arXiv subjects

Pradip Roy

Publications and source records attributed to Pradip Roy.

86 records · Page 5Linked to original sources

arXiv2002

Space-time evolution of ultra-relativistic heavy ion collisions and hadronic spectra

The space-time evolution of the hot and dense matter formed after the collisions of heavy nuclei at ultra-relativistic energies is investigated using (3+1) dimensional hydrodynamical models. The effects of the spectral shift of the hadronic properties are incorporated in the equation of state (EOS) of the evolving matter. In-medium shift of hadronic properties are considered for Quantum Hadrodynamics (QHD) and universal scaling scenarios. It is found that the EOS for the hadronic matter for universal scaling of hadronic masses (except pseudoscalar mesons) is similar to the recent lattice results. We observe that the space-time volume of the hadronic matter at the freeze-out is considerably different from the one when medium effects on the hadrons are ignored. The sensitivity of the results on the initial radial velocity profile is investigated. The transverse mass spectra of pions and protons of NA49 collaboration are analyzed.

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arXiv2001

Photons from Nucleus-Nucleus Collisions at Ultra-Relativistic Energies

We compare the photon emission rates from hot hadronic matter with in-medium mass shift and Quark Gluon Plasma (QGP). It is observed that the WA98 data can be well reproduced by hadronic initial state with initial temperature $\sim 200$ MeV if the universal scaling of temperature dependent hadronic masses are assumed and the evolution of temperature with time is taken from transport model or (3+1) dimensional hydrodynamics. The data can also be reproduced by QGP initial state with similar initial temperature and non-zero initial radial velocity.

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arXiv2000

Thermal Photons and Lepton Pairs from Quark Gluon Plasma and Hot Hadronic Matter

The formulation of the real and virtual photon production rate from strongly interacting matter is presented in the framework of finite temperature field theory. The changes in the hadronic spectral function induced by temperature are discussed within the ambit of the Walecka type model, gauged linear and non-linear sigma models, hidden local symmetry approach and QCD sum rule approach. Possibility of observing the direct thermal photon and lepton pair from quark gluon plasma has been contrasted with those from hot hadronic matter with and without medium effects for various mass variation scenarios. At SPS energies, in-medium effects of different magnitude on the hadronic properties for the Walecka model, Brown-Rho scaling and Nambu scaling scenarios are conspicuously visible through the low invariant mass distribution of dilepton and transverse momentum spectra of photon. However, at RHIC energies the thermal photon (dilepton) spectra originating from Quark Gluon Plasma overshines those from hadronic matter for large transverse momentum (invariant mass) irrespective of the models used for evaluating the finite temperature effects on the hadronic properties. It is thus expected that both at RHIC and LHC energies the formation of Quark Gluon Plasma in the initial stages may indeed turn out to be a realistic scenario.

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arXiv1999

Electromagnetic Signals of Hot Hadronic Matter

The photon and dilepton emission rates from quark gluon plasma and hot hadronic matter have been evaluated. The in-medium modifications of the particles appearing in the internal loop of the self energy diagram are taken into account by using a phenomenological effective Lagrangian approach, Brown-Rho and Nambu scaling scenarios. We note that the in-medium effects on the low invariant mass distribution of dilepton and transverse momentum spectra of photon are clearly visible.

hep-ph↗

arXiv1999

Cosmological QCD Phase Transition and Dark Matter

We calculate the size distribution of quark nuggets, which could be formed due to first order QCD phase transition in the early universe. We find that there are a large number of stable Quark Nuggets which could be a viable candidate for cosmological dark matter.

hep-ph↗

arXiv1999

Electromagnetic Radiation from Hot and Dense Hadronic Matter

The modifications of hadronic masses and decay widths at finite temperature and baryon density are investigated using a phenomenological model of hadronic interactions in the Relativistic Hartree Approximation. We consider an exhaustive set of hadronic reactions and vector meson decays to estimate the photon emission from hot and dense hadronic matter. The reduction in the vector meson masses and decay widths is seen to cause an enhancement in the photon production. It is observed that the effect of $ρ$-decay width on photon spectra is negligible. The effects on dilepton production from pion annihilation are also indicated.

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arXiv1999

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 $ω\ra ρπ$, for the omega meson, which is impossible in free space. This along with the channel $ωπ\ra ππ$ 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 $ρ$ and $ω$ mesons. The Walecka model is used for the underlying calculation for the sake of illustration.

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arXiv1999

Electromagnetic probes of hot and dense hadronic matter

The formalism of photon and dilepton productions in a thermal bath are discussed in the frame work of finite temperature field theory. The emission rate has been expressed in terms of the discontinuities or imaginary part of the photon self energy in the thermal bath. The photon and dilepton emissions have been computed by taking into account the in-medium modifications of the particles appearing in the internal loop of the self energy diagram using a phenomenological effective lagrangian approach. The results has been compared with the photon spectra measured by WA 98 collaboration at CERN SPS energies.

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arXiv1998

Photons from Pb + Pb and S + Au Collisions at CERN SPS Energies

The effects of the variation of vector meson masses and decay widths on photon production from hot strongly interacting matter formed after Pb + Pb and S + Au collisions at CERN SPS energies are considered. It has been shown that the present photon spectra measured by WA80 and WA98 Collaborations can not distinguish between the formation of quark matter and hadronic matter in the initial state.

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arXiv1998

Unstable particles in matter at a finite temperature: the rho and omega mesons

Unstable particles (such as the vector mesons) have an important role to play in low mass dilepton production resulting from heavy ion collisions and this has been a subject of several investigations. Yet subtleties, such as the implications of the generalization of the Breit-Wigner formula for nonzero temperature and density, e.g. the question of collisional broadening, the role of Bose enhancement, etc., the possibility of the kinematic opening (or closing) of decay channels due to environmental effects, the problem of double counting through resonant and direct contributions, are often given insufficient emphasis. The present study attempts to point out these features using the rho and omega mesons as illustrative examples. The difference between the two versions of the Vector Meson Dominance Model in the present context is also presented. Effects of non-zero temperature and density, through vector meson masses and decay widths, on dilepton spectra are studied, for concreteness within the framework of a Walecka-type model, though most of the basic issues highlighted apply to other scenarios as well.

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arXiv1998

Photons from Hadronic Matter at Finite Temperature

Temperature dependence of hadronic decay widths and masses are studied within the framework of an effective Lagrangian approach. At finite temperature the hadronic masses do not seem to follow a universal scaling law. Considering an exhaustive set of hadronic reactions and vector meson decays we have estimated the photon spectrum emitted from hot hadronic matter taking into account medium effects through thermal loop corrections on the hadronic decay widths and masses. An enhancement in photon emission rate is obtained when we use the in-medium masses of vector mesons in our calculations. It is observed that the effect of $ρ$ decay width on the photon spectra is negligible.

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arXiv1997

Quark Gluon Plasma Diagnostics in a Successive Equilibrium Scenario

The relativistic Fokker Planck equation has been used to study the evolution of the quark distribution in the quark gluon phase expected to be formed in ultra-relativistic heavy ion collisions. The effect of thermal masses for quarks and gluons is incorporated to take account of the in-medium properties. We find that the kinetic equilibrium is achieved before the system reaches the critical temperature of quark hadron phase transition. We find that chemical equilibrium is not achieved during this time. We have evaluated the electromagnetic probes of quark gluon plasma from the non-equilibrated quark gluon phase and compared them with those in completely equilibrated scenario. The hard QCD production rates for the electromagnetic ejectiles as well as the heavy quark production rates are also calculated.

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arXiv1996

Thermal Masses and Equilibrium Rates in the Quark Gluon Phase

We apply the momentum integrated Boltzmann transport equation to study the time evolution of various quark flavours in the central region of ultra-relativistic heavy ion collisions. The effects of thermal masses for quarks and gluons are incorporated to take into account the in-medium properties of these ingredients of the putative quark gluon plasma. We find that even under very optimistic conditions, complete chemical equilibration in the quark gluon plasma appears unlikely.

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arXiv1996

Dissipative Effects in Photon Diagnostics of Quark Gluon Plasma

The effects of dissipation on the space time evolution of matter formed in ultra-relativistic heavy ion collision is discussed. The thermal photon spectra for RHIC and LHC energies with viscous flow is considered. The effects of viscosity in the thermal single photon spectra is seen to be important in QGP phase as compared to the hadronic phase. Recently available data from WA80 group at SPS energies for $S+Au$ collision are compared with theoretical calculations. The experimental data do not appear to be compatible with the formation of matter in the pure hadronic phase.

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