729 research outputs found
Dead cone due to parton virtuality
A general expression for the dead cone of gluons radiated by virtual partons
has been derived. The conventional dead cone for massive on-shell quarks and
the dead cone for the massless virtual partons have been obtained by using
different limits of the general expression. Radiative suppression due to the
virtuality of initial parton jets in Heavy-Ion Collisions (HIC) has been
discussed. It is observed that the suppression caused by the high virtuality is
overwhelmingly large as compared to that on account of conventional dead-cone
of heavy quarks. The dead cone due to virtuality may play a crucial role in
explaining the observed similar suppression patterns of light and heavy quarks
jets in heavy ion collisions at Relativistic Heavy Ion Collider (RHIC)
Matter induced charge symmetry breaking and pion form factor in nuclear medium
Medium modification of pion form factor has been evaluated in asymmetric
nuclear matter. It is shown that both the shape and the pole position of the
pion form factor in dense asymmetric nuclear matter is different from its
vacuum counterpart with - mixing. This is due to the density and
asymmetry dependent - mixing which could even dominate over its
vacuum counterpart in matter. Effect of the in-medium pion factor on
experimental observables {\it e.g.}, invariant mass distribution of lepton
pairs has been demonstrated.Comment: Final Version to appear in Jour. Phys.
Quenching of light hadrons at RHIC in a collisional energy loss scenario
We evaluate the nuclear suppression factor, for light hadrons
by taking into account the collisional energy loss. We show that in the
measured domain of RHIC the elastic process is the dominant mechanism for
the partonic energy loss.Comment: 4 pages with 3 figures, Quark Matter 2008 Proceeding
Elliptic flow of thermal dileptons as a probe of QCD matter
We study the variation of elliptic flow of thermal dileptons with transverse
momentum and invariant mass of the pairs for Pb+Pb collisions at
= 2.76 TeV. The dilepton productions from quark gluon plasma
(QGP) and hot hadrons have been considered including the spectral change of
light vector mesons in the thermal bath. The space time evolution has been
carried out within the frame work of 2+1 dimensional ideal hydrodynamics with
lattice+hadron resonance gas equation of state. We find that a judicious
selection of invariant mass(M) and transverse momentum (p_T) windows can be
used to extract the collective properties of quark matter, hadronic matter and
also get a distinct signature of medium effects on vector mesons. Our results
indicate a reduction of elliptic flow (v_2) for M beyond phi mass, which if
observed experimentally would give the measure of v_2 of the partonic phase.Comment: To appear in Phys. Rev. C (Rapid Comm.
Thermal Radiation from Nucleons and Mesons
Thermal photon emission rates due to meson-nucleon interactions have been
evaluated. An exhaustive set of reactions involving p(\bar p), n(\bar n), rho,
omega, a_1, pi and eta is seen to provide a sizeable contribution to the
emission rate from hot hadronic matter. Contributions from baryonic resonances
are found to be negligibly small
Photons from Quark Gluon Plasma and Hot Hadronic Matter
The productions of real photons from quark gluon plasma and hot hadronic
matter formed after the nucleus - nucleus collisions at ultra-relativistic
energies are discussed. The effects of the spectral shift of the hadrons at
finite temperature on the production of photons are investigated. On the basis
of the present analysis it is shown that the photon spectra measured by WA98
collaboration in Pb + Pb collisions at CERN SPS energies can be explained by
both QGP as well as hadronic initial states if the spectral shift of hadrons at
finite temperature is taken into account. Several other works on the analysis
of WA98 photon data have also been briefly discussed.Comment: Latex file, six eps figures include
Axion-Dilaton Cosmology and Dark Energy
We discuss a class of flat FRW cosmological models based on D=4 axion-dilaton
gravity universally coupled to cosmological background fluids. In particular,
we investigate the possibility of recurrent acceleration, which was recently
shown to be generically realized in a wide class of axion-dilaton models, but
in absence of cosmological background fluids. We observe that, once we impose
the existence of radiation -and matter- dominated earlier stages of cosmic
evolution, the axion-dilaton dynamics is altered significantly with respect to
the case of pure axion-dilaton gravity. During the matter dominated epoch the
scalar fields remain either frozen, due to the large expansion rate, or enter a
cosmological scaling regime. In both cases, oscillations of the effective
equation of state around the acceleration boundary value are impossible. Models
which enter an oscillatory stage in the low redshift regime, on the other hand,
are disfavored by observations. We also comment on the viability of the
axion-dilaton system as a candidate for dynamical dark energy. In a certain
subclass of models, an intermediate scaling regime is succeeded by eternal
acceleration. We also briefly discuss the issue of dependence on initial
conditions.Comment: 28 pages, 11 figure
Omega meson as a chronometer and thermometer in hot-dense hadronic matter
Changes in the properties of the vector mesons in hot and dense hadronic
matter, as produced in heavy ion collisions, lead to the intriguing possibility
of the opening of the decay channel \omega \ra \rho \pi, for the omega meson,
which is impossible in free space. This along with the channel \omega \pi \ra
\pi \pi would result in a decrease in its effective life-time enabling it to
decay within the hot zone and act as a chronometer in contradiction to the
commonly held opinion and would have implications vis a vis determination of
the size of the region through pion interferometry. A new peak and a radically
altered shape of the low invariant mass dilepton spectra appears due to
different shift in the masses of and mesons. The Walecka model
is used for the underlying calculation for the sake of illustration.Comment: To appear in Phys. ReV.
Thermal Radiation from Au + Au Collisions at \sqrt{s} = 200 GEV/A Energy
The transverse momentum distribution of the direct photons measured by the
PHENIX collaboration in collisions at GeV/A has been
analyzed. It has been shown that the data can be reproduced reasonably well
assuming a deconfined state of thermalized quarks and gluons with initial
temperature more than the transition temperature for deconfinement inferred
from lattice QCD. The value of the initial temperature depends on the equation
of state of the evolving matter. The sensitivities of the results on various
input parameters have been studied. The effects of the modifications of
hadronic properties at non-zero temperature have been discussed.Comment: minor modifications in the text, accepted for publicatio
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