flat direction of scalar vacuum
expectation values is non-perturbatively chosen to cancel the FI D-term, xi,
generated by the anomalous U(1)-breaking Green-Schwarz-Dine-Seiberg-Wittten
mechanism. A study of perturbative flat directions and their phenomenological
implications for this model is underway.
This paper is a product of the NFS Research Experiences for Undergraduates
and the NSF High School Summer Science Research programs at Baylor University.Comment: 16 pages. Standard Late
Background Dependent Lorentz Violation: Natural Solutions to the Theoretical Challenges of the OPERA Experiment
To explain both the OPERA experiment and all the known phenomenological
constraints/observations on Lorentz violation, the Background Dependent Lorentz
Violation (BDLV) has been proposed. We study the BDLV in a model independent
way, and conjecture that there may exist a "Dream Special Relativity Theory",
where all the Standard Model (SM) particles can be subluminal due to the
background effects. Assuming that the Lorentz violation on the Earth is much
larger than those on the interstellar scale, we automatically escape all the
astrophysical constraints on Lorentz violation. For the BDLV from the effective
field theory, we present a simple model and discuss the possible solutions to
the theoretical challenges of the OPERA experiment such as the Bremsstrahlung
effects for muon neutrinos and the pion decays. Also, we address the Lorentz
violation constraints from the LEP and KamLAMD experiments. For the BDLV from
the Type IIB string theory with D3-branes and D7-branes, we point out that the
D3-branes are flavour blind, and all the SM particles are the conventional
particles as in the traditional SM when they do not interact with the
D3-branes. Thus, we not only can naturally avoid all the known phenomenological
constraints on Lorentz violation, but also can naturally explain all the
theoretical challenges. Interestingly, the energy dependent photon velocities
may be tested at the experiments.Comment: RevTex4, 14 pages, minor corrections, references adde
Minimal Standard Heterotic String Models
Three generation heterotic-string vacua in the free fermionic formulation
gave rise to models with solely the MSSM states in the observable Standard
Model charged sector. The relation of these models to Z_2 x Z_2 orbifold
compactifications dictates that they produce three pairs of untwisted Higgs
multiplets. The reduction to one pair relies on the analysis of supersymmetric
flat directions, that give superheavy mass to the dispensable Higgs states. We
explore the removal of the extra Higgs representations by using the free
fermion boundary conditions and hence directly at the string level, rather than
in the effective low energy field theory. We present a general mechanism that
achieves this reduction by using asymmetric boundary conditions between the
left- and right-moving internal fermions. We incorporate this mechanism in
explicit string models containing three twisted generations and a single
untwisted Higgs doublet pair. We further demonstrate that an additional effect
of the asymmetric boundary conditions is to substantially reduce the
supersymmetric moduli space.Comment: 20 pages, LaTeX; added reference
Relic Neutralino Densities and Detection Rates with Nonuniversal Gaugino Masses
We extend previous analyses on the interplay between nonuniversalities in the
gaugino mass sector and the thermal relic densities of LSP neutralinos, in
particular to the case of moderate to large tan beta. We introduce a set of
parameters that generalizes the standard unified scenario to cover the complete
allowed parameter space in the gaugino mass sector. We discuss the physical
significance of the cosmologically preferred degree of degeneracy between
charginos and the LSP and study the effect this degree of degeneracy has on the
prospects for direct detection of relic neutralinos in the next round of dark
matter detection experiments. Lastly, we compare the fine tuning required to
achieve a satisfactory relic density with the case of universal gaugino masses,
as in minimal supergravity, and find it to be of a similar magnitude. The
sensitivity of quantifiable measures of fine-tuning on such factors as the
gluino mass and top and bottom masses is also examined.Comment: Uses RevTeX; 14 pages, 16 figure
A novel hybrid material with calcium and strontium release capability
The preparation of PDMS–TEOS–CaO hybrid materials by sol–gel techniques has been widely described in
previous works. Calcium nitrate is the most common source of calcium used in these preparations.
However, to remove possible toxic nitrate by-products a thermal treatment is necessary at temperatures
above 500 1C, which leads to the degradation of the polymeric components of the hybrids. Strontium has
already shown some promising results in the therapeutic area, being used in cases of osteoporosis and low
bone density. In this study a new potential bioactive hybrid material was prepared, by sol–gel techniques,
using calcium acetate as a novel calcium source. Also, for the first time, incorporation of strontium in a
PDMS–TEOS hybrid system was evaluated. Samples were characterized before and after immersion in
Kokubo’s Simulated Body Fluid (SBF) by SEM, EDS, ICP and FT-IR spectroscopy
Fluctuation Theorems for Entropy Production and Heat Dissipation in Periodically Driven Markov Chains
Asymptotic fluctuation theorems are statements of a Gallavotti-Cohen symmetry
in the rate function of either the time-averaged entropy production or heat
dissipation of a process. Such theorems have been proved for various general
classes of continuous-time deterministic and stochastic processes, but always
under the assumption that the forces driving the system are time independent,
and often relying on the existence of a limiting ergodic distribution. In this
paper we extend the asymptotic fluctuation theorem for the first time to
inhomogeneous continuous-time processes without a stationary distribution,
considering specifically a finite state Markov chain driven by periodic
transition rates. We find that for both entropy production and heat
dissipation, the usual Gallavotti-Cohen symmetry of the rate function is
generalized to an analogous relation between the rate functions of the original
process and its corresponding backward process, in which the trajectory and the
driving protocol have been time-reversed. The effect is that spontaneous
positive fluctuations in the long time average of each quantity in the forward
process are exponentially more likely than spontaneous negative fluctuations in
the backward process, and vice-versa, revealing that the distributions of
fluctuations in universes in which time moves forward and backward are related.
As an additional result, the asymptotic time-averaged entropy production is
obtained as the integral of a periodic entropy production rate that generalizes
the constant rate pertaining to homogeneous dynamics
Investigation of Quasi--Realistic Heterotic String Models with Reduced Higgs Spectrum
Quasi--realistic heterotic-string models in the free fermionic formulation
typically contain an anomalous U(1), which gives rise to a Fayet-Iliopolous
term that breaks supersymmetry at the one--loop level in string perturbation
theory. Supersymmetry is restored by imposing F- and D-flatness on the vacuum.
In Phys. Rev. D 78 (2008) 046009, we presented a three generation free
fermionic standard-like model which did not admit stringent F- and D-flat
directions, and argued that the all the moduli in the model are fixed. The
particular property of the model was the reduction of the untwisted Higgs
spectrum by a combination of symmetric and asymmetric boundary conditions with
respect to the internal fermions associated with the compactified dimensions.
In this paper we extend the analysis of free fermionic models with reduced
Higgs spectrum to the cases in which the SO(10) symmetry is left unbroken, or
is reduced to the flipped SU(5) subgroup. We show that all the models that we
study in this paper do admit stringent flat directions. The only examples of
models that do not admit stringent flat directions remain the strandard-like
models of reference Phys. Rev. D 78 (2008) 046009.Comment: 38 pages, 1 figur
A Minimal Superstring Standard Model II: A Phenomenological Study
Recently, we demonstrated the existence of heterotic--string solutions in
which the observable sector effective field theory just below the string scale
reduces to that of the MSSM, with the standard observable gauge group being
just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x U(1)_Y-charged
spectrum of the observable sector consisting solely of the MSSM spectrum.
Associated with this model is a set of distinct flat directions of vacuum
expectation values (VEVs) of non-Abelian singlet fields that all produce solely
the MSSM spectrum. In this paper, we study the effective superpotential induced
by these choices of flat directions. We investigate whether sufficient degrees
of freedom exist in these singlet flat directions to satisfy various
phenomenological constraints imposed by the observed Standard Model data. For
each flat direction, the effective superpotential is given to sixth order. The
variations in the singlet and hidden sector low energy spectrums are analyzed.
We then determine the mass matrices (to all finite orders) for the three
generations of MSSM quarks and leptons. Possible Higgs mu-terms are
investigated. We conclude by considering generalizations of our flat directions
involving VEVs of non-Abelian fields.Comment: 41 pages. Standard Late
Perspectives for the detection and measurement of Supersymmetry in the focus point region of mSUGRA models with the ATLAS detector at LHC
This paper discusses the ATLAS potential to study Supersymmetry for the
"Focus-Point" region of the parameter space of mSUGRA models. The potential to
discovery a deviation from Standard Model expectations with the first few
of LHC data was studied using the parametrized simulation of the
ATLAS detector. Several signatures were considered, involving hard jets, large
missing energy, and either -tagged jets, opposite-sign isolated electron or
muon pairs, or top quarks reconstructed exploiting their fully hadronic decays.
With only 1 of data each of these signatures may allow to observe
an excess of events over Standard Model expectation with a statistical
significance exceeding 5 standard deviations. An analytical expression was
derived for the shape of the distribution of the dilepton invariant mass
arising from the three-body leptonic decay of the neutralinos under the
hypothesis of heavy scalars, which is appropriate for the focus-point scenario.
The resulting function was used to fit the distribution of the dilepton
invariant mass obtained with simulated LHC data, and to extract the value of
two kinematic endpoints measuring the and
the mass differences. This information was
used to constrain the MSSM parameter space compatible with the data
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