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.

Abstract. 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.

The effects of gluon radiation by charm quarks on the transport coefficients 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.

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.Comment: One LaTeX file for the text and three eps files for figure

We study the effect of the magnetic field on the collisional energy loss of heavy quark (HQ) moving in a magnetized thermal partonic medium. This is investigated in the strong field approximation where the lowest Landau level (LLL) becomes relevant. We work in the limit g √ eB T √ eB which is relevant for heavy ion collisions. Effects of the magnetic field are incorporated through the resummed gluon propagator in which the dominant contribution arises from the quark loop. We also take the approximation √ eB M , M being the HQ mass, so that the HQ is not Landau quantized. It turns out that there are only two types of scatterings that contribute to the energy loss of HQ; the Coulomb scattering of HQ with light quarks/anti-quarks and the t-channel Compton scattering. It is observed that for a given magnetic field, the dominant contribution to the collisional energy loss arises from Compton scattering process i.e., Qg → Qg. On the other hand, of the two processes, the Coulomb scattering i.e., Qq → Qq is more sensitive to the magnetic field. The net collisional energy loss is seen to increase with increase in the magnetic field. For a reasonable strength of the magnetic field, the field dependent contribution to the collisional energy loss is of the same order as to the case without magnetic field which can be important for the jet quenching phenomena in the heavy ion collision experiments.

We demonstrate that spin flipping transitions occur between various quarkonium spin states due to transient magnetic field produced in non central heavy ion collisions (HICs). The inhomogeneous nature of the magnetic field results in non adiabatic evolution of (spin)states of quarkonia moving inside the transient magnetic environment. Our calculations explicitly show that the consideration of azimuthal inhomogeneity gives rise to dynamical mixing between different spin states owing to Majorana spin flipping. Notably, this effect of non-adiabaticity is novel and distinct from previously predicted mixing of the singlet and one of the triplet states of quarkonia in the presence of a static and homogeneous magnetic field.

We study the effects of the equation of state on the nuclear suppression of heavy flavours in quark gluon plasma and estimate the initial entropy density of the system produced at the highest RHIC energy. For this purpose we have used the experimental data on the charged particle multiplicity and the nuclear suppression of single electron spectra originating from the semi-leptonic decays of open charm and beauty mesons. 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 ∼ 20/fm 3 with corresponding initial temperature ∼ 210 MeV well above the value of the transition temperature predicted by lattice QCD. PACS numbers: 12.38.Mh,24.85.+p,25.75.Nq A thermalized system of quarks and gluons, called quark gluon plasma (QGP) is expected to be formed in the collisions of two nuclei at ultra-relativistic energies [1]. Rigorous experimental and theoretical efforts are on to create and characterize this novel, deconfined phase of quarks and gluons. Lattice QCD (LQCD) calculations indicate that at a temperature ∼ 175 MeV the entropy density (s) of the hadronic matter rises significantly due to the release of colour degrees of freedom which are confined within the hadrons at zero temperature. Therefore, it is of foremost importance to determine the value of the initial entropy density (s i ) / initial temperature (T i ) for the system formed in nuclear collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) and assess whether the system is formed in colour deconfined phase or not. The focus of the present study is to estimate s i or T i of the system formed at Au+Au collisions at √ s NN = 200 GeV. For this purpose we strict to take inputs from experimental data and LQCD calculations to minimize the model dependence of the outcome of the present analysis.One of the possible way to estimate the value of the initial entropy density is the extrapolation of the measured (final) observables backward in time through a suitable dynamical model. In absence of viscous loss the time reversal symmetry of the system is valid, therefore, the measured multiplicity at the freeze-out of the system can be used to estimate s i . The s i and the thermalization time (τ i ) are constrained by the measured (final) hadron multiplicity (dN/dy) by the following relation [2]:where A ⊥ is the transverse area of the system can be determined from the collision geometry and κ is a known constant (=3.7 for massless bosons like pions). The value of dN/dy which is connected to s i through Eq. 1 is readily available for different collision centralities [1]. In Eq. 1 there are two unknown quantities, τ i and s i both of which can not be determined from a single equation involving a single measured the dN/dy. Therefore, we choose another experimentally measur...

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