420 research outputs found
Partition Functions in Statistical Mechanics, Symmetric Functions, and Group Representations
Partition functions for non-interacting particles are known to be symmetric
functions. It is shown that powerful group-theoretical techniques can be used
not only to derive these relationships, but also to significantly simplify
calculation of the partition functions for particles that carry internal
quantum numbers. The partition function is shown to be a sum of one or more
group characters. The utility of character expansions in calculating the
partition functions is explored. Several examples are given to illustrate these
techniques.Comment: 16 pages of RevTe
Zero temperature phases of the frustrated J1-J2 antiferromagnetic spin-1/2 Heisenberg model on a simple cubic lattice
At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg
antiferromagnet on a simple cubic lattice with competing first and second
neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave
theory. We find existence of two phases: a two sublattice Neel phase for small
J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain
the sublattice magnetizations and ground state energies for the two phases and
find that there exists a first order phase transition from the AF-phase to the
CAF-phase at the critical transition point, pc = 0.28. Our results for the
value of pc are in excellent agreement with results from Monte-Carlo
simulations and variational spin wave theory. We also show that the quartic 1/S
corrections due spin-wave interactions enhance the sublattice magnetization in
both the phases which causes the intermediate paramagnetic phase predicted from
linear spin wave theory to disappear.Comment: 19 pages, 4 figures, Fig. 1b modified, Appendix B text modifie
Partner bereavement and risk of chronic urticaria, alopecia areata and vitiligo:cohort studies in the UK and Denmark
The pathogeneses of skin diseases are not fully understood. Psychological stress has been proposed to be associated with skin diseases, but the epidemiological evidence is limited [1, 2]. We have recently reported the associations between partner bereavement (an extreme life stressor) and psoriasis, atopic eczema and melanoma [3, 4]. In this study, we further investigated whether partner bereavement was associated with urticaria, alopecia areata, or vitiligo
Two-Loop -Diagrams from String Theory
Using the {\em cutting and sewing} procedure we show how to get Feynman
diagrams, up to two-loop order, of -theory with an internal SU(N)
symmetry group, starting from tachyon amplitudes of the open bosonic string
theory. In a properly defined field theory limit, we easily identify the
corners of the string moduli space reproducing the correctly normalized field
theory amplitudes expressed in the Schwinger parametrization.Comment: 28 pages, 12 figure
Quantum state transformation by dispersive and absorbing four-port devices
The recently derived input-output relations for the radiation field at a
dispersive and absorbing four-port device [T. Gruner and D.-G. Welsch, Phys.
Rev. A 54, 1661 (1996)] are used to derive the unitary transformation that
relates the output quantum state to the input quantum state, including
radiation and matter and without placing frequency restrictions. It is shown
that for each frequency the transformation can be regarded as a well-behaved
SU(4) group transformation that can be decomposed into a product of U(2) and
SU(2) group transformations. Each of them may be thought of as being realized
by a particular lossless four-port device. If for narrow-bandwidth radiation
far from the medium resonances the absorption matrix of the four-port device
can be disregarded, the well-known SU(2) group transformation for a lossless
device is recognized. Explicit formulas for the transformation of Fock-states
and coherent states are given.Comment: 24 pages, RevTe
In-medium relativistic kinetic theory and nucleon-meson systems
Within the model of coupled nucleon-meson systems, a
generalized relativistic Lenard--Balescu--equation is presented resulting from
a relativistic random phase approximation (RRPA). This provides a systematic
derivation of relativistic transport equations in the frame of nonequilibrium
Green's function technique including medium effects as well as flucuation
effects. It contains all possible processes due to one meson exchange and
special attention is kept to the off--shell character of the particles. As a
new feature of many particle effects, processes are possible which can be
interpreted as particle creation and annihilation due to in-medium one meson
exchange. In-medium cross sections are obtained from the generalized derivation
of collision integrals, which possess complete crossing symmetries.Comment: See nucl-th/9310032 for revised version which the authors
incompetently resubmitted rather than correctly replacing thi
A Planck-scale axion and SU(2) Yang-Mills dynamics: Present acceleration and the fate of the photon
From the time of CMB decoupling onwards we investigate cosmological evolution
subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale
eV (masquerading as the factor of the SM at
present). The viability of this postulate is discussed in view of cosmological
and (astro)particle physics bounds. The gauge theory is coupled to a spatially
homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by
Frieman et al., such an axion is a viable candidate for quintessence, i.e.
dynamical dark energy, being associated with today's cosmological acceleration.
A prediction of an upper limit for the duration of the
epoch stretching from the present to the point where the photon starts to be
Meissner massive is obtained: billion years.Comment: v3: consequences of an error in evolution equation for coupling
rectified, only a minimal change in physics results, two refs. adde
Semiclassical force for electroweak baryogenesis: three-dimensional derivation
We derive a semiclassical transport equation for fermions propagating in the
presence of a CP-violating planar bubble wall at a first order electroweak
phase transition. Starting from the Kadanoff-Baym (KB) equation for the
two-point (Wightman) function we perform an expansion in gradients, or
equivalently in the Planck constant h-bar. We show that to first order in h-bar
the KB equations have a spectral solution, which allows for an on-shell
description of the plasma excitations. The CP-violating force acting on these
excitations is found to be enhanced by a boost factor in comparison with the
1+1-dimensional case studied in a former paper. We find that an identical
semiclassical force can be obtained by the WKB method. Applications to the MSSM
are also mentioned.Comment: 19 page
Changes in examination practices reduce procrastination in university students
The purpose of the study reported here was to observe the effects of examination practices on the extent to which university students procrastinate. These examination practices were: (1) limiting the number of resits, (2) compensatory rather than conjunctive decision-making about student progress, and (3) restricting the time available for completing the first bachelor year. Study success in the first academic year (successful completion within one year, delay, or dropout) of 12,432 students entering a Dutch university before the introduction of the new examination practices was compared with that of 17,036 students admitted after its introduction. After the implementation of the new examination practices successful completion increased with 23% and delay decreased with 25%. The data were collected using an interrupted time series design. Three attempts were made to deal with possible threats to its internal validity. (1) Potential confounding variables were demonstrated not to play a role in explaining the effect of the new examination practices. (2) Interrupted time series regression demonstrated that the intervention, not other changes over time, contributed to study success. And (3), extraneous events interfering with the effect of the intervention were shown to be unlikely. In conclusion, the study presented here is the first to demonstrate the effect of examination rules on study delay. The findings indicate that delays, as usually observed in higher education, are not necessarily
The reporting of studies conducted using observational routinely collected health data statement for pharmacoepidemiology (RECORD-PE).
In pharmacoepidemiology, routinely
collected data from electronic health
records (including primary care
databases, registries, and
administrative healthcare claims) are a
resource for research evaluating the
real world effectiveness and safety of
medicines. Currently available
guidelines for the reporting of research
using non-randomised, routinely
collected data—specifically the
REporting of studies Conducted using
Observational Routinely collected
health Data (RECORD) and the
Strengthening the Reporting of
OBservational studies in Epidemiology
(STROBE) statements—do not
capture the complexity of
pharmacoepidemiological research.
We have therefore extended the
RECORD statement to include
reporting guidelines specific to
pharmacoepidemiological research
(RECORD-PE). This article includes the
RECORD-PE checklist (also available on
www.record-statement.org) and
explains each checklist item with
examples of good reporting. We
anticipate that increasing use of the
RECORD-PE guidelines by researchers
and endorsement and adherence by
journal editors will improve the
standards of reporting of
pharmacoepidemiological research
undertaken using routinely collected
data. This improved transparency will
benefit the research community,
patient care, and ultimately improve
public health
- …