6,002 research outputs found
Direct photons from relativistic heavy ion collisions at CERN SPS and at RHIC
Assuming QGP as the initial state, we have analyzed the direct photon data,
obtained by the WA98 collaboration, in 158 A GeV Pb+Pb collisions at CERN SPS.
It was shown, that for small thermalisation time, two loop rate contribute
substantially to high photons. We argue that for extremely short
thermalisation time scale, the higher loop contribution should not be
neglected. For thermalisation time 0.4 fm or greater, when higher loop
contribution are not substantial, the initial temperature of the QGP is not
large and the system does not produce enough hard photons to fit the WA98
experiment. For initial time in the ranges of 0.4-1.0 fm, WA98 data could be
fitted only if the fluid has initial radial velocity in the range of 0.3-0.5c.
The model was applied to predict photon spectrum at RHIC energy.Comment: 5 pages, 5 figure
Elastic amplitudes studied with the LHC measurements at 7 and 8 TeV
Recent measurements of the differential cross sections in the forward region
of pp elastic scattering at 7 and 8 TeV show precise form of the
dependence. We propose a detailed analysis of these measurements including the
structures of the real and imaginary parts of the scattering amplitude. A good
description is achieved, confirming in all experiments the existence of a zero
in the real part in the forward region close to the origin, in agreement with
the prediction of a theorem by A. Martin, with important role in the observed
form of . Universal value for the position of this zero and
regularity in other features of the amplitudes are found, leading to
quantitative predictions for the forward elastic scattering at 13 TeV.Comment: 22 pages, 17 figures and 4 table
Homogeneity of Stellar Populations in Early-Type Galaxies with Different X-ray Properties
We have found the stellar populations of early-type galaxies are homogeneous
with no significant difference in color or Mg2 index, despite the dichotomy
between X-ray extended early-type galaxies and X-ray compact ones. Since the
X-ray properties reflect the potential gravitational structure and hence the
process of galaxy formation, the homogeneity of the stellar populations implies
that the formation of stars in early-type galaxies predat es the epoch when the
dichotomy of the potential structure was established.Comment: 6 pages, 5 figures, accepted for publication in Ap
Perturbative analysis of wave interactions in nonlinear systems
This work proposes a new way for handling obstacles to asymptotic
integrability in perturbed nonlinear PDEs within the method of Normal Forms -
NF - for the case of multi-wave solutions. Instead of including the whole
obstacle in the NF, only its resonant part is included, and the remainder is
assigned to the homological equation. This leaves the NF intergable and its
solutons retain the character of the solutions of the unperturbed equation. We
exploit the freedom in the expansion to construct canonical obstacles which are
confined to te interaction region of the waves. Fo soliton solutions, e.g., in
the KdV equation, the interaction region is a finite domain around the origin;
the canonical obstacles then do not generate secular terms in the homological
equation. When the interaction region is infifnite, or semi-infinite, e.g., in
wave-front solutions of the Burgers equation, the obstacles may contain
resonant terms. The obstacles generate waves of a new type, which cannot be
written as functionals of the solutions of the NF. When an obstacle contributes
a resonant term to the NF, this leads to a non-standard update of th wave
velocity.Comment: 13 pages, including 6 figure
Effects of Ram-Pressure from Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies
Using a simple model of molecular cloud evolution, we have quantitatively
estimated the change of star formation rate (SFR) of a disk galaxy falling
radially into the potential well of a cluster of galaxies. The SFR is affected
by the ram-pressure from the intracluster medium (ICM). As the galaxy
approaches the cluster center, the SFR increases to twice the initial value, at
most, in a cluster with high gas density and deep potential well, or with a
central pressure of because the ram-pressure
compresses the molecular gas of the galaxy. However, this increase does not
affect the color of the galaxy significantly. Further into the central region
of the cluster ( Mpc from the center), the SFR of the disk
component drops rapidly due to the effect of ram-pressure stripping. This makes
the color of the galaxy redder and makes the disk dark. These effects may
explain the observed color, morphology distribution and evolution of galaxies
in high-redshift clusters. By contrast, in a cluster with low gas density and
shallow potential well, or the central pressure of ,
the SFR of a radially infalling galaxy changes less significantly, because
neither ram-pressure compression nor stripping is effective. Therefore, the
color of galaxies in poor clusters is as blue as that of field galaxies, if
other environmental effects such as galaxy-galaxy interaction are not
effective. The predictions of the model are compared with observations.Comment: 19 pages, 9 figures, to appear in Ap
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