arXiv subjects

Sandeep Chatterjee

Publications and source records attributed to Sandeep Chatterjee.

At least 19 recordsLinked to original sources

arXiv2026

Heavy flavour production in the presence of a weak magnetic field

We calculate an analytic expression for the cross section, up to order $(eB)^2$, for heavy quark ($Q$) production of mass $m_Q$ through the gluon ($g$) fusion process $g+g \to Q+\bar{Q}$ in the presence of a constant weak external magnetic field $B$ with strength $eB \ll m_Q^2$. This is done using the weak-field expansion of the fermion propagator, calculated with Schwinger's proper-time formalism. Owing to the hierarchy of scales ($m_Q^2 \gg eB$), we neglect the effects of the magnetic field on the quark and antiquark spinors in the external lines. Since the magnetic field breaks rotational invariance, the cross section becomes anisotropic and depends on the orientation angle $θ$ between the magnetic field and the incoming gluons in the center of mass frame. The lowest-order magnetic correction to the vacuum cross section scales as $(eB)^2$ and has $\cos 2θ$ and $\cos 4θ$ dependence, reflecting the breaking of rotational symmetry by the magnetic field. The cross section is maximized when the gluons propagate along the field direction ($θ= 0, π$) and minimized in the perpendicular configuration ($θ= π/2$). The resulting asymmetry peaks near the production threshold and falls with increasing center of mass energy, scaling as $(eB)^2/m_Q^4$ with the quark mass. These results provide direct, calculable signatures of the initial-stage magnetic field in relativistic heavy-ion collisions, which can in principle be imprinted on heavy-flavour observables measured in the final state.

hep-ph↗

arXiv2026

Baryon Number Susceptibilities of Hot and Dense Nuclear Matter in Relativistic Mean-Field Models

We develop a fully analytical framework for calculating baryon number susceptibilities in relativistic mean-field (RMF) models of hot and dense nuclear matter at finite temperature and baryon chemical potential. The formalism is applied to representative nonlinear RMF parameterization sets to investigate the model dependence of baryon number fluctuations over a wide range of temperatures and baryon chemical potentials. All the considered RMF parameterizations reproduce the overall qualitative beam-energy dependence of the cumulant ratios. The observed skewness and kurtosis within the RMF models satisfy the experimental values. Specifically, the FSU-family parameterizations exhibit relatively small deviations from the measured heavy-ion observables i.e. cumulant ratios.

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arXiv2026

Superradiant and dynamical spin-down of neutron stars with gravitational wave implications

Neutron stars such as pulsars and magnetars lose angular momentum primarily through electromagnetic dipole radiation, gravitational waves, $r$-mode oscillation. However, spin variations, particularly enhanced spin-down rates, indicate additional spin-down mechanisms. We propose superradiant spin-down as a potential faster spin-down process. By modelling the interplay between conventional and superradiant spin-down channels, we evaluate their impact on neutron star rotational evolution. We also discuss gravitational-wave emission produced by quadrupole deformation, $r$-mode oscillations, and axion-induced bosonic clouds around an isolated neutron star, highlighting their potential as distinct multi-messenger probes in upcoming detectors.

astro-ph.HE↗

arXiv2026

First flow harmonic of net baryon from directed diffusion of stopped baryons

We propose a new ansatz for baryon deposition that incorporates salient features from the baryon junction picture. The deposited baryon is coupled to a tilted fireball and deployed to study the directed flow of identified hadrons in Au+Au collisions at $\sqrt{s_{NN}}=7.7-200$ GeV. We find a parameter space in the model where the resulting hydrodynamic evolution generates a suitable flow to describe the directed flow $v_1$ of identified hadrons including the observed double sign change between $\sqrt{s_{NN}}=7.7-39$ GeV of the rapidity slopes of the net proton and the net lambda $v_1$. Our model that includes stopping of baryons and their consequent diffusion within a relativistic hydrodynamic framework along with a crossover equation of state as obtained from lattice QCD calculations provides a noncritical baryonic baseline that is crucial in our ongoing searches for the QCD critical point and the initial strong electromagnetic field.

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arXiv2026

Dipolar flow of identified hadrons at mid-rapidity using transport models

We report a transport model study of the rapidity even component of dipolar flow, $v_{1}^{\mathrm{even}}$, for identified charged hadrons at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}} = 7.7-200$ GeV. The analysis is performed using the AMPT model. In addition, HIJING model is used to quantify residual non-flow contributions in $v_{1}^{\mathrm{even}}$. The $v_{1}^{\mathrm{even}}$ of identified hadrons ($π$, $K$, and $p$) shows no significant difference between particles and anti-particles at $\sqrt{s_{NN}} = 200$ GeV. However, a clear splitting between proton and anti-proton $v_{1}^{\mathrm{even}}$ develops with decreasing beam energy, while no corresponding difference is observed for mesons ($π^{\pm}$ and $K^{\pm}$). These results indicate that the proton-antiproton difference in $v_{1}^{\mathrm{even}}$ is sensitive to baryon transport and antibaryon annihilation in the hadronic medium. Our study highlights the potential of identified-particle $v_{1}^{\mathrm{even}}$ measurements at RHIC Beam Energy Scan energies as a novel probe of baryon stopping and the evolution of the hadronic medium.

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arXiv2026

Rapidity-even directed flow splitting of protons and antiprotons as a probe of baryon stopping in relativistic heavy-ion collisions

We compare the rapidity-even directed flow $v_1^{\rm even}$ in Au+Au collisions at Beam Energy Scan (BES) energies for baryons and anti-baryons within a (3+1)-dimensional viscous relativistic hydrodynamics coupled to hadronic transport framework. The double-junction baryon stopping picture motivates a rapidity-even component in the baryon deposition in the initial state. We demonstrate that the split in the $v_1^{\rm even}$ of protons and anti-protons is sensitive to the rapidity extension of the baryon deposition that we associate with the double junction baryon stopping. Particularly, we find that the mid-rapidity curvature $\frac{d^2 Δv_1^{\rm even} (p-\bar{p})}{dy^2}\vert_{y=0}$ is a robust discriminator of the initial state baryon rapidity profiles. A simultaneous measurement of $Δv_1^{\rm even}$ and its curvature at mid-rapidity could constrain both the baryon diffusion strength and the baryon stopping profile, providing access to the physics of baryon stopping in relativistic heavy ion collisions.

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arXiv2026

Diffusion of multiple conserved charges from entropy production

We derive dissipative relativistic hydrodynamic equations in the presence of multiple conserved charges, i.e., baryon number ($B$), electric charge ($Q$), and strangeness ($S$), using the Chapman-Enskog (CE) method within the kinetic theory approach. The relativistic Boltzmann equation is solved within the relaxation-time approximation with a momentum-independent relaxation time in the collision term. We derive both first-order (Navier-Stokes limit) and second-order dissipative hydrodynamic equations. Within the kinetic theory framework, using the Boltzmann's H-theorem, and by demanding that for a dissipative system, the entropy must be produced, we find different transport coefficients at the first-order and second-order gradient expansion of the out-of-equilibrium distribution function around the local equilibrium. Apart from the well-known transport coefficients, the shear ($η$) and the bulk ($ζ$) viscosities , we also find the diffusion matrix elements ($κ_{qq^{\prime}}$) for the conserved charges $B$, $Q$ and $S$. The diffusion matrix elements ($κ_{qq^{\prime}}$) are important to model the multi-component diffusion dynamics sourced by inhomogeneous baryon stopping in the initial state of heavy-ion collisions. We estimate the temperature ($T$) and chemical potential dependence of diagonal and off-diagonal elements of the diffusion matrix elements for the (2+1) flavor quark-gluon plasma. We further estimate the ratio $κ_{qq^{\prime}}T/η$ for a wide range of temperature and chemical potentials to show the relative importance of the diffusion matrix elements compared to other transport coefficients.

hep-ph↗

arXiv2026

Disentangling Flow Contributions from the Chiral Magnetic Effect in U+U Collisions with Forward-Backward Multiplicity Asymmetry

The observation of the Chiral Magnetic Effect (CME) in heavy-ion collisions remains challenging because of large flow-induced backgrounds and experimental constraints. We demonstrate that the forward-backward multiplicity asymmetry (FBMA) provides a robust and experimentally accessible control parameter to separate the flow background from CME signal in the collisions of deformed nuclei, such as prolate uranium where FBMA is naturally enhanced and correlated with the initial-state geometry. Monte Carlo Glauber simulations indicate that varying FBMA within a fixed centrality class modulates ellipticity largely independently of the magnetic-field correlator, establishing FBMA as a practical tool for disentangling CME signals from flow driven background.

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arXiv2026

Probing Nuclear Geometry through Multi-Particle Azimuthal Correlations and Rapidity-Even Dipolar Flow in ${}^{16}$O+${}^{16}$O Collisions

We study symmetric and asymmetric cumulants as well as rapidity-even dipolar flow in ${}^{16}$O+${}^{16}$O collisions at $\sqrt{s_{NN}} = 200$~GeV to explore $α$-clustering phenomena in light nuclei within the viscous relativistic hydrodynamics framework. Signatures of $α$-clustering manifest in the anisotropic flow coefficients and their correlations -- particularly in observables involving elliptic-triangular flow correlations. We show that final-state symmetric and asymmetric cumulants -- especially $\mathrm{NSC}(2,3)$ and $\mathrm{NAC}_{2,1}(2,3)$ -- are sensitive to the initial nuclear geometry. Additionally, we observe a significant difference in rapidity-even dipolar flow, $v_1^{\text{even}}$, between $α$-clustered and Woods--Saxon configurations in high-multiplicity events. These findings underscore the pivotal role of nuclear structure in heavy-ion collision dynamics and provide observables for distinguishing nuclear geometries, particularly in ultra-central collisions.

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arXiv2025

Flow harmonic correlations via multi-particle symmetric and asymmetric cumulants in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

We study multi-particle azimuthal correlations in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We use initial conditions obtained from a Monte-Carlo Glauber model and evolve them within a viscous relativistic hydrodynamics framework that eventually gives way to a transport model in the late hadronic stage of the evolution. We compute the multi-particle symmetric and asymmetric cumulants and present the results for their sensitivity to the shear and bulk viscosities during the hydrodynamic evolution. We also check their sensitivity to resonance decay and hadronic interactions. We demonstrate that while some of these observables are more sensitive to transport properties than traditional flow observables, others are less sensitive, making them suitable for studying different stages of the evolution.

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arXiv2025

Charge dependent directed flow splitting from baryon inhomogeneity and electromagnetic field

This work aims to understand the recent experimental data from the STAR collaboration on the system size dependence of directed flow splitting between oppositely charged hadrons [arXiv:2412.18326]. Previously, we have studied the role of baryon inhomogeneity on charge dependent directed flow. We now incorporate the effects of the electromagnetic (EM) field albeit perturbatively, as implemented in Ref. [arXiv:1806.05288]. This enables us to compare the relative contributions between baryon inhomogeneity and EM field on charge dependent directed flow. Our model calculation describes the experimental data on the centrality and system size dependence of the mid-rapidity directed flow slope splitting, $Δdv_1/dy$, between protons and anti-protons. Our results indicate that in central collisions, where the EM field strength is negligible, the inclusion of EM field effects does not influence the splitting between protons and anti-protons. This suggests that the observed system size dependence of $Δdv_1/dy (p-\bar{p})$ in central collisions arises solely from enhanced baryon stopping in larger collision systems. However, in semi-central and peripheral collisions, both baryon diffusion and EM field effects contribute to the splitting. Furthermore, the centrality dependence of $Δdv_1/dy (p-\bar{p})$ is highly sensitive to the electrical conductivity of the medium, making it a potential probe for extracting this transport coefficient in the QCD medium through model-to-data comparisons. However, achieving this requires a precise determination of the background baseline originating from baryon diffusion. Additionally, further investigation is needed to understand $Δdv_1/dy$ for oppositely charged kaons and pions, particularly by incorporating the diffusion of other conserved charges.

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arXiv2024

Charm balance function in relativistic heavy-ion collisions

We calculate the balance function for charm in relativistic heavy-ion collisions. The distribution of pairs of charm-anticharm quarks produced in hard processes in the early stages of the nucleus-nucleus collision evolves in the dense fireball formed in the collision. The evolution of the dense matter is described using a relativistic viscous hydrodynamic model and the quark diffusion with a Langevin equation. The evolution of the charm quark balance function from the formation of the charm-anticharm pair up to the freeze-out traces the partial thermalization of the heavy quarks in the dense matter. For the balance function in azimuthal angle we reproduce the collimation effect due to the transverse flow. The evolution in rapidity shows the thermalization of the longitudinal velocity of the quark in the fluid. We provide predictions for the one and two-dimensional balance functions for $D^0$-$\bar{D^0}$ mesons produced in ultarelativistic Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. The shape of the charm balance function in relative rapidity is sensitive to the rescattering of heavy quarks in the early stages of the collision, while the shape of the balance function in azimuthal angle is sensitive to the rescattering in the latter stages.

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arXiv2023

Effect of hadronic interaction on the flow of $K^{*0}$

We explore the implications of the late stage hadronic rescattering phase on the flow of $K^{*0}$. The model calculations are done using a (3+1)-dimensional hybrid framework, incorporating both hydrodynamic evolution and hadronic transport that is calibrated to agree with bulk observables including the elusive rapidity differential $v_1$ of light-flavor hadrons. We find that the late stage hadronic rescattering phase causes significant qualitative modification of the $K^{*0}$ $v_1$ resulting in $\frac{dv_1}{dy}(K^{*0})-\frac{dv_1}{dy}(K^{+})$ and $\frac{dv_1}{dy}(ϕ)-\frac{dv_1}{dy}(K^{+})$ to have opposite signs with the effect being more pronounced in central collisions as compared to peripheral ones due to the larger multiplicity as well as longer duration of the hadronic phase. Further, this effect is enhanced in low-energy collisions owing to a stronger breaking of boost invariance. On the contrary, the influence of the hadronic phase on the $K^{*0}$ elliptic flow $v_2$ is found to be less significant and quantitative.

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arXiv2023

Baryon diffusion coefficient of the strongly interacting medium

We propose that the transverse momentum ($p_T$) differential splitting of directed flow ($Δv_1$) between proton and anti-proton can serve as a sensitive observable to extract the baryon diffusion coefficient ($κ_B$) of the hot and dense strongly interacting matter produced in relativistic heavy ion collisions. We use relativistic dissipative hydrodynamics framework with Glauber model based initial condition for the energy as well as baryon deposition that is calibrated to capture the rapidity dependence of charged particle multiplicity, net proton yield as well as the elusive $v_1$ splitting between proton and anti-proton. We employ the commonly used kinetic theory motivated ansatz: $κ_B= C_B \frac{n_B}{T} \left( \frac{1}{3} \text{coth}\left(\frac{μ_B}{T} \right) - \frac{n_BT}{ε+P} \right)$ where $n_B$, $ε$, $P$, $T$ and $μ_B$ are baryon number density, energy density, pressure, temperature and baryon chemical potential respectively while $C_B$ is an arbitrary constant which is largely unknown for the Quantum Chromodynamics (QCD) medium. We find that the variation of $Δv_1$ with $p_T$ is strongly influenced by the choice of $C_B$. Further, we find that the recent STAR measurement of the centrality dependence of the rapidity slope of $Δv_1$ prefers $0.5<C_B<1.5$.

nucl-th↗

arXiv2023

Baryon inhomogeneities driven charge dependent directed flow in heavy ion collisions

Electromagnetic field in heavy ion collisions are expected to cause charge dependent directed flow splitting ($Δv_1$). Such charge dependent $Δv_1$ has been observed by the STAR collaboration. We demonstrate that relativistic dissipative fluid dynamic simulations with baryon diffusion that include realistic model of baryon stopping in the initial condition and no contribution from electromagnetic field describe the measured $Δv_1$ for observables involving baryons and anti-baryons. This suggests strong background contribution from baryon current as a response to initial state baryon inhomogeneities to such charge dependent $Δv_1$ involving baryons and anti-baryons. Our current model calculations that only account for the evolution of the baryon charge and not electric charge and strangeness miss the observed $Δv_1$ of mesons leaving their interpretation open.

nucl-th↗

arXiv2022

Directed flow of light flavor hadrons for Au+Au collisions at $\sqrt{S_{NN}}=$ 7.7-200 GeV

We have studied the directed flow of light-flavor hadrons for Au + Au collisions at $\sqrt{S_{NN}}=$ 7.7 - 200 GeV. The initial condition is taken from a suitable Glauber model which is further evolved within the framework of relativistic hydrodynamics. Model calculations of the rapidity-odd directed flow ($v_1$) of identified light-flavor hadrons are compared with the available experimental data after suitably calibrating the initial condition to describe the rapidity dependence of charged particle multiplicity and net-proton yield. For reasonable choice of the initial condition, we are able to describe the measured rapidity and beam energy dependence of identified hadron $v_{1}$ including the observed $v_1$ splitting between baryons and anti-baryons.

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arXiv2022

Splitting of elliptic flow in a tilted fireball

The splitting of elliptic flow measured in different regions of the momentum space of produced hadrons has been recently studied in transport models and proposed as a sensitive probe of the angular momentum carried by the fireball produced in a relativistic heavy ion collision. The initial state angular momentum also gives rise to rapidity odd directed flow which has been measured. We consider a relativistic hydrodynamic framework with the initial matter distribution suitably calibrated to describe the observed directed flow and apply it to study the spilt in the elliptic flow. Our study suggests that the split in the elliptic flow is mostly driven by directed and triangular flows and may be used to constrain models of initial state rapidity distribution of matter in the fireball.

nucl-th↗

arXiv2021

Multiplicity dependence freeze-out scenarios in pp collisions at $\sqrt{s}$ = 7 TeV

The data on transverse momentum integrated hadron yields in different multiplicity classes of p+p collisions at $\sqrt{s}=7$ TeV have been analyzed to extract the chemical freeze-out parameters using a thermal model. The chemical freeze-out parameters have been extracted for three different freeze-out schemes: i. unified freeze-out for all hadrons in complete thermal equilibrium (1CFO), ii. unified freeze-out for all hadrons with an additional parameter $γ_S$ which accounts for possible out-of-equilibrium production of strange hadrons (1CFO$+γ_S$), and iii. separate freeze-out for hadrons with and without strangeness content (2CFO). It has been observed that 1CFO$+γ_S$ scheme gives the best description of the hadronic yields at midrapidity when multiplicity ($\langle dN_{ch}/dη\rangle$) of the collision is less than 10. This indicates that the strangeness is out of equilibrium in most of the multiplicity classes of p+p collisions. All the three parameters of this CFO scheme, temperature ($T$), radius of the fireball ($R$) and strangeness suppression factor ($γ_S$) increase with the increase of $\langle dN_{ch}/dη\rangle$. Further, we have compared applicability of different CFO schemes considering two more colliding system p+Pb at $\sNN$ = 5.02 and Pb+Pb at $\sNN$ = 2.76 TeV along with p+p collisions at $\sqrt{s}=7$ TeV. We observe a freeze-out volume (or multiplicity) dependence of CFO schemes regardless of colliding ions. The 1CFO+$γ_S$, 1CFO and 2CFO schemes provide best description of the data when the dimension less quantity $VT^3$ approximately satisfies the conditions $VT^3 <50$, $50 < VT^3 < 100$ and $VT^3 > 100$ respectively or the corresponding multiplicity satisfies the conditions $\langle dN_{ch}/dη\rangle<30$, $30 < dN_{ch}/dη< 60$ and $\langle dN_{ch}/dη\rangle>100$ respectively.

hep-ph↗