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 $\rho$-$\omega$ mixing. This is due to the density and asymmetry dependent $\rho$-$\omega$ 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, $R_{AA}(p_T)$ for light hadrons by taking into account the collisional energy loss. We show that in the measured $p_T$ 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 $\sqrt{s_{NN}}$ = 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 $\rho$ and $\omega$ 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 $Au + Au$ collisions at $\sqrt{s}=200$ 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