17,287 research outputs found
Pion and Quark Annihilation Mechanisms of Dilepton Production in Relativistic Heavy-Ion Collisions
We revise the pion-pion and quark-quark annihilation mechanisms of dilepton
production during relativistic heavy-ion collisions. We focus on the
modifications caused by the specific features of intramedium pion states rather
than by medium modification of the rho-meson spectral density. The main
ingredient emerging in our approach is a form-factor of the multi-pion
(multi-quark) system. Replacing the usual delta-function the form-factor plays
the role of distribution which, in some sense, "connects" the 4-momenta of the
annihilating and outgoing particles. The difference between the c.m.s.
velocities attributed to annihilating and outgoing particles is a particular
consequence of this replacement and results in the appearance of a new factor
in the formula for the dilepton production rate. We obtained that the
form-factor of the multi-pion (multi-quark) system causes broadening of the
rate which is most pronounced for small invariant masses, in particular, we
obtain a growth of the rate for the invariant masses below two masses of the
annihilating particles.Comment: 6 pages, 6 figures, LaTex; to appear in Mod. Phys. Lett.
Assessment of Metabolic Parameters For Autism Spectrum Disorders
Autism is a brain development disorder that first appears during infancy or childhood, and generally follows a steady course without remission. Impairments result from maturation-related changes in various systems of the brain. Autism is one of the five pervasive developmental disorders (PDD), which are characterized by widespread abnormalities of social interactions and communication, and severely restricted interests and highly repetitive behavior. The reported incidence of autism spectrum disorders (ASDs) has increased markedly over the past decade. The Centre for Disease Control and Prevention has recently estimated the prevalence of ASDs in the United States at approximately 5.6 per 1000 (1 of 155 to 1 of 160) children. Several metabolic defects, such as phenylketonuria, are associated with autistic symptoms. In deciding upon the appropriate evaluation scheme a clinician must consider a host of different factors. The guidelines in this article have been developed to assist the clinician in the consideration of these factors
Mode-locking in vertical external-cavity surface-emitting lasers with type-II quantum-well configurations
A microscopic study of mode-locked pulse generation is presented for vertical
external-cavity surface-emitting lasers utilizing type-II quantum well
configurations. The coupled Maxwell semiconductor Bloch equations are solved
numerically where the type-II carrier replenishment is modeled via suitably
chosen reservoirs. Conditions for stable mode-locked pulses are identified
allowing for pulses in the \unit[100]{fs} range. Design strategies for type-II
configurations are proposed that avoid potentially unstable pulse dynamics.Comment: Main paper with supplementary material
On the temperature dependence of correlation functions in the space like direction in hot QCD
We study the temperature dependence of quark antiquark correlations in the
space like direction. In particular, we predict the temperature dependence of
space like Bethe-Salpeter amplitudes using recent Lattice gauge data for the
space like string potential. We also investigate the effect of the space like
string potential on the screening mass and discuss possible corrections which
may arise when working with point sources.Comment: 15 pages 8 figures (not included, will be sent on request),
(SUNY-NTG-94-3
Theory of the Franck-Condon blockade regime
Strong coupling of electronic and vibrational degrees of freedom entails a
low-bias suppression of the current through single-molecule devices, termed
Franck-Condon blockade. In the limit of slow vibrational relaxation, transport
in the Franck-Condon-blockade regime proceeds via avalanches of large numbers
of electrons, which are interrupted by long waiting times without electron
transfer. The avalanches consist of smaller avalanches, leading to a
self-similar hierarchy which terminates once the number of transferred
electrons per avalanche becomes of the order of unity. Experimental signatures
of self-similar avalanche transport are strongly enhanced current (shot) noise,
as expressed by giant Fano factors, and a power-law noise spectrum. We develop
a theory of the Franck-Condon-blockade regime with particular emphasis on
effects of electron cotunneling through highly excited vibrational states. As
opposed to the exponential suppression of sequential tunneling rates for
low-lying vibrational states, cotunneling rates suffer only a power-law
suppression. This leads to a regime where cotunneling dominates the current for
any gate voltage. Including cotunneling within a rate-equation approach to
transport, we find that both the Franck-Condon blockade and self-similar
avalanche transport remain intact in this regime. We predict that cotunneling
leads to absorption-induced vibrational sidebands in the Coulomb-blockaded
regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.
Low Mass Dilepton Rate from the Deconfined Phase
We discuss low mass dilepton rates ( GeV) from the deconfined phase of
QCD using both perturbative and non-perturbative models and compare with those
from lattice gauge theory and in-medium hadron gas. Our analysis suggests that
the rate at very low invariant mass ( MeV) using the nonperturbative
gluon condensate in a semiempirical way within the Green function dominates
over the Born-rate and independent of any uncertainty associated with the
choice of the strong coupling in perturbation theory. On the other hand the
rate from interaction in the deconfined phase is important between 200
MeV M\ge 200M\ge 500$ MeV, constraining on the broad resonance structures
in the dilepton rate at large invariant mass. We also discuss the lattice
constraints on the low mass dilepton rate. Furthermore, we discuss a more
realistic way to advocate the quark-hadron duality hypothesis based on the
dilepton rates from QGP and hadron gas than it is done in the literature.Comment: 24 pages, 9 figures; Discussion added, Accepted in Phys. Rev.
Surprisingly Simple Spectra
The large N limit of the anomalous dimensions of operators in
super Yang-Mills theory described by restricted Schur polynomials, are studied.
We focus on operators labeled by Young diagrams that have two columns (both
long) so that the classical dimension of these operators is O(N). At large N
these two column operators mix with each other but are decoupled from operators
with columns. The planar approximation does not capture the large N
dynamics. For operators built with 2, 3 or 4 impurities the dilatation operator
is explicitly evaluated. In all three cases, in a certain limit, the dilatation
operator is a lattice version of a second derivative, with the lattice emerging
from the Young diagram itself. The one loop dilatation operator is diagonalized
numerically. All eigenvalues are an integer multiple of and there
are interesting degeneracies in the spectrum. The spectrum we obtain for the
one loop anomalous dimension operator is reproduced by a collection of harmonic
oscillators. This equivalence to harmonic oscillators generalizes giant
graviton results known for the BPS sector and further implies that the
Hamiltonian defined by the one loop large dilatation operator is
integrable. This is an example of an integrable dilatation operator, obtained
by summing both planar and non-planar diagrams.Comment: 34 page
Training requirements for the administration of intravenous contrast media by radiographers: Radiologists’ perspective
Background. The administration of intravenous contrast media (IVCM) is one of the key areas currently under investigation for inclusion in the South African (SA) radiographers’ scope of practice. However, for the radiographers to legally administer IVCM, training guidelines must first be identified, developed and accredited by the Health Professions Council of SA.Objective. To investigate the radiologists’ perspective of the knowledge, skills and medicolegal training required of radiographers for the administration of IVCM to provide input for the development of national training guidelines.Methods. A quantitative, cross-sectional research study using an online survey, administered by SurveyMonkey, was conducted. The target population included all radiologists residing and practising in the province of KwaZulu-Natal, SA.Results. Fifty-nine participants (60.8%) completed the online survey. Twelve were excluded owing to incomplete surveys, resulting in a final response rate of 48.5% (n=47). The study revealed that various theoretical, clinical/practical and medicolegal study units should be included in the training, i.e. the study of the pharmacology of contrast media, practical training on cardiopulmonary resuscitation and basic life support, as well as the rights and responsibilities of a healthcare professional. In addition, both theory and practical/clinical assessments need to be included.Conclusion. Key data have been provided for the development of national training guidelines for radiographers to administer IVCM, based on scientific evidence that is relevant to the SA context. The study may be of value to other related health professions where scopes of practice are expanded through transforming the education and training curricula
Chiral Prediction for the Scattering Length to Order
We evaluate the S-wave pion--nucleon scattering length in the framework
of heavy baryon chiral perturbation theory up--to--and--including terms of
order . We show that the order piece of the isovector
amplitude at threshold, , vanishes exactly. We predict for the
isovector scattering length, .Comment: 5 pp, LaTeX file, 2 figures (appended as separate compressed tar
file, amin.uu
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