Search

The drag and diffusion coefficients of heavy quarks propagating through quark–gluon plasma (QGP) have been estimated by shielding the infra-red divergences using electric and magnetic screening masses. The electric-type screening in perturbative quantum chromodynamics (pQCD) has been widely studied and used in evaluating the diffusion coefficient of heavy quarks (HQs). The impact of magnetic screening on diffusion coefficients of HQs is not studied before to the best of our knowledge. We explore the effect of magnetic screening mass on the drag and diffusion coefficients of HQs and found it to be non-negligible. Therefore, the effect of magnetic screening should be taken into consideration to characterize hot and dense matter formed in the collisions of nuclei at ultra-relativistic energies. We estimate the suppression of heavy flavored mesons in heavy ion collisions compared to proton+proton collisions at high transverse momenta and found that the suppression is less with the inclusion of magnetic screening. The value of the magnetic screening mass is not known exactly because of its nonperturbative nature. Moreover, it may not be possible to single out the effect of magnetic mass because of the uncertainties in other parameters involved in the diffusion process of HQs. Still it is important to include the effects of magnetic screening because of its physical origin in QCD.

The drag and diffusion coefficients of a hot hadronic medium have been evaluated by using hidden charm mesons as probes. The matrix elements for the evaluation of these coefficients are calculated using an effective theory as well as from scattering lengths. Although the transport coefficients show a significant rise with temperature its effects on the suppression of $J/\psi$ in hadronic matter is not significant., Comment: Accepted in NPA

The two-point correlation functions of kaonic viscous stress tensors and electro-magnetic currents have been calculated respectively to estimate the contributions of the strange sector in the shear viscosity and electrical conductivity of hadronic medium. In the one-loop correlators, kaon propagators contain a non-zero thermal width that leads to non-divergent values of transport coefficients. With the help of effective Lagrangian densities of strange hadrons, we have calculated in-medium self-energy of kaon for different possible mesonic loops, whose imaginary part provide the estimation of kaon thermal width. It is observed that near the quark-hadron transition temperature, the contribution of kaons to shear viscosity is larger and increases faster with temperature than pionic contribution. In case of electrical conductivity the trend due kaon component appears to be opposite in nature with respect to pion component.

We present a study of $\tau^{+}\tau^{-}$ lepton pair production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.5 TeV. The larger $\tau^{\pm}$ mass ($\sim$ 1.77 GeV) compared to $e^{\pm}$ and $\mu^{\pm}$ leads to considerably small hadronic contribution to the $\tau^{+}\tau^{-}$ pair invariant mass ($M$) distribution relative to the production from thermal partonic sources. The quark-anti-quark annihilation processes via intermediary virtual photon, Z and Higgs bosons have been considered for the production of $ \tau^{+}\tau^{-}$. We observe that the contribution from Drell-Yan process dominates over thermal yield for $\tau^{+}\tau^{-}$ pair mass from 4 to 20 GeV at the LHC energy. We also present the ratio of $\tau$ lepton pair yields for nucleus-nucleus collisions relative to yields from p+p collisions scaled by number of binary collisions at LHC energies as a function $\tau$ pair mass. The ratio is found to be significantly above unity for the mass range 4 to 6 GeV. This indicates the possibility of detecting $\tau^{+}\tau^{-}$ pair from quark gluon plasma (QGP) in the mass window $4\leq M$(GeV)$\leq 6$., Comment: Five pages with one LaTeX file and three eps files for figures

A novel attempt has been made to probe the QCD phase boundary by using the experimental data for transverse momenta of {\phi} mesons produced in nuclear collisions at AGS, SPS and RHIC energies. The data are confronted with simple thermodynamic expectations and lattice QCD results. The experimental data indicate a first-order phase transition, with a mixed phase stretching the energy density between \sim1 and 3.2 GeV/fm3 corresponding to SPS energies., Comment: 8-pages, 3-figs, Replaced with the published version

The hydrodynamic expansion rate of quark gluon plasma (QGP) is evaluated and compared with the scattering rate of quarks and gluons within the system. Partonic scattering rates evaluated within the ambit of perturbative Quantum Choromodynamics (pQCD) are found to be smaller than the expansion rate evaluated with ideal equation of state (EoS) for the QGP. This indicate that during the space-time evolution the system remains out of equilibrium. Enhancement of pQCD cross sections and a more realistic EoS keep the partons closer to the equilibrium., Comment: To be published in the Quark Matter 2008 poster proceedings

It has been shown that the evolution of collectivity in ultra-relativistic heavy ion collision is manifested in the variation of various HBT radii with invariant mass ($M$) extracted from the correlation functions of two lepton pairs. The value of the radial velocity ($v_r$) can be estimated from the ratio of the $p_T$ distributions of single photons to lepton pairs for various $M$ windows. It has been argued that the variation of radial flow with appropriate kinematic variables can be used as an indicator of a phase transition from initially produced partons to hadrons. We also consider the elliptic flow ($v_2^{HF}$) of the matter as probed by the single electron spectra originating from the semileptonic decays of heavy mesons. The measured values of $v_2^{HF}$ and the nuclear suppression factor ($R_{\mathrm AA}$) at RHIC energy have been reproduced simultaneously by including both the collisional and radiative processes within the scope of perturbative quantum chromodynamics. The $R_{\mathrm AA}$ and $v_2^{HF}$ have been predicted for LHC energy., Plenary talk at ICPAQGP 2010, December 6-10, 2011, Goa, India

The direct photon spectrum measured by the PHENIX collaboration in Au + Au collisions at sqrt{s_{NN}}=200 GeV has been analyzed. It has been shown that the data can be reproduced reasonably well by assuming a deconfined state of thermalized quarks and gluons. The effects of the equation of state on the value of the initial temperature have been studied. The modifications of hadronic properties at non-zero temperature have been taken in to account., To appear in Quark Matter 2006 proceedings

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.

We calculate nuclear suppression factor ($R_{AA}$) for light hadrons by taking only the elastic processes and argue that in the measured $p_T$ domain of RHIC, collisional rather than the radiative processes is the dominant mechanism for partonic energy loss., Presented at the international conference on strong and electroweak matter 2006, May 10-13, Brookhaven National Laboratory

Effect of gluon radiation off charm quark on transport coefficients and the equilibrium distribution of charm quark

The drag and diffusion coefficients of heavy quarks (HQs) have been evaluated in the pre-equilibrium phase of the evolving fireball produced in heavy ion collisions at RHIC and LHC energies. The KLN and classical Yang-Mills spectra have been used for describing the momentum distributions of the gluons produced just after the collisions but before they thermalize. The interaction of the HQs with these gluons has been treated within the framework of perturbative QCD. We have observed that the HQs are dragged almost equally by the kinetically equilibrated and out-of-equilibrium gluonic systems. We have also noticed that the HQs diffusion in the pre-equilibrium gluonic phase is as fast as in the kinetically equilibrated gluons. Moreover, the diffusion is faster in the pre-equilibrium phase than in the chemically equilibrated quark-gluon plasma. These findings may have significant impact on the analysis of experimental results on the elliptic flow and the high momentum suppression of the open charm and beauty hadrons., Comment: 11 pages,11 figures, Accepted for publication in Journal of Physics G

We study the effect of in-medium hadronic properties in photon nucleus interactions in the context of shadowing as well as the dilepton spectrum for incident photon energies in the range 1.1- 3 GeV. A reasonable agreement with the experimental data for shadowing is obtained in a scenario of downward spectral shift of the hadrons. We show that distinguishable features for in-medium changes of the hadronic properties can be observed experimentally through the dilepton spectrum by judicious choice of target nuclei and incident energy of photons., Comment: 20 pages,RevTex 16 eps files for figs

We study the dilepton spectra measured by the CERES Collaboration in Pb+Au collisions at various values of centrality/charge multiplicity with particular emphasis on characterizing the medium effects of hadrons. The full in-medium spectral function of the vector mesons calculated from quantum hadrodynamics has been used along with other scenarios of medium modifications in the presence of baryons. We show that the CERES acceptance cuts considerably reduce baryonic effects on the spectra of dileptons. The sensitivity on different versions of vector meson dominance has been contrasted. Medium effects have also been incorporated in the equation of state used in the hydrodynamic evolution of the fireball. We observe that a thermal source with initial temperature ~200 MeV can reproduce the observed enhancement in the low mass region of the dilepton spectra. We also estimate the number of π–π collisions near the ρ-peak of the dilepton spectra and argue that thermal equilibrium may have been achieved in the system, justifying the use of a hydrodynamical model to describe the spacetime evolution. The dilepton spectra in Au+Au collisions at RHIC energies are evaluated with and without medium effects on hadrons.

The medium modification of vector meson properties in hot/dense hadronic matter and its role in explaining the CERES/NA45 dilepton data at different centralities are discussed., Talk given at ICPA-QGP, Jaipur, November 2001

The bulk viscosity ($\zeta$) of the hadronic medium has been estimated within the ambit of the Hadron Resonance Gas (HRG) model including the Hagedorn density of states. The HRG thermodynamics within a grand canonical ensemble provides the mean hadron number as well as its fluctuation. The fluctuation in the chemical composition of the hadronic medium in the grand canonical ensemble can result in non-zero divergence of the hadronic fluid flow velocity, allowing us to estimate the $\zeta$ of the hadronic matter upto a relaxation time. We study the influence of the hadronic spectrum on $\zeta$ and find its correlation with the conformal symmetry breaking (CSB) measure, $\epsilon-3P$. We estimate $\zeta$ along the contours with constant, $S/N_B$ (total entropy/net baryon number) in the $T-\mu$ plane (temperature-baryonic chemical potential) for $S/N_B=30, 45$ and 300. We also assess the value of $\zeta$ on the chemical freezeout curve for various centre of mass energy ($\sqrt{s_{NN}}$) and find that the bulk viscosity to entropy density ratio, $\zeta/s$ is larger at FAIR than LHC energies., Comment: Two more figures added. One LaTex for the text and 9 eps files for the figures

The microsecond old universe was too hot for the nucleons to survive as bound states of quarks. When the temperature of the universe fell below ∼10 K as a result of cooling due to expansion, the quarks got confined inside the nucleons to reside there ever after—making them inaccessible to any direct observation in isolation. Natural objects like the core of the compact astrophysical objects may also contain deconfined quark matter. Therefore, creation of a deconfined phase of thermal quarks and gluons will be very important for understanding the evolution of the early universe and the physics of compact astrophysical objects like neutron star. Calculations based on lattice QCD indicate that hadronic matter at high temperatures and densities undergoes a phase transition to a thermalized system of deconfined quarks and gluons, called quark gluon plasma (QGP). It is expected that such high densities and temperatures may be achieved by colliding two nuclei at RHIC, LHC, or FAIR energies. The present volume addresses some of the experimental and theoretical issues related to quark-hadron phase transition in the relativistic heavy ion collisions by the world experts in the field. On the experimental sides it contains results from CERN’s Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) and Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). Comparison of experimental results from RHIC and LHC is extremely useful to gain insights into the properties of the matter formed in these experiments. The present volume offers a review on this subject. On the theoretical side the present volume contains papers on the space-time evolution of the system formed in heavy ion collisions including the effects of viscosities through hydrodynamic equations, issues of equilibration, electromagnetic and heavy quark probes of QGP, quarkonia suppression in QGP, ridge, and transverse correlations. Recent progress in the femtoscopy and particle production in strong electromagnetic fields in heavy ion collisions is also addressed adequately. In summary, the present volume brings out recent developments on almost all the relevant issues of heavy ion collisions like creation, evolution, and signals of QGP. We are sure that the volume will be useful for the current practitioners and young new entrants to this exciting field of research.

The cosmic first order phase transition from quarks to hadrons, occurring a few microseconds after the Big Bang, would lead to the formation of quark nuggets which would be stable on a cosmological time scale, if the associated baryon number is larger than a critical value. We examine the possibility that these surviving quark nuggets may not only be viable candidates for cold dark matter but even close the universe., Latex file

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., text and figures modified

The QCD phase transition occurring a few microseconds after the Big Bang, would lead to the formation of quark nuggets which would be stable on a cosmological time scale, if the initial baryon number of the quark nuggets are larger than a critical value. We examine the possibility that these surviving quark nuggets may not only be viable candidates for cold dark matter but could even close the universe.

Effects of temperature dependence of hadronic form factors on the electromagnetic probes of Quark–Gluon Plasma (QGP) have been looked into. It has been observed that in the kinematic domain thought to be most relevant for Quark–Gluon Plasma diagnostics with electromagnetic probes (M ~ 1–4 GeV ), the space–time integrated emission rates (i.e. count rates) of dileptons and photons from the hot hadronic sector are not very sensitive to the finite temperature corrections of the form factors.

The effects of gluon radiation by charm quarks on the transport coefficients {\it e.g.} drag, longitudinal and transverse diffusion and shear viscosity have been studied within the ambit of perturbative quantum chromodynamics (pQCD) and kinetic theory. We found that while the soft gluon radiation has substantial effects on the transport coefficients of the charm quarks in the quark gluon plasma its effects on the equilibrium distribution function is insignificant., 8 pages, 10 figures

In very energetic collisions of heavy ions, a novel state of matter — quark gluon plasma — is expected to be transiently formed. We review the suitability of electromagnetically interacting particles, dilepton pairs, single photons, diphotons, etc., as diagnostic tools for quark gluon plasma.

NA60 collaboration has extracted the inverse slope parameters, T_{eff} of the dimuon spectra originating from the In+In collisions at root(s_NN)=17.3 GeV for various invariant mass region. They have observed that the inverse slope parameter as a function of invariant mass of the lepton pair drops beyond the rho-peak. In the present work, first we reproduce the observed invariant mass and transverse momentum spectra of the muon pairs. Then show that the slope parameters extracted from the transverse momentum distributions for various invariant mass region windows can be explained by assuming formation of a partonic phase initially which reverts to hadronic phase through a weak first order phase transition at a temperature T_c ~ 175 MeV. It is observed that a scenario without the formation of a partonic phase does not reproduce the non-monotonic behaviour of the inverse slope parameter non typical of radial flow.

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).

The spectrum of emitted gluons from the process g+g -> g+g+g has been evaluated by relaxing some of the approximations used in earlier works. The difference in the results from earlier calculations have been pointed out. The formula obtained in the present work has been applied to estimate physical quantities like equilibration rate of gluons and the energy loss of fast gluon in the gluonic plasma., One LaTeX file for the text and three eps files for figures

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., To appear in Phys. Rev. C (Rapid Comm.)

A measurement for studying the mass dependence of the dilepton interferometry in relativistic heavy-ion collision experiments as a tool to characterize the quark-gluon phase is proposed. In calculations involving dileptons, we show that the mass dependence of radii extracted from the virtual photon (dilepton) interferometry provide access to the development of collective flow with time. It is argued that the non-monotonic variation of HBT radii with invariant mass of the lepton pairs signals the formation of quark gluon plasma in heavy ion collisions. Our proposal of experimentally measuring the ratio, $R_{\mathrm out}/R_{\mathrm side}$ for dileptons can be used to estimate the average life times of the partonic as well as the hadronic phases., Comment: One LaTeX file for text and four eps files for figures

The shear ($\eta$) and bulk ($\zeta$) viscous coefficients have been evaluated for a gluonic fluid. The elastic, $gg \rightarrow gg$ and the inelastic, number non-conserving, $gg\rightarrow ggg$ processes have been considered as the dominant perturbative processes in evaluating the viscous co-efficients to entropy density ($s$) ratios. Recently the processes: $gg \rightarrow ggg$ has been revisited and a correction to the widely used Gunion-Bertsch (GB) formula has been obtained. The $\eta$ and $\zeta$ have been evaluated for gluonic fluid with the formula derived recently. The sensitivity of the quantity, $\eta/s$ on the running coupling constant is also discussed. At $\alpha_s=0.3$ we get $\eta/s=0.24$ which is close to the value obtained from the analysis of the elliptic flow at RHIC experiments.

The structure of the one loop self-energy graphs of the $\rho$ meson is analyzed in the real time formulation of thermal field theory. The modified spectral function of the $\rho$ meson in hot hadronic matter leads to a large enhancement of lepton pair production below the bare peak of the $\rho$. It has been shown that the effective temperature extracted from the inverse slope of the transverse momentum distributions for various invariant mass ($M$) windows of the pair can be used as an efficient tool to characterize different phases of the evolving matter., Comment: Presented at the 6th International Conference on the Physics and Astrophysics of Quark Gluon Plasma, ICPAQGP2010, Dec. 6-10, Goa, India

We study the effects of the equation of state on the nuclear suppression of heavy flavors in quark gluon plasma and estimate the initial entropy density of the system produced in Au + Au collision at the highest Relativistic Heavy Ion Collider (RHIC) energy. For this purpose the experimental data on the charged-particle multiplicity and the nuclear suppression of single-electron spectra originating from the semileptonic decays of open charm and beauty mesons have been employed. We have used inputs from lattice QCD to minimize the model dependence of the results. We obtain the value of the initial entropy density, which varies from 20 to 59/fm${}^{3}$ depending on the value of the velocity of sound that one uses for the analysis. Our investigation leads to a conservative value of the initial entropy density of $\ensuremath{\sim}$20/fm${}^{3}$ with a corresponding initial temperature of $\ensuremath{\sim}$210 MeV, which is well above the value of the transition temperature predicted by lattice QCD.

The momentum dependence of the drag coefficient of heavy quarks propagating through quark gluon plasma (QGP) has been evaluated. The results have been used to estimate the nuclear suppression factor of charm and bottom quarks in QGP. We observe that the momentum dependence of the transport coefficients plays crucial role in the suppression of the heavy quarks and consequently in discerning the properties of QGP using heavy flavours as a probe. We show that the large suppression of the heavy quarks observed at RHIC and LHC is predominantly due to the radiative losses. The suppression of $D^0$ in Pb+Pb collisions at LHC energy - recently measured by the ALICE collaboration has also been studied., Comment: Minor changes in the text

High-energy photons emitted in ultrarelativistic nucleus-nucleus collisions at energies reached at the CERN SPS and LHC and the BNL RHIC are discussed as a source of information on the collision dynamics and as a signature of the deconfining phase transition. The energy density and velocity fields of the expanding matter have been computed assuming cylindrical symmetry along the transverse direction and boost invariance along the longitudinal direction. The emission of photons from an expanding quark-gluon plasma undergoing a first-order phase transition has been evaluated. When the transverse flow of the system is accounted for, it is found that the photons having their origin in the hadronic matter are comparable to those originating in the quark-gluon plasma for [ital p][sub [ital T]] greater than 2 GeV, at energies reached at the SPS and RHIC. At energies reached at the LHC, photons having a [ital p][sub [ital T]] larger than about 4 GeV are shown to have their origin predominantly in the quark-gluon plasma. The location of the [ital p][sub [ital T]] window is shown to be very sensitive to the value of the freeze-out temperature chosen. The feasibility of evaluating the direct (QCD) photons has been studied. Photons fragmented off final more » state quarks in parton-parton scattering are shown to provide the largest background beyond [ital p][sub [ital T]][approx]3 GeV at energies reached at the LHC. Accounting for hadronic degrees of freedom beyond the pion is found to be important. « less

A first attempt has been made to extract the evolution of radial flow from the analysis of the experimental data on electromagnetic probes experimentally measured at SPS and RHIC energies. The $p_T$ spectra of photons and dileptons measured by WA98 and NA60 collaborations respectively at CERN-SPS and the photon spectra obtained by PHENIX collaboration at BNL-RHIC have been used to constrain the theoretical models, rendering the outcome of the analysis largely model independent. We argue that the variation of the radial velocity with invariant mass is indicative of a phase transition from initially produced partons to hadrons at SPS and RHIC energies., One LaTeX and 9 eps files, to appear in Phys. Rev. C

The drag and diffusion coefficients of charm and bottom quarks propagating through quark gluon plasma (QGP) have been evaluated for conditions relevant to nuclear collisions at the Large Hadron Collider (LHC). The dead cone and Landau-Pomeronchuk-Migdal (LPM) effects on radiative energy loss of heavy quarks have been considered. Both radiative and collisional processes of energy loss are included in the effective drag and diffusion coefficients. With these effective transport coefficients, we solve the Fokker-Plank (FP) equation for the heavy quarks executing Brownian motion in the QGP. The solution of the FP equation has been used to evaluate the nuclear suppression factor, R{sub AA}, for the nonphotonic single-electron spectra resulting from the semileptonic decays of hadrons containing charm and bottom quarks. The effects of mass on R{sub AA} have also been highlighted.

A microscopic approach has been employed to study the kaon productions in heavy ion collisions. The momentum integrated Boltzmann equation has been used to study the evolution of strangeness in the system formed in heavy ion collision at relativistic energies. The kaon productions have been calculated for different centre of mass energies ($\sqrt{s_{\mathrm {NN}}}$) ranging from AGS to RHIC. The results have been compared with available experimental data. We obtain a non-monotonic horn like structure for $K^+/\pi^+$ when plotted with $\sqrt{s_{\mathrm {NN}}}$ with the assumption of an initial partonic phase beyond a certain threshold in $\sqrt{s_{NN}}$. However, a monotonic rise of $K^+/\pi^+$ is observed when a hadronic initial state is assumed for all $\sqrt{s_{\mathrm {NN}}}$. Experimental values of $K^-/\pi^-$ are also reproduced within the ambit of the same formalism. Results from scenarios where the strange quarks and hadrons are formed in equilibrium and evolves with and without secondary productions have also been presented., Comment: Published in Phys. Rev. C 82 (2010)024914

In this paper the soft gluon radiation from partonic interaction of the type: $2 \to 2$ + gluon has been revisited and a correction term to the widely used Gunion-Bertsch (GB) formula is obtained., Comment: Few typos corrected, to appear in Phys. Rev. D (rapid communication)

In this work, we extract the QCD Equation of State (EoS) using experimental results of the $\phi$ meson produced in nuclear collisions at AGS, SPS and RHIC energies. The data are confronted to simple thermodynamic expectations and lattice results. The experimental data indicate a first order phase transition, with a mixed phase stretching energy density between $\sim$ 1 and 3.2 GeV/$fm^3$.

It has been shown that the thermal photon and the lepton pair spectra can be used to estimate the radial velocity of different phases of the matter formed in nuclear collisions at ultra-relativistic energies. We observe a non-monotonic variation of the flow velocity with invariant mass of the lepton pair which is indicative of two different sources of thermal dilepton sources at early and late stage of the dynamically evolving system. We also show that the study of radial velocity through electromagnetic probes may shed light on the nature of the phase transition from hadrons to QGP., 0ne figure added, minor modification in text

In this work a first attempt is made to extract the Equation of State (EoS) using experimental results of the $\phi$ meson produced in nuclear collisions at AGS, SPS and RHIC energies. The data are confronted to simple thermodynamic expectations and lattice results. The experimental data indicate a first order phase transition, with a mixed phase stretching energy density between $\sim$ 1 and 3.2 GeV/$fm^3$., Comment: 7 pages, 7 figures, Presented in SQM08. To appear in Journal of Physics G: Nucl. and Part. Phys

Theoretical calculation of transverse momentum($p_T$) distribution of thermal photons and dileptons originating from ultra-relativistic heavy ion collisions suffer from several uncertainties since the evaluation of these spectra needs various inputs which are not yet known unambiguously. In the present work the ratio of the $p_T$ spectra of thermal photons to lepton pairs has been evaluated and it is shown that the ratio is insensitive to some of these parameters., Comment: Figs replaced

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. G

Average energy loss of light quarks has been calculated in a two stage equilibrium scenario where the quarks are executing Brownian motion in a gluonic heat bath. The evolution of the quark $p_T$ spectra is studied by solving Fokker-Planck equation in an expanding plasma. Results are finally compared with experimentally measured pion $p_T$ spectrum at RHIC., 4 pages, 2 figures, typos corrected, Phys. Rev. C in press

Thermal photon emission rates due to meson-nucleon interactions have been evaluated. An exhaustive set of reactions involving $p(\overline{p}),$ $n(\mathrm{n\ifmmode \bar{}\else \={}\fi{}}),$ $\ensuremath{\rho},$ $\ensuremath{\omega},$ ${a}_{1},$ $\ensuremath{\pi},$ and $\ensuremath{\eta}$ is seen to provide a sizable contribution to the emission rate of photons from hot hadronic matter. Contributions from the decays of baryonic resonances are found to be negligibly small.

The variation of average transverse mass of identified hadrons with charge multiplicity have been studied for AGS, SPS and RHIC energies. The observation of a plateau in the average transverse mass for multiplicities corresponding to SPS energies is attributed to the formation of a co-existence phase of quark gluon plasma and hadrons. A subsequent rise for RHIC energies may indicate a deconfined phase in the initial state. Several possibilities which can affect the average transverse mass are discussed. Constraints on the initial temperature and thermalization time have been put from the various experimental data available at SPS energies., 4 pages and 2 figures, title changed and draft modified

We have studied the rapidity distribution of secondary hadrons produced in nucleus-nucleus collisions at ultra-relativistic energies within the ambit of the Landau's hydrodynamical model. A reasonable description of the data can also be obtained by using the Bjorken's hydrodynamical model if the boost invariance is restricted to a finite rapidity range. The sensitivity of the hadronic spectra on the equation of state vis- a -vis the velocity of sound has been discussed. The correlation between the velocity of sound and the freeze-out temperature has been indicated. The effects of the non-zero widths of various mesonic and baryonic degrees of freedom up to the mass value ~ 2.5 GeV is seen to be small., Comment: 9 pages and 11 figures. Major changes. To appear in Physical Review C

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., Total 17 pages with 18 figures

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 included

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., Comment: Talk given in the International Nuclear Physics Conference, at the University of California, Berkeley, USA, during July 30 - August 3, 2001

We estimate the photon and dilepton emission rates from hot hadronic matter with in-medium spectral shift and broadening of vector mesons. It is observed that both the WA98 photon data and CERES/NA45 dilepton data can be well reproduced with similar initial conditions. The freeze-out condition has been constrained by the transverse mass spectra of pions and protons measured by the NA49 collaboration. We argue that simultaneous measurement of the $p_T$ spectra of single photons as well as invariant mass distribution of dileptons is crucial to understand the in-medium spectral function of the vector mesons., Title and contents changed