Department of Physics
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Item Thermoelectric effect and seebeck coefficient of hot and dense hadronic matter in the hadron resonance gas model(Springer, 2021-05) Das, ArpanWe study the thermoelectric effect of a baryon-rich plasma, with a temperature gradient, produced in heavy-ion collision experiments. We estimate the Seebeck coefficient for the hot and dense hadronic matter within the framework of relativistic kinetic theory using relaxation time approximation. For quantitative analysis, we use the hadron resonance gas model with hadrons and resonance states up to a cutoff in the mass as 2.25 GeV. The current produced due to temperature gradient can be a source of a magnetic field in heavy-ion collision experiments.Item Electrical conductivity and Hall conductivity of a hot and dense quark gluon plasma in a magnetic field: a quasiparticle approach(APS, 2020-02) Das, ArpanWe estimate here the electrical and Hall conductivity using a quasiparticle approach for quark matter. We use a Boltzmann kinetic approach in the presence of external magnetic field. We confront the results of model calculations with lattice QCD simulations for vanishing magnetic field. In general, electrical conductivity decreases with magnetic field. The Hall conductivity on the other hand can show a nonmonotonic behavior with magnetic field due to an intricate interplay of behavior of relaxation time and strength of the magnetic field. We argue for vanishing quark chemical potential Hall conductivity vanishes and quark gluon plasma with finite quark chemical potential can show Hall effect. Both electrical conductivity and Hall conductivity increase with increasing quark chemical potential.Item Hawking radiation from acoustic black holes in relativistic heavy ion collisions(Elsevier, 2021-06) Das, ArpanWe propose a new analogue model of gravity - the evolving quark gluon plasma (QGP) produced in relativistic heavy ion collisions. This quark gluon plasma is the “most inviscid” fluid known. Such low kinematic viscosity is believed to reflect strongly correlated nature for QGP in these experiments. Hence, it may provide a good example of a quantum fluid naturally suited to studies of acoustic Hawking radiation. Due to rapid longitudinal expansion, presence of a sonic horizon is also naturally guaranteed here, though, in general, this horizon is not static. Using Ultra relativistic quantum molecular dynamics (UrQMD) simulations, we show that, under certain conditions, the longitudinal velocity of the plasma, near the sonic horizon, can become time independent for a short span during the evolution of the system. During this period, we can have a conformally static acoustic metric with a (conformal) Killing horizon coinciding with the apparent horizon. An asymptotic observer will then see a thermal flux of phonons, constituting the Hawking radiation, coming from the horizon. For the relatively low energy collision considered here, where the resulting QCD system is governed by non-relativistic hydrodynamics, we estimate the Hawking temperature to be about 4-5 MeV (with the temperature of the QCD fluid being about 135 MeV). We discuss the experimental signatures of this Hawking radiation in terms of a thermal component in the rapidity dependence of the transverse momentum distribution of detected particles. We also discuss extension to ultra-relativistic case which should lead to a higher Hawking temperature, along with the effects of dynamical horizon leading to blue/red shift of the temperature.Item Magneto-Seebeck coefficient and Nernst coefficient of a hot and dense hadron gas(APS, 2020-07) Das, ArpanWe discuss the thermoelectric effect of hot and dense hadron gas within the framework of the hadron resonance gas model. Using the relativistic Boltzmann equation within the relaxation time approximation we estimate the Seebeck coefficient of the hot and dense hadronic medium with a gradient in temperature and baryon chemical potential. The hadronic medium in this calculation is modeled by the hadron resonance gas (HRG) model with hadrons and their resonances up to a mass cutoff Λ ∼2.6 GeV . We also extend the formalism of the thermoelectric effect for a nonvanishing magnetic field. The presence of magnetic field also leads to a Hall type thermoelectric coefficient (Nernst coefficient) for the hot and dense hadronic matter apart from a magneto-Seebeck coefficient. We find that generically in the presence of a magnetic field the Seebeck coefficient decreases while the Nernst coefficient increases with the magnetic field. At higher temperature and/or baryon chemical potential these coefficients approach to their values at vanishing magnetic field.Item Equivalence between first-order causal and stable hydrodynamics and Israel-Stewart theory for boost-invariant systems with a constant relaxation time(Elsevier, 2020-07) Das, ArpanWe show that the recently formulated causal and stable first-order hydrodynamics has the same dynamics as Israel-Stewart theory for boost-invariant, Bjorken expanding systems with an ideal gas equation of state and a regulating sector determined by a constant relaxation time. In this case, the general solution of the new first-order formulation can be determined analytically.Item Correspondence between Israel-Stewart and first-order casual and stable hydrodynamics for the boost-invariant massive case with zero baryon density(APS, 2020-08) Das, ArpanExact correspondence between Israel-Stewart theory and first-order causal and stable hydrodynamics is established for the boost-invariant massive case with zero baryon density and the same constant relaxation times used in the shear and bulk sectors. Explicit expressions for the temperature dependent regulators are given for the case of a relativistic massive gas. The stability and causality conditions known in the first-order approach are applied, and one finds that one of them is violated in this case.Item Quantum fluctuations of energy in subsystems of a hot relativistic gas(2020-12) Das, ArpanWe derive a formula that defines quantum fluctuations of energy in subsystems of a hot relativistic gas. For small subsystem sizes we find substantial increase of fluctuations compared to those known from standard thermodynamic considerations. However, if the size of the subsystem is sufficiently large, we reproduce the result for energy fluctuations in the canonical ensemble. Our results are subsequently used in the context of relativistic heavy-ion collisions to introduce limitations of the concepts such as classical energy density or fluid element. In the straightforward way, our formula can be applied in other fields of physics, wherever one deals with hot and relativistic matter.Item Correspondence between Israel-Stewart and first-order causal and stable hydrodynamics for Bjorken-expanding baryon-rich systems with vanishing particle masses(APS, 2021-01) Das, ArpanWe obtain an exact correspondence between the dynamical equations in Israel-Stewart (IS) theory and first-order causal and stable (FOCS) hydrodynamics for a boost-invariant system with an ideal gas equation of state at finite baryon chemical potential. Explicit expressions for the temperature and chemical potential dependence of the regulators in the FOCS theory are given in terms of the kinetic coefficients and constant relaxation time of the IS theory. Using the correspondence between the IS and FOCS theory, stability conditions for a charged fluid which are known in the FOCS approach are applied and one finds that the IS theory considered is unstable.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 In medium properties of an axion within a flavor Polyakov loop enhanced Nambu–Jona-Lasinio model(APS, 2021-04) Das, ArpanWe estimate in medium properties of axion i.e., its mass and self-coupling within a three flavor Polyakov loop extended Nambu–Jona-Lasinio (PNJL) model with Kobayashi-Maskawa-t’Hooft determinant interaction. We also estimate the topological susceptibility of strong interaction within the same model. It is observed that (statistical) confinement effects simulated by Polyakov loop potential play an important role in the estimation of all these quantities, particularly, near the critical temperature. Both the mass and the self-coupling of the axion get correlated with the chiral and deconfinement transition. The results for all these quantities obtained within the PNJL model is compared with chiral perturbation theory, Nambu–Jona-Lasinio (NJL) model and lattice QCD simulation results wherever available. The results for properties of axions at finite baryon densities are also presented.