arXiv subjects

Sushant K. Singh

Publications and source records attributed to Sushant K. Singh.

17 recordsLinked to original sources

arXiv2026

Bayesian constraints on the transport coefficients $η/s$ and $ζ/s$ from spin polarization in relativisitic heavy-ion collisions

Bayesian analyses in the context of relativistic heavy-ion collisions have so far relied almost exclusively on bulk hadronic observables constructed from momentum degrees of freedom to constrain the transport properties of the quark-gluon plasma. In this work, we perform the Bayesian inference after incorporating the longitudinal spin polarization of $Λ$ hyperons alongside conventional bulk measurements in Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV to constrain the shear and bulk viscosity to entropy density ratios, $η/s$ and $ζ/s$. We demonstrate that the inclusion of spin polarization, which provides complementary sensitivity to the space-time structure and vorticity of the medium, shifts the posterior distribution of $ζ/s$ toward larger values, although current uncertainties do not allow a statistically significant separation at the 68% credibility level. Nevertheless, the results establish spin polarization as a valuable probe in quantitative studies of QGP transport properties and indicate that it should be incorporated in comprehensive and systematically constrained Bayesian extractions of the medium's dynamical parameters.

nucl-th↗

arXiv2025

Spin Polarization of $Λ$ hyperons from Dissipative Spin Hydrodynamics

We present a framework for spin dynamics in the quark-gluon plasma created in relativistic heavy-ion collisions. Under the approximation of small polarization, macroscopic spin degrees of freedom decouple from the background, and their evolution equations and transport coefficients have been computed using quantum kinetic theory of massive particles with nonlocal collisions. Employing this theory, we numerically solve dissipative relativistic spin hydrodynamics. We explore three interaction scenarios between constituent particles and apply this framework to compute both global and local spin polarization of $Λ$ hyperons in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. Our results show that the initially vanishing spin potential relaxes toward thermal vorticity, driving global polarization. Furthermore, we demonstrate that the sign of longitudinal polarization is sensitive to the interaction type, emphasizing the need for a consistent treatment of dissipative effects in spin hydrodynamics to describe experimental data.

hep-ph↗

arXiv2025

Spin dynamics with realistic hydrodynamic background for relativistic heavy-ion collisions

The equations of perfect spin hydrodynamics are solved for the first time using a realistic (3+1)-dimensional hydrodynamic background, calibrated to reproduce a comprehensive set of hadronic observables, including rapidity distributions, transverse momentum spectra, and elliptic flow coefficients for Au+Au collisions at the beam energy of $\sqrt{s_{\rm NN}} = 200$ GeV. The spin dynamics is governed by the conservation of the spin tensor, describing spin-$\frac{1}{2}$ particles, with particle mass in the spin tensor treated as an effective parameter. We investigate several scenarios, varying both the effective mass and the initial evolution time for the spin polarization tensor. The model predictions are then compared with experimental measurements of global and longitudinal spin polarization of Lambda hyperons. Our results indicate that a successful description of the data requires a delayed initial evolution time for the perfect spin hydrodynamics of about 4 fm/$c$ (in contrast to the standard initial time of 1 fm/$c$ used for the hydrodynamic background). This delay marks a transition from the phase where spin-orbit interaction is significant to the regime where spin-conserving processes dominate. Our findings suggest that the spin-orbit dissipative interaction plays a significant role only in the very early stages of the system's evolution.

hep-ph↗

arXiv2025

Hadron momentum spectra from analytical solutions of relativistic hydrodynamics

We present analytical solution of relativistic hydrodynamics for a system having cylindrical symmetry with boost-invariant longitudinal expansion and Hubble-like transverse expansion. We also consider analytical solution for Hubble-like spherically expanding system. For these two cases, we calculate analytical expression for transverse momentum spectra of hadrons, at constant temperature freeze-out hypersurface using Cooper-Frye prescription. We compare our results for transverse momentum spectra with experimental results from Large Hadron Collider and CERN SPS where one expects cylindrical and spherical geometry of the fireball, respectively. In the case of low-energy collisions with spherical geometry, we calculate rapidity spectra and compare with the results from CERN SPS.

hep-ph↗

arXiv2022

Suppression of thermal vorticity as an indicator of QCD critical point

We study the impact of the QCD critical point (CP) on the spin polarization of $Λ$-hyperon generated by the thermal vorticity in viscous quark gluon plasma (QGP). The equations of the relativistic causal viscous hydrodynamics have been solved numerically in (3+1) dimensions to evaluate the thermal vorticity. The effects of the CP have been incorporated through the equation of state (EoS) and the scaling behavior of the transport coefficients. A significant reduction in the global polarization has been found as the CP is approached. A drastic change induced by the CP in the rapidity dependence of the spin polarization is observed which can be used as a signature of the CP.

hep-ph↗

arXiv2022

Hyperon productions from Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV

We evaluate centrality dependence of hyperon, $Λ, Ξ, Ω$ and $K$ meson yields from Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV using rate equation and compare with experimental observations. An increase in the yield per participating nucleon is observed with centrality. Higher rate of multistrange productions compared to single-strange are also observed in case of central collisions. Using rate equation or momentum integrated Boltzmann equation we discuss the yield microscopically which are then normalised with thermal pions to get the yield ratio,($H_s+\bar{H_s}$)/($π^++π^-$) at various centralities. We have also evaluated the relative yield of anti hyperons per participant at various centralities and compared with observations made at RHIC. Increase in the rate of $Ξ$ yield compared to $Λ$ is realised in most central collisions. We find multi strange $Ξ,Ω$ freeze-out at a temperature very close to $T_c$ and $Λ, K$ do little later.

nucl-th↗

arXiv2022

The effects of the QCD critical point on the spectra and flow coefficients of hadrons

The space-time evolution of the hot and dense fireball of quarks and gluons produced in ultra-relativistic heavy-ion collisions at non-zero baryonic chemical potential and temperature has been studied by using relativistic viscous causal hydrodynamics. For this purpose a numerical code has been developed to solve the relativistic viscous causal hydrodynamics in (3+1)-dimensions with the inclusion of QCD critical point (CP) through the equation of state and scaling behaviour of the transport coefficients. We have evaluated the transverse momentum spectra, directed and elliptic flow coefficients of pions and protons to comprehend the effect of CP on these observables by using this code. It is found that the integration over the entire space-time history of the fireball largely obliterates the effects of CP on the spectra and flow coefficients.

nucl-th↗

arXiv2020

Chiral symmetry breaking and chemical equilibrium in a heavy-ion collisions

We examine the thermalization of an ensemble of the octet of pseudoscalar mesons, in the isospin symmetric limit, whose interactions are constrained through chiral symmetry, unitarity, and measurements. The reaction amplitudes generate all resonances up to masses of about 2 GeV, with twelve input parameters, namely f_pi, three masses, and eight low energy constants (LECs) of chiral perturbation theory. In linear response theory, we find that matter takes an extremely long time to thermalize. These long relaxation times are directly related to the fact that these mesons are pseudo-Goldstone bosons of chiral symmetry breaking. This result indicates that fireballs created with zero baryon number in heavy-ion collisions will drop out of chemical equilibrium once they enter the chiral symmetry broken phase.

hep-ph↗

arXiv2020

Study of freeze-out dynamics of strange hadrons

We study the chemical freeze-out dynamics of strange particles ($K,\, Λ,\, Σ$) from a homogeneous and isotropically expanding hadronic system of $π, K, ρ, N, Λ$ and $Σ$ with zero net baryon density. We use the momentum integrated Boltzmann equation and study their evolution over the bulk hadronic matter, a condition being similar to the one created at top RHIC and LHC energies. The cross-sections, which are input to the equations, are taken either from phenomenological models or parameterized by comparing against experimental data. From this microscopic calculation we find that these strange particles freeze-out near transition temperature $\approx T_c$ due to large relaxation time. The continuous cease of the inelastic processes due to gradual fall in the temperature and decrease in the number density, thus lead to early freeze out of strange hadrons $K, Λ$ and $Σ$ which happens sequentially near $T_c$. However, freeze-out of these strange species near Tc appears as a sudden and simultaneous process, which is mostly predicted by thermal model while explaining the yield of identified particles at RHIC and LHC energies.

nucl-th↗

arXiv2020

Microscopic study of strange hadron productions at the LHC with$\sqrt{s_{NN}}$=2.76 TeV

Ratio of the yield of strange hadrons to pions is considered as an important observable in studying the properties of the system produced in relativistic heavy ion collisions. Production of strange hadrons $K, \bar{K}, Λ, Σ, Ξ$ and $Ω$ have been evaluated microscopically using rate equations by considering their hadronic interaction cross sections in an expanding medium. The yields obtained from rate equations are normalized with thermal pions and compared with the measurements from Pb-Pb collisions at various multiplicities at LHC energy. The calculation has been done for various initial and freeze out conditions. At 2760 GeV, LHC energy, $Λ, Ξ, Ω$ freeze out close to $T_C$ and $K^0_s$ freezes out little later. But there is a subtle difference in freeze out temperatures of different species which may be distinguishable at lower colliding energies.

nucl-th↗

arXiv2020

HTL effective action of topologically massive gluons in 3+1 dimensions

We construct an effective action for "soft" gluons by integrating out hard thermal modes of topologically massive vector bosons at one loop order. The loop carrying hard gluons (momentum $\sim T$) are known as hard thermal loop (HTL). The gluons are massive in the non-Abelian topologically massive model (TMM) due to a quadratic coupling $B\wedge F$ where a 2-form field $B$ is coupled quadratically with the field strength $F$ of Yang-Mills (YM) field. The mass of the gluons plays an important role in the perturbative analysis of thermal field theory. Due to the presence of this infrared cut-off in the model, the color diffusion constant and conductivity can be analyzed in perturbative regime.

hep-th↗

arXiv2019

Cascade and Omega productions from Pb+Pb Collisions at LHC energy

Production of multi strange hadrons like cascade ($Ξ$) and omega($Ω$) baryons are studied microscopically using rate equation at $\sqrt{s_{NN}}$=2.76 TeV, Large Hadron Collider(LHC) energy. The rate equations for $Ξ$, $Ω$ are solved simultaneously with other strange hadrons in an expanding medium. The results for $Ξ$ and $Ω$ are compared with the data obtained from Pb-Pb collisions at $\sqrt{s_{NN}}$= 2.76 TeV from ALICE experiments. The ratio of yields of $Ξ$ and $Ω$ to $π$ are analysed with various initial conditions and compared with the experimental observations made for various charge particle multiplicities.

nucl-th↗

arXiv2018

Effect of initial-state nucleon shadowing on the elliptic flow of thermal photons

Recently the effect of nucleon shadowing on the Monte-Carlo Glauber initial condition was studied and its role on the centrality dependence of elliptic flow ($v_2$) and fluctuations in initial eccentricity for different colliding nuclei were explored. It was found that the results with shadowing effects are closer to the QCD based dynamical model as well as to the experimental data. Inspired by this outcome, in this work we study the transverse momentum ($p_T$) spectra and elliptic flow of thermal photons for Au+Au collisions at RHIC and Pb+Pb collisions at LHC by incorporating the shadowing effects in deducing the initial energy density profile required to solve the relativistic hydrodynamical equations. We find that the thermal photon spectra remain almost unaltered, however, the elliptic flow of photon is found to be enhanced significantly due to shadowing effects.

nucl-th↗

arXiv2018

Calculation of Power Spectrum in the Little Bangs

The power spectrum of fluctuations in the momentum distributions of particles have been estimated with optical Glauber and Monte-Carlo Glauber initial conditions for relativistic heavy ion collisions. The evaluation procedure adopted in this work is analogous to the one used in the calculation of power spectrum in Cosmic Microwave Background Radiation (CMBR). The power spectrum due to perturbations in the phase space distribution of the particles has also been evaluated. The perturbation in phase space has been evolved through the Boltzmann transport equation in an expanding quark gluon plasma (QGP) background. The expansion of the QGP has been treated within the purview of (3+1) dimensional relativistic hydrodynamics. We observe that the non-equlibrium effects introduced as perturbations in the phase space distributions can be traced from the enhancement of the power spectrum as well as through its variation with temperature which is distinctly different from the case of vanishing perturbation. A relation has been derived between the power spectrum and the flow harmonics.

nucl-th↗

arXiv2016

Initial conditions from the shadowed Glauber model

The two component Monte-Carlo Glauber model predicts a knee-like structure in the centrality dependence of elliptic flow $v_2$ in Uranium+Uranium collisions at $\sqrt{s_{NN}}=193$ GeV. It also produces a strong anti-correlation between $v_2$ and $dN_{ch}/dy$ in the case of top ZDC events. However, none of these features have been observed in data. We address these discrepancies by including the effect of nucleon shadowing to the two component Monte-Carlo Glauber model. Apart from addressing successfully the above issues, we find that the nucleon shadow suppresses the event by event fluctuation of various quantities, e.g. $\varepsilon_2$ which is in accordance with expectation from the dynamical models of initial condition based on gluon saturation physics.

nucl-th↗

arXiv2016

Initial conditions from the shadowed Glauber model for Pb+Pb at $\sqrt{s_{\rm NN}}=2.76$ TeV

We study the initial conditions for Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV using the two component Monte-Carlo Glauber model with shadowing of the nucleons in the interior by the leading ones. The model parameters are fixed by comparing to the multiplicity data of p+Pb and Pb+Pb at $\sqrt{s_{\rm NN}}=5.02$ and $2.76$ TeV respectively. We then compute the centrality dependence of the eccentricities upto the fourth order as well as their event by event distributions. The inclusion of shadowing brings the Monte-Carlo Glauber model predictions in agreement with data as well as with results from other dynamical models of initial conditions based on gluon saturation at high energy nuclear collisions. Further, we find that the shadowed Glauber model provides the desired relative magnitude between the ellipticity and triangularity of the initial energy distribution required to explain the data on the even and odd flow harmonics $v_2$ and $v_3$ respectively at the LHC.

nucl-th↗

arXiv2015

Effect of random field disorder on the first order transition in $p$-spin interaction model

We study the random field $p$-spin model with Ising spins on a fully connected graph using the theory of large deviations in this paper. This is a good model to study the effect of quenched random field on systems which have a sharp first order transition in the pure state. For $p=2$, the phase-diagram of the model, for bimodal distribution of the random field, has been well studied and is known to undergo a continuous transition for lower values of the random field ($h$) and a first order transition beyond a threshold, $h_{tp}(\approx 0.439)$. We find the phase diagram of the model, for all $p \ge 2$, with bimodal random field distribution, using large deviation techniques. We also look at the fluctuations in the system by calculating the magnetic susceptibility. For $p=2$, beyond the tri-critical point in the regime of first order transition, we find that for $h_{tp} h_o=1/p!$), the system does not show ferromagnetic order even at zero temperature. We find that the magnetic susceptibility for $p \ge 3$ is discontinuous at the transition point for $h<h_o$.

cond-mat.stat-mech↗