141 research outputs found
Structure functions and form factors close to the chiral limit from lattice QCD
Results for nucleon matrix elements (arising from moments of structure
functions) and form factors from a mixture of runs using Wilson, clover and
overlap fermions (both quenched and unquenched) are presented and compared in
an effort to explore the size of the chiral `regime', lattice spacing errors
and quenching artefacts. While no run covers this whole range of effects the
partial results indicate a picture of small lattice spacing errors, small
quenching effects and only reaching the chiral regime at rather light quark
masses.Comment: 7 pages, 7 figures; contribution to the 2003 Workshop on Lattice
Hadron Physics, Cairns, Australia; error in Fig. 4 corrected; minor text
change
GeV Photons from Ultra High Energy Cosmic Rays accelerated in Gamma Ray Bursts
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a
highly relativistic fireball, via the formation of a collisionless shock. When
this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show
in this paper that these particles produce, via synchrotron emission as they
cross the acceleration region, photons up to 300 GeV which carry away a small,
~0.01, but non-negligible fraction of the total burst energy. I show that, when
the shock occurs with the interstellar medium, the optical depth to
photon-photon scattering, which might cause energy degradation of the photons,
is small. The burst thusly produced would be detected at Earth simultaneoulsy
with the parent gamma-ray burst, although its duration may differ significantly
from that of the lower energy photons. The expected fluences, ~10^{-5}-10^{-6}
erg/cm^2 are well within the range of planned detectors. A new explanation for
the exceptional burst GRB 940217 is discussed.Comment: Accepted for publication in The Physical Review Letters. 4 pages,
RevTeX needed, no figure
Novel study design to assess the utility of the copd assessment test in a primary care setting
The quality of a consultation provided by a physician can have a profound impact on the quality of care and
patient engagement in treatment decisions. When the COPD Assessment Test (CAT) was developed, one of its aims
was to aid the communication between physician and patient about the impact of COPD. We developed a novel
study design to assess this in a primary care consultation.
Primary care physicians across five countries in Europe conducted videoed consultations with six standardised
COPD patients (played by trained actors) which had patient-specific issues that the physician needed to identify
through questioning. Half the physicians saw the patients with the completed CAT, and half without. Independent
assessors scored the physicians on their ability to identify and address the patient-specific issues, review standard COPD aspects, their understanding of the case and their overall performance. This novel study design presented
many challenges which needed to be addressed to achieve an acceptable level of robustness to assess the utility of
the CAT. This paper discusses these challenges and the measures adopted to eliminate or minimise their impact on
the study results
Precision Gauge Unification from Extra Yukawa Couplings
We investigate the impact of extra vector-like GUT multiplets on the
predicted value of the strong coupling. We find in particular that Yukawa
couplings between such extra multiplets and the MSSM Higgs doublets can resolve
the familiar two-loop discrepancy between the SUSY GUT prediction and the
measured value of alpha_3. Our analysis highlights the advantages of the
holomorphic scheme, where the perturbative running of gauge couplings is
saturated at one loop and further corrections are conveniently described in
terms of wavefunction renormalization factors. If the gauge couplings as well
as the extra Yukawas are of O(1) at the unification scale, the relevant
two-loop correction can be obtained analytically. However, the effect persists
also in the weakly-coupled domain, where possible non-perturbative corrections
at the GUT scale are under better control.Comment: 26 pages, LaTeX. v6: Important early reference adde
Effects of 126 dimensional Higgs scalar on Bottom-Tau unification and quasi-infrared fixed point
In the presence of Higgs multiplets in a SO(10)
theory, the fermion masses get contributions from an induced vacuum expectation
value (VEV) of a doublet residing in which differentiates
between quarks and leptons by a relative sign leading to a significant
correction to the prediction of the mass ratio of the bottom quark and the tau
lepton for ranges of the mass of this extra doublet. We perform a two-loop
renormalization group analysis of the minimal version of the one-step
supersymmetric SO(10) model to display this and re-calculate the corrections to
the top quark mass in the presence of such an induced VEV. We show that these
effects make the infra-red fixed point scenario consistent with experimental
results.Comment: revised version with same conclusions. To appear in Phys. Rev.
Finite Theories and the SUSY Flavor Problem
We study a finite SU(5) grand unified model based on the non-Abelian discrete
symmetry A_4. This model leads to the democratic structure of the mass matrices
for the quarks and leptons. In the soft supersymmetry breaking sector, the
scalar trilinear couplings are aligned and the soft scalar masses are
degenerate, thus solving the SUSY flavor problem.Comment: 17 pages, LaTeX, 1 figur
Predictive fermion mass matrix ansatzes in non-supersymmetric SO(10) grand unification
We investigate the status of predictive fermion mass ansatzes which make use
of the grand unification scale conditions , , and in non-supersymmetric SO(10) grand unification.
The gauge symmetry below an intermediate symmetry breaking scale is
assumed to be that of the standard model with either one Higgs doublet or two
Higgs doublets . We find in both cases that a maximum of 5 standard model
parameters may be predicted within experimental ranges. We find that
the standard model scenario predicts the low energy to be in
a range which includes its experimental mid-value 0.044 and which for a large
top mass can extend to lower values than the range resulting in the
supersymmetric case. In the two Higgs standard model case, we identify the
regions of parameter space for which unification of the bottom quark and tau
lepton Yukawa couplings is possible at grand unification scale. In fact, we
find that unification of the top, bottom and tau Yukawa couplings is possible
with the running b-quark mass within the preferred range provided is near the low end of its allowed
range. In this case, one may make 6 predictions which include
within its confidence limits. However unless the running mass , third generation Yukawa coupling unification requires the top mass to be
greater thanComment: 30 pages, 8 figures available on request from
[email protected], Late
Planetary Dynamics and Habitable Planet Formation In Binary Star Systems
Whether binaries can harbor potentially habitable planets depends on several
factors including the physical properties and the orbital characteristics of
the binary system. While the former determines the location of the habitable
zone (HZ), the latter affects the dynamics of the material from which
terrestrial planets are formed (i.e., planetesimals and planetary embryos), and
drives the final architecture of the planets assembly. In order for a habitable
planet to form in a binary star system, these two factors have to work in
harmony. That is, the orbital dynamics of the two stars and their interactions
with the planet-forming material have to allow terrestrial planet formation in
the habitable zone, and ensure that the orbit of a potentially habitable planet
will be stable for long times. We have organized this chapter with the same
order in mind. We begin by presenting a general discussion on the motion of
planets in binary stars and their stability. We then discuss the stability of
terrestrial planets, and the formation of potentially habitable planets in a
binary-planetary system.Comment: 56 pages, 29 figures, chapter to appear in the book: Planets in
Binary Star Systems (Ed. N. Haghighipour, Springer publishing company
Uncertainties in Coupling Constant Unification
The status of coupling constant unification in the standard model and its
supersymmetric extension are discussed. Uncertainties associated with the input
coupling constants, , threshold corrections at the low and high scales,
and possible nonrenormalizable operators are parametrized and estimated. A
simple parametrization of a general supersymmetric new particle spectrum is
given. It is shown that an effective scale can be defined, but for a
realistic spectrum it may differ considerably from the typical new particle
masses. The implications of the lower (higher) values of
suggested by low-energy (-pole) experiments are discussed.Comment: LaTex, 51 pages, 6 figures (available upon request), UPR-0513
The Supersymmetric Particle Spectrum
We examine the spectrum of supersymmetric particles predicted by grand
unified theoretical (GUT) models where the electroweak symmetry breaking is
accomplished radiatively. We evolve the soft supersymmetry breaking parameters
according to the renormalization group equations (RGE). The minimization of the
Higgs potential is conveniently described by means of tadpole diagrams. We
present complete one-loop expressions for these minimization conditions,
including contributions from the matter and the gauge sectors. We concentrate
on the low fixed point region (that provides a natural explanation
of a large top quark mass) for which we find solutions to the RGE satisfying
both experimental bounds and fine-tuning criteria. We also find that the
constraint from the consideration of the lightest supersymmetric particle as
the dark matter of the universe is accommodated in much of parameter space
where the lightest neutralino is predominantly gaugino. The supersymmetric mass
spectrum displays correlations that are model-independent over much of the GUT
parameter space.Comment: 62 pages + 10 PS figures included (uuencoded), MAD/PH/80
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