Browsing by Author "Das, Arpan"

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Axion mass in a hot QCD plasma
(Sissa Medialab, 2023-03) Das, Arpan
We estimate axion mass in a QCD medium. To take into consideration the non-perturbative nature of the QCD we model the QCD medium using three (2+1) flavor Polyakov loop extended Nambu Jona Lasinio (PNJL) model. Axion is incorporated within the framework of the PNJL model through the Kobayashi-Maskawa-t'hooft determinant interaction. We argue that chiral transition and confinement-deconfinement transition affect the mass of axion in a QCD medium. Our results suggest that the in-medium mass of axion is correlated with the chiral and confinement-deconfinement transition. We compare our results with the Nambu Jona Lasinio (NJL) model results and Lattice QCD calculations.
Azimuthal anisotropy of dijet events in PbPb collisions at = 5.02 TeV
(Springer, 2023) Das, Arpan
The path-length dependent parton energy loss within the dense partonic medium created in lead-lead collisions at a nucleon-nucleon center-of-mass energy of = 5.02 TeV is studied by determining the azimuthal anisotropies for dijets with high transverse momentum. The data were collected by the CMS experiment in 2018 and correspond to an integrated luminosity of 1.69 nb−1. For events containing back-to-back jets, correlations in relative azimuthal angle and pseudorapidity (η) between jets and hadrons, and between two hadrons, are constructed. The anisotropies are expressed as the Fourier expansion coefficients vn, n = 2–4 of these azimuthal distributions. The dijet vn values are extracted from long-range (1.5 < |∆η| < 2.5) components of these correlations, which suppresses the background contributions from jet fragmentation processes. Positive dijet v2 values are observed which increase from central to more peripheral events, while the v3 and v4 values are consistent with zero within experimental uncertainties.
Boost invariant spin hydrodynamics within the first order in derivative expansion
(APS, 2022-11) Das, Arpan
Recent evidence of the spin polarization of weakly decaying Lambda hyperons has opened up a newpathway for investigating nontrivial vortical structures of strongly interacting matter generated in heavy-ion experiments [1–10]. Several approaches for describing relativistic hydrodynamics for spin-polarized fluids have been developed as a result of the successes of the relativistic dissipative hydrodynamic framework in heavy-ion phenomenological research [11,12]. Different frameworks have been constructed using entropy current analysis [13–19], relativistic kinetic theory [20–38], effective Lagrangian approach [39–42], quantum statistical density operators [43–47], equilibrium partition functions [48], and holography [49,50]. To allow for future dynamic simulations of spin polarization [51–53], a consistent framework of relativistic hydrodynamics with spin degrees of freedom (spin hydrodynamics) is currently being built. In this work, we present an analysis of boost-invariant solutions of the spin hydrodynamic equations formulated by Hattori et al. in Ref. [13] and investigated later in a series of publications [14,18,19,54,55]. The approach of Ref. [13] is based on the gradient expansion and requirement of positive entropy production.
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, Arpan
The 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.
Canonical and phenomenological formulations of spin hydrodynamics
(APS, 2023-08) Das, Arpan
Two formulations of relativistic hydrodynamics of particles with spin 1/2 are compared. The first approach, dubbed the canonical one, uses expressions for the energy-momentum and spin tensors that have properties that follow a direct application of Noether’s theorem, which yields a totally antisymmetric spin tensor. The other one is based on a simplified form of the spin tensor and is commonly used in the current literature under the name of a phenomenological approach. We show that these two frameworks are equivalent, i.e., they can be directly connected by a suitably defined pseudogauge transformation, only if the first framework is initially improved by a suitable modification of the energy-momentum tensor (addition of a divergence-free term that cannot be interpreted as a pseudogauge). Our analysis uses arguments related to the positivity of entropy production. The latter turns out to be equivalent for the improved canonical and phenomenological frameworks
Categorical-symmetry resolved entanglement in conformal field theory
(APS, 2024-05) Das, Arpan
We propose a symmetry resolution of entanglement for categorical noninvertible symmetries (CaT-SREE) in (1+1)-dimensional conformal field theories. The definition parallels that of grouplike invertible symmetries, employing the concept of symmetric boundary states with respect to a categorical symmetry. Our examination extends to rational conformal field theories, where the behavior of CaT-SREE mirrors that of grouplike invertible symmetries. We find that CaT-SREE can be defined if there is no obstruction to gauging the categorical symmetry, as happens in the case of grouplike symmetries. We also provide instances of the breakdown of entanglement equipartition at the next-to-leading order in the cutoff expansion. Our findings shed light on how the interplay between conformal boundary conditions and categorical symmetries lead to specific patterns in the entanglement entropy.
Chiral susceptibility in the Nambu–Jona-Lasinio model: a wigner function approach
(APS, 2019-11) Das, Arpan
We estimate here chiral susceptibility at finite temperature within the framework of the Nambu–Jona-Lasinio model (NJL) using the Wigner function approach. We also estimate it in the presence of chiral chemical potential (𝜇5 ) as well as a nonvanishing magnetic field (𝐵 ). We use a medium separation regularization scheme (MSS) in the precence of magnetic field to calculate the chiral condensate and corresponding susceptibility. It is observed that for a fixed value of chiral chemical potential (𝜇5 ), transition temperature increases with the magnetic field. While for the fixed value of the magnetic field, transition temperature decreases with chiral chemical potential. For a strong magnetic field, we observe nondegeneracy in susceptibility for up and down type quarks.
Confronting nuclear equation of state in the presence of dark matter using GW170817 observation in relativistic mean field theory approach
(APS, 2019-02) Das, Arpan
We confront the admixture of dark matter inside a neutron star using gravitational wave constraints coming from binary neutron star merger. We consider a relativistic mean field model including 𝜎 −𝜔 −𝜌 meson interaction with NL3 parametrization. We study fermionic dark matter interacting with nucleonic matter via Higgs portal mechanism. We show that admixture of dark matter inside the neutron star softens the equation state and lowers the value of tidal deformability. Gravitational wave GW170817 observation puts an upper bound on tidal deformability of a binary neutron star with low spin prior at 90% confidence level, which disfavors stiff equation of state such as the Walecka model with NL3 parametrization. However, we show that the Walecka model with NL3 parametrization with a fermionic dark matter component satisfies the tidal deformability bound coming from the GW170817 observation.
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, Arpan
Exact 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.
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, Arpan
We 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.
Correspondence between Israel–stewart theory and first-order causal and stable hydrodynamics for the boost-invariant flow
(Springer, 2022-10) Das, Arpan
In this proceeding, we discuss a mapping between the well-explored Israel–Stewart (IS) theory of dissipative relativistic hydrodynamics and the recently formulated causal and stable first-order hydrodynamics (FOCS), for Bjorken expanding systems with an ideal gas equation of state. Due to such correspondence, an analytical solution of the new first-order formulation can be determined.
Dark matter admixed neutron star properties in light of gravitational wave observations: a two fluid approach
(APS, 2022-06) Das, Arpan
We consider the effect of density dependent dark matter on the neutron star mass, radius, and tidal deformability. Nuclear matter (normal matter) as well as the fermionic dark matter sector is considered in a mean field model. We adopt the two fluid formalism to investigate the effect of dark matter on the neutron star properties. In the two fluid picture, there is no direct interaction between the dark matter and the nuclear matter. Rather these two sectors interact only through gravitational interaction. The nuclear matter sector is described by the 𝜎−𝜔−𝜌 meson interaction in the “FSU2R” parametrization. In the dark matter sector, we use the Bayesian parameter optimization technique to fix the unknown parameters in the dark matter equation of state. In the two fluid picture, we solve the coupled Tolman-Oppenheimer-Volkoff (TOV) equations to obtain the mass and radius of dark matter admixed neutron stars (DANSs). We also estimate the effect of the density dependent dark matter sector on the tidal deformability of dark matter admixed neutron stars (DANSs).
Diffusion matrix associated with conserved charges in relativistic heavy ion collisions
(Elsevier, 2025-12) Das, Arpan
Matter produced in heavy ion collisions has multiple conserved numbers like baryon number, strangeness and electric charge. The diffusion process of these charges can be described by a diffusion matrix describing the interdependence of diffusion of different charges. Incorporating the Landau-Lifshitz frame condition, we have estimated the diffusion coefficient matrix using Boltzmann kinetic theory for the hadronic phase within relaxation time approximation.
Diffusion matrix associated with the diffusion processes of multiple conserved charges in a hot and dense hadronic matter
(APS, 2022) Das, Arpan
Bulk matter produced in heavy ion collisions can have multiple conserved quantum numbers like baryon number, strangeness and electric charge. The diffusion process of these charges can be described by a diffusion matrix describing the interdependence of diffusion of different charges. The diffusion coefficient matrix is estimated here from the Boltzmann kinetic theory for the hadronic phase within relaxation time approximation. In the derivation for the same, we impose the Landau-Lifshitz conditions of fit. This leads to, e.g., the diagonal diffusion coefficients to be manifestly positive definite. The explicit calculations are performed within the ambit of hadron resonance gas model with and without excluded volume corrections. It is seen that the off-diagonal components can be significant to affect the charge diffusion in a fluid with multiple conserved charges. The excluded volume correction effects is seen to be not significant in the estimation of the elements of the diffusion matrix.
A dissipative spin hydrodynamic framework and Kubo formula: Spin chemical potential as the leading order term in the hydrodynamic gradient expansion
(Elsevier, 2025-12) Das, Arpan
Using the entropy current analysis, we develop a Naiver-Stokes theory of dissipative spin hydrodynamic framework. Here, we consider that the spin chemical potential () is a leading order term () in the hydrodynamic gradient expansion. For consistency of the framework, the energy–momentum tensor needs to be symmetric up to first order in the gradient expansion. The spin tensor is antisymmetric only in the last two indices. Moreover, by applying Zubarev’s non-equilibrium statistical operator method, we find the Kubo relations for various transport coefficients, including spin transport coefficients appearing in the spin hydrodynamic description.
Dynamics of hot QCD matter 2024 — bulk properties
(World Scientific, 2025) Das, Arpan
The second Hot QCD Matter 2024 conference at IIT Mandi focused on various ongoing topics in high-energy heavy-ion collisions, encompassing theoretical and experimental perspectives. This proceedings volume includes 19 contributions that collectively explore diverse aspects of the bulk properties of hot QCD matter. The topics encompass the dynamics of electromagnetic fields, transport properties, hadronic matter, spin hydrodynamics, and the role of conserved charges in high-energy environments. These studies significantly enhance our understanding of the complex dynamics of hot QCD matter, the quark–gluon plasma (QGP) formed in high-energy nuclear collisions. Advances in theoretical frameworks, including hydrodynamics, spin dynamics and fluctuation studies, aim to improve theoretical calculations and refine our knowledge of the thermodynamic properties of strongly interacting matter. Experimental efforts, such as those conducted by the ALICE and STAR collaborations, play a vital role in validating these theoretical predictions and deepening our insight into the QCD phase diagram, collectivity in small systems, and the early-stage behavior of strongly interacting matter. Combining theoretical models with experimental observations offers a comprehensive understanding of the extreme conditions encountered in relativistic heavy-ion and proton-proton collisions.
Dynamics of QCD matter — current status
(World Scientific, 2021) Das, Arpan
In 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.
Effects of magnetic field on plasma evolution in relativistic heavy-ion collisions
(APS, 2017-09) Das, Arpan
Very strong magnetic fields can arise in noncentral heavy-ion collisions at ultrarelativistic energies, and may not decay quickly in a conducting plasma. We carry out relativistic magnetohydrodynamics (RMHD) simulations to study the effects of this magnetic field on the evolution of the plasma and on resulting flow fluctuations in the ideal RMHD limit. Our results show that the magnetic field leads to enhancement in elliptic flow for small impact parameters while it suppresses it for large impact parameters (which may provide a signal for the initial stage magnetic field). Interestingly, we find that magnetic field in localized regions can temporarily increase in time as evolving plasma energy density fluctuations lead to reorganization of magnetic flux. This can have important effects on the chiral magnetic effect. The magnetic field has nontrivial effects on the power spectrum of flow fluctuations. For the very strong magnetic field case, one sees a pattern of even-odd difference in the power spectrum of flow coefficients arising from reflection symmetry about the magnetic field direction if initial state fluctuations are not dominant. We discuss the situation of nontrivial magnetic field configurations arising from collision of deformed nuclei and show that it can lead to anomalous elliptic flow. Special (crossed body-body) configurations of deformed nuclei collisions can lead to the presence of a quadrupolar magnetic field, which can have very important effects on the rapidity dependence of transverse expansion (similar to beam focusing from quadrupole fields in accelerators).
Effects of phase transition induced density fluctuations on pulsar dynamics
(Elsevier, 2015-07) Layek, Biswanath; Das, Arpan
We show that density fluctuations during phase transitions in pulsar cores may have non-trivial effects on pulsar timings, and may also possibly account for glitches and anti-glitches. These density fluctuations invariably lead to non-zero off-diagonal components of the moment of inertia, leading to transient wobbling of star. Thus, accurate measurements of pulsar timing and intensity modulations (from wobbling) may be used to identify the specific pattern of density fluctuations, hence the particular phase transition, occurring inside the pulsar core. Changes in quadrupole moment from rapidly evolving density fluctuations during the transition, with very short time scales, may provide a new source for gravitational waves.
Electrical conductivity and Hall conductivity of a hot and dense hadron gas in a magnetic field: a relaxation time approach
(APS, 2019-05) Das, Arpan
We estimate the electrical conductivity and the Hall conductivity of a hot and dense hadron gas using the relaxation time approximation for the solution of the Boltzmann transport equation in the presence of an electromagnetic field. We investigate the temperature and the baryon chemical potential dependence of these transport coefficients in the presence of a magnetic field. The explicit calculation is performed within the ambit of the hadron resonance gas model. In general, it is observed that the electrical conductivity decreases in the presence of a magnetic field. While at vanishing magnetic field the electrical conductivity decreases monotonically with temperature, in the presence of a magnetic field the same shows a nonmonotonic behavior with a peak. The Hall conductivity, on the other hand, shows a nonmonotonic behavior with respect to the dependence on a magnetic field as well as with temperature. We argue that for a pair plasma (particle-antiparticle plasma) where 𝜇𝐵 =0 , Hall conductivity vanishes. Only for a nonvanishing baryon chemical potential, Hall conductivity has a nonzero value. We also estimate the electrical conductivity and the Hall conductivity as a function of the center of mass energy along the freeze-out curve as may be relevant for relativistic heavy-ion collision experiments.