Department of Physics
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Item Dynamics of QCD matter — current status(World Scientific, 2021) Das, ArpanIn this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. More specifically, Sec. 2 discusses mass of π0 in the linear sigma model coupled to quarks at zero temperature. In Sec. 7, one-loop calculation of the anisotropic pressure are discussed in the presence of strong magnetic field. In Sec. 9, chiral transition and chiral susceptibility in the NJL model is discussed for a chirally imbalanced plasma in the presence of magnetic field using a Wigner function approach. Sections 10 discusses electrical conductivity and Hall conductivity of hot and dense hadron gas within Boltzmann approach and Sec. 11 deals with electrical resistivity of quark matter in presence of magnetic field. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12 and 15–17 of this article probe some aspects of this exciting field. In Sec. 12, analytical solutions of viscous Landau hydrodynamics in 1+1D are discussed. Section 15 deals with derivation of hydrodynamics from effective covariant kinetic theory. Sections 16 and 17 discuss hydrodynamics with spin and analytical hydrodynamic attractors, respectively. Transport coefficients together with their temperature- and density-dependence are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deal with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark–gluon plasma using Higgs–quark interaction and calculate the Higgs decays into quark and anti-quark, which shows a dominant on-shell contribution in the bottom-quark channel. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model to study hadron production from a dissipative quark–gluon plasma.Item Quarkonia suppression in small systems with a nonadiabatic evolution(APS, 2024-07) Das, ArpanIn high multiplicity proton-proton (𝑝−𝑝) collisions, the formation of a deconfined state of quarks and gluons akin to heavy ion collisions (HIC) has been a subject of significant interest. In proton-proton (𝑝 −𝑝) collisions, the transverse size of the system is comparable to the longitudinal (Lorentz contracted) dimension, unlike the case in a nucleus-nucleus (𝐴 −𝐴) collision, leading to a hitherto unexplored effect of rapid decrease of temperature of the medium on quark-antiquark bound states. Starting with a bottom-up thermalization framework for preequilibrium stage, we model the hydrodynamic expansion of the resulting fireball based on the Gubser flow with both inviscid theory and viscous corrections up to the third order. We find that the temperature evolution in small systems is rather fast even with viscous evolution equations for energy density, thereby introducing sudden changes in the time evolution of the Hamiltonian. This scenario prompts the consideration of nonadiabatic evolution, justifying the need in the present case to go beyond the traditional adiabatic framework. We demonstrate that nonadiabatic evolution may suppress the dissociation of 𝐽/Ψ substantially in 𝑝 −𝑝 collisions, even at higher multiplicities, offering new insights into the dynamics of strongly interacting matter produced in smaller collision systems.Item Bottom quark energy loss and hadronization with B^+ and B^0_\mathrm{s} nuclear modification factors using pp and PbPb collisions at \sqrt{s_\mathrm{NN}} = 5.02 TeV(2025) Das, ArpanThe production cross sections of B and B mesons are reported in proton-proton (pp) collisions recorded by the CMS experiment at the CERN LHC with a center-of-mass energy of 5.02 TeV. The data sample corresponds to an integrated luminosity of 302 pb . The cross sections are based on measurements of the B J/ (1020) (K K ) and B J/ K decay channels. Results are presented in the transverse momentum ( ) range 7-50 GeV/ and the rapidity interval 2.4 for the B mesons. The measured -differential cross sections of B and B in pp collisions are well described by fixed-order plus next-to-leading logarithm perturbative quantum chromodynamics calculations. Using previous PbPb collision measurements at the same nucleon-nucleon center-of-mass energy, the nuclear modification factors, , of the B mesons are determined. For 10 GeV/ , both mesons are found to be suppressed in PbPb collisions (with values significantly below unity), with less suppression observed for the B mesons. In this range, the values for the B mesons are consistent with those for inclusive charged hadrons and D mesons. Below 10 GeV/ , both B and B are found to be less suppressed than either inclusive charged hadrons or D mesons, with the B value consistent with unity. The values found for the B and B are compared to theoretical calculations, providing constraints on the mechanism of bottom quark energy loss and hadronization in the quark-gluon plasma, the hot and dense matter created in ultrarelativistic heavy ion collisions.Item Search for jet quenching with dijets from high-multiplicity pPb collisions at = 8.16 TeV(Springer, 2025-07) Das, ArpanThe first measurement of the dijet transverse momentum balance xj in proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of = 8.16 TeV is presented. The xj observable, defined as the ratio of the subleading over leading jet transverse momentum in a dijet pair, is used to search for jet quenching effects. The data, corresponding to an integrated luminosity of 174.6 nb−1, were collected with the CMS detector in 2016. The xj distributions and their average values are studied as functions of the charged-particle multiplicity of the events and for various dijet rapidity selections. The latter enables probing hard scattering of partons carrying distinct nucleon momentum fractions x in the proton- and lead-going directions. The former, aided by the high-multiplicity triggers, allows probing for potential jet quenching effects in high-multiplicity events (with up to 400 charged particles), for which collective phenomena consistent with quark-gluon plasma (QGP) droplet formation were previously observed. The ratios of xj distributions for high- to low-multiplicity events are used to quantify the possible medium effects. These ratios are consistent with simulations of the hard-scattering process that do not include QGP production. These measurements set an upper limit on medium-induced energy loss of the subleading jet of 1.26% of its transverse momentum at the 90% confidence level in high multiplicity pPb events.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.