Department of Physics
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Item Comment on the paper "Energy Loss of Charm Quarks in the Quark-Gluon Plasma : Collisional vs Radiative"(ARXIV, 2007-08) Mishra, MadhukarIn the article by M. G. Mustafa published in Phys. Rev. C {\bf 72}, 014905 (2005) the author has estimated the total energy loss of a charm quark and quenching of hadron spectra due to the collisional energy loss of energetic partons in an expanding quark-gluon plasma employing Fokker-Planck equation. We wish to point out through this Comment that some of conceptual and numerical results of the said paper are unreliable. For the sake of clarity our discussion will focus on the massless case (although a few remarks on the m≠0 case are also made).Item J/ψsuppression at forward rapidity as a potential probe for QGP formation in colour screening scenario(ARXIV, 2008) Mishra, MadhukarIn order to study the properties of J/ψ (1S) in the deconfining medium, we extend our previous formalism [Phys. Lett. B {\bf 656}, 45 (2007)] on J/ψ suppression at mid-rapidity using the colour screening framework. Our formalism is more general as the complete rapidity, transverse momentum and centrality dependence including J/ψ suppression at forward as well as mid-rapidity can be computed directly from it. Careful attention is paid to the role of the medium's proper time in determining the locus of the screening region where J/ψ gets suppressed. Other important ingredients in the calculation are bag model equation of state for QGP, the longitudinal expansion of the QGP fluid obeying Bjorken's boost invariant scaling law and non-sequential/sequential melting of χc (1P) as well as ψ′ (2S) higher resonances. Upon comparison with the recent data of PHENIX collaboration on J/ψ suppression at forward and mid-rapidity regions, we find that our model shows a reasonable agreement with the data without incorporating any sequential decay mechanism of higher charmonia states. Furthermore, we observe a larger suppression at forward rapidity in our model which is again well supported by the PHENIX data and also gives a hint that a scenario based on directly produced J/ψ's is preferable.Item Quenching of Hadron Spectra in a chemically equilibrating Quark-Gluon Plasma(ARXIV, 2007-03) Mishra, MadhukarUsing the Fokker-Planck equation we have studied the drag co-efficient A(t) and the consequent shift Δp⊥(L) in the transverse momentum due to collisional energy loss of energetic partons while passing through a chemically equilibrating quark-gluon plasma. Based on these we estimate the quenching factor Q(p⊥) when the medium is undergoing longitudinal expansion governed by master rate equations. In contrast to the case of chemically equilibrated plasma investigated earlier by Mustafa and Thoma \cite{mus} we find less quenching because our calculated Q(p⊥) is always greater at all momenta. This result is attributed to the weak drag coefficient operating during initial state interactions.Item Temperature-dependent formation-time approach for Υ suppression at energies available at the CERN Large Hadron Collider(APS, 2015-03) Mishra, MadhukarWe present here a comprehensive model to describe the bottomonium suppression data obtained from the CERN Large Hadron Collider (LHC) at a center-of-mass energy of √sNN=2.76 TeV. We employ a quasiparticle model (QPM) equation of state for the quark-gluon plasma (QGP) expanding under Bjorken's scaling law. The current model includes the modification of the formation time based on the temperature of the QGP, color screening during bottomonium production, gluon-induced dissociation, and collisional damping due to the imaginary part of the potential between the b¯b pair. We propose a method for determining the temperature-dependent formation time of bottomonia using the solution of the time-independent Schrödinger equation and compare it with another approach based on time-dependent Schrödinger wave equation simulation. We find that these two independent methods based on different axioms give similar results for the formation time. Cold nuclear matter effects and feed-down from higher resonance states of Υ have also been included in the present work. The suppression of the bottomonium states at midrapidity is determined as a function of centrality. The results compare closely with the recent centrality-dependent suppression data at the energies available at the CERN LHC in the midrapidity region.Item Unified description of charmonium suppression in a quark-gluon plasma medium at RHIC and LHC energies(APS, 2015-09) Mishra, MadhukarRecent experimental and theoretical studies suggest that the quarkonium suppression in a thermal QCD medium created in heavy ion collisions is a complex interplay of various physical processes. In this article we put together most of these processes in a unified way to calculate the charmonium survival probability (nuclear modification factor) at energies available at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments. We include shadowing as the dominant cold-nuclear-matter effect. Further, gluonic dissociation and collision damping are included, which provide width to the spectral function of charmonia in a thermal medium and cause the dissociation of charmonium along with the usual color screening. We include color screening by using our recently proposed modified Chu–Matsui model. Furthermore, we incorporate the recombination of uncorrelated charm and anticharm quarks for the regeneration of charmonium over the entire temporal evolution of the QGP medium. Finally, we do a feed-down correction from the excited states to calculate the survival probability of charmonium. We find that our unified model suitably and simultaneously describes the experimental nuclear modification data of J/ψ at RHIC and LHC.Item Bottomonium suppression at √sNN=2.76 TeV using a model based on color screening and gluonic dissociation with collisional damping(APS, 2013-10) Mishra, MadhukarWe present a model to explain the bottomonium suppression in Pb+Pb collisions at midrapidity obtained from Large Hadron Collider (LHC) energy, √sNN=2.76 TeV. The model consists of two decoupled mechanisms, namely, color screening during bottomonium production followed by gluon induced dissociation along with collisional damping. The quasiparticle model (QPM) is used as equation of state (EOS) for the quark-gluon plasma (QGP) medium. The feed-down from higher Υ states, such as Υ(1P), Υ(2S), and Υ(2P), dilated formation times for bottomonium states, and viscous effect of the QGP medium are other ingredients included in the current formulation. We further assume that the QGP is expanding according to (1+1)-dimensional Bjorken's boost invariant scaling law. The net suppression (in terms of pT integrated survival probability) for bottomonium states at midrapidity is obtained as a function of centrality, and the result is then compared both quantitatively and qualitatively with the recent LHC experimental data in the midrapidity region recently published by the CMS Collaboration. We find that the current model, based on Debye color screening plus gluonic dissociation along with collisional damping, better describes the centrality dependence of bottomonium suppression at LHC energy as compared to the color screening model alone.Item Color screening scenario for quarkonia suppression in a quasiparticle model compared with data obtained from experiments at the CERN SPS, BNL RHIC, and CERN LHC(APS, 2013-03) Mishra, MadhukarWe present a modified color screening model for J/ψ suppression in the quark-gluon plasma (QGP) using the quasiparticle model (QPM) as the equation of state (EOS). Other theoretical ingredients incorporated in the model are feed-down from higher resonances, namely, χc, and ψ′, dilated formation time for quarkonia, and viscous effects of the QGP medium. By assuming further that the QGP is expanding with Bjorken's hydrodynamical expansion, the present model is used to analyze the centrality dependence of the J/ψ suppression in the mid-rapidity region and compare it with the data obtained from Super Proton Synchrotron, Relativistic Heavy Ion Collider, and Large Hadron Collider experiments. We find that the centrality dependence of the data for the survival probability at all energies is well reproduced by our model. We further compare our model predictions with the results obtained from the bag model EOS for QGP which has usually been used earlier in all such calculations