1,007 research outputs found
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.
Screening of a Moving Parton in the Quark-Gluon Plasma
The screening potential of a parton moving through a quark-gluon plasma is
calculated using the semi-classical transport theory. An anisotropic potential
showing a minimum in the direction of the parton velocity is found. As
consequences possible new bound states and J/psi dissociation are discussed.Comment: 4 pages, 2 figures, final, extended version, to be published in
Phys.Rev.
Wakes in the quark-gluon plasma
Using the high temperature approximation we study, within the linear response
theory, the wake in the quark-gluon plasma by a fast parton owing to dynamical
screening in the space like region. When the parton moves with a speed less
than the average speed of the plasmon, we find that the wake structure
corresponds to a screening charge cloud traveling with the parton with one sign
flip in the induced charge density resulting in a Lennard-Jones type potential
in the outward flow with a short range repulsive and a long range attractive
part. On the other hand if the parton moves with a speed higher than that of
plasmon, the wake structure in the induced charge density is found to have
alternate sign flips and the wake potential in the outward flow oscillates
analogous to Cerenkov like wave generation with a Mach cone structure trailing
the moving parton. The potential normal to the motion of the parton indicates a
transverse flow in the system. We also calculate the potential due to a color
dipole and discuss consequences of possible new bound states and
suppression in the quark-gluon plasma.Comment: 20 pages, 14 figures (high resolution figures available with
authors); version accepted for publication in Phys. Rev.
Can Van Hove singularities be observed in relativistic heavy-ion collisions ?
Based on general arguments the in-medium quark propagator in a quark-gluon
plasma leads to a quark dispersion relation consisting of two branches, of
which one exhibits a minimum at some finite momentum. This results in a
vanishing group velocity for collective quark modes. Important quantities such
as the production rate of low mass lepton pairs and mesonic correlators depend
inversely on this group velocity. Therefore these quantities, which follow from
self energy diagrams containing a quark loop, are strongly affected by Van Hove
singularities (peaks and gaps). If these sharp structures could be observed in
relativistic heavy-ion collisions it would reveal the physical picture of the
QGP as a gas of quasiparticles.Comment: 12 pages including nine figures and style files, invited talk given
at the ICPAQGP-2001, November 26-30, 2001, Jaipur, Indi
Racism, ethnic density and psychological well-being through adolescence: evidence from the Determinants of Adolescent Social well-being and Health longitudinal study
Objective. To investigate the effect of racism, own-group ethnic density, diversity and deprivation on adolescent trajectories in psychological well-being
Non-Perturbative Dilepton Production from a Quark-Gluon Plasma
The dilepton production rate from the quark-gluon plasma is calculated from
the imaginary part of the photon self energy using a quark propagator that
contains the gluon condensate. The low mass dilepton rate obtained in this way
exhibits interesting structures (peaks and gaps), which might be observable at
RHIC and LHC.Comment: 16 pages, REVTEX, 8 PostScript figure
Distinguishing among Scalar Field Models of Dark Energy
We show that various scalar field models of dark energy predict degenerate
luminosity distance history of the Universe and thus cannot be distinguished by
supernovae measurements alone. In particular, models with a vanishing
cosmological constant (the value of the potential at its minimum) are
degenerate with models with a positive or negative cosmological constant whose
magnitude can be as large as the critical density. Adding information from CMB
anisotropy measurements does reduce the degeneracy somewhat but not
significantly. Our results indicate that a theoretical prior on the preferred
form of the potential and the field's initial conditions may allow to
quantitatively estimate model parameters from data. Without such a theoretical
prior only limited qualitative information on the form and parameters of the
potential can be extracted even from very accurate data.Comment: 15 pages, 5 figure
Skewness in the Cosmic Microwave Background Anisotropy from Inflationary Gravity Wave Background
In the context of inflationary scenarios, the observed large angle anisotropy
of the Cosmic Microwave Background (CMB) temperature is believed to probe the
primordial metric perturbations from inflation. Although the perturbations from
inflation are expected to be gaussian random fields, there remains the
possibility that nonlinear processes at later epochs induce ``secondary''
non-gaussian features in the corresponding CMB anisotropy maps. The
non-gaussianity induced by nonlinear gravitational instability of scalar
(density) perturbations has been investigated in existing literature. In this
paper, we highlight another source of non-gaussianity arising out of higher
order scattering of CMB photons off the metric perturbations. We provide a
simple and elegant formalism for deriving the CMB temperature fluctuations
arising due to the Sachs-Wolfe effect beyond the linear order. In particular,
we derive the expression for the second order CMB temperature fluctuations. The
multiple scattering effect pointed out in this paper leads to the possibility
that tensor metric perturbation, i.e., gravity waves (GW) which do not exhibit
gravitational instability can still contribute to the skewness in the CMB
anisotropy maps. We find that in a flat universe, the skewness in
CMB contributed by gravity waves via multiple scattering effect is comparable
to that from the gravitational instability of scalar perturbations for equal
contribution of the gravity waves and scalar perturbations to the total rms CMB
anisotropy. The secondary skewness is found to be smaller than the cosmic
variance leading to the conclusion that inflationary scenarios do predict that
the observed CMB anisotropy should be statistically consistent with a gaussian
random distribution.Comment: 10 pages, Latex (uses revtex), 1 postscript figure included. Accepted
for publication in Physical Review
Some Applications of Thermal Field Theory to Quark-Gluon Plasma
The lecture provides a brief introduction of thermal field theory within
imaginary time formalism, the Hard Thermal Loop perturbation theory and some of
its application to the physics of the quark-gluon plasma, possibly created in
relativistic heavy ion collisions.Comment: 17 pages, 12 figures : Lectures given in "Workshop on Hadron Physics"
during March 7-17, 2005, Puri, Indi
A novel radio imaging method for physical spectral index modelling
We present a new method, called "forced-spectrum fitting", for
physically-based spectral modelling of radio sources during deconvolution. This
improves upon current common deconvolution fitting methods, which often produce
inaccurate spectra. Our method uses any pre-existing spectral index map to
assign spectral indices to each model component cleaned during the
multi-frequency deconvolution of WSClean, where the pre-determined spectrum is
fitted. The component magnitude is evaluated by performing a modified weighted
linear least-squares fit. We test this method on a simulated LOFAR-HBA
observation of the 3C196 QSO and a real LOFAR-HBA observation of the 4C+55.16
FRI galaxy. We compare the results from the forced-spectrum fitting with
traditional joined-channel deconvolution using polynomial fitting. Because no
prior spectral information was available for 4C+55.16, we demonstrate a method
for extracting spectral indices in the observed frequency band using
"clustering". The models generated by the forced-spectrum fitting are used to
improve the calibration of the datasets. The final residuals are comparable to
existing multi-frequency deconvolution methods, but the output model agrees
with the provided spectral index map, embedding correct spectral information.
While forced-spectrum fitting does not solve the determination of the spectral
information itself, it enables the construction of accurate multi-frequency
models that can be used for wide-band calibration and subtraction.Comment: 17 pages, 9 figures, 5 tables. Accepted for publication in MNRA
- …