arXiv subjects

Kishora Nayak

Publications and source records attributed to Kishora Nayak.

8 recordsLinked to original sources

arXiv2026

Driver and damping of the directed-flow response in heavy-ion collisions

The initial-state geometry plays a crucial role in driving directed flow, while the dissipative response of the medium dampens it. Both of these factors influence how the directed-flow slope varies with system size. We developed a method to differentiate between these driving and damping effects on charged-hadron directed flow in O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at $\sqrt{s_{\mathrm{NN}}} = 200$~GeV using an improved version of the string-melting AMPT model. We formulated three scaling observables based on the entropy density, the number of participants, and the mass number. A dimensionless ratio was constructed, revealing the threshold mass number $A \approx 35$ in central collisions from the entropy and participant scaling, indicating the onset of collective behavior. We constructed a kinetic-theory Knudsen-number ($\mathrm{Kn}$) map to analyze the contributions of the initial-state driver, which grows as $\mathrm{Kn}^κ$ with $κ\approx 2$, and a final-state viscous damping of the hydrodynamic response with characteristic scale $\mathrm{Kn}_0 \approx 0.27$ for the directed flow slope. This damping scale is found to be about a factor of 2.5 smaller than the $\mathrm{Kn}_0 \approx 0.7$ extracted from the elliptic flow. Furthermore, we determined the ratio of shear viscosity to entropy density, $η/s$, to be between 0.10 and 0.20, using an alternative method that does not rely on fitting flow harmonics.

hep-ph↗

arXiv2026

Charge-Dependent Directed Flow in Symmetric Nuclear Collisions

The directed flow ($v_1$) of identified hadrons ($π^{\pm}, K^{\pm}, p, \bar{p}, ϕ, Λ$, and $\barΛ$) is studied in symmetric nuclear collisions (O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U) at $\sqrt{s_{NN}} = 200$ GeV using the string-melting version of a multiphase transport model with improved quark coalescence. The mid-rapidity $v_1$-slope ($dv_1/dy$) and its charge-dependent splitting ($Δdv_1/dy$) between particles and anti-particles are investigated as a function of nuclear mass number ($A$) and collision centrality in both low-$p_\mathrm{T}$ (0.2$-$2.0 GeV/$c$) and high-$p_\mathrm{T}$ (2.0$-$5.0 GeV/$c$) regions. At low-$p_\mathrm{T}$, the $v_1$-slope shows weak system-size dependence, while at high-$p_\mathrm{T}$ strong system-size dependence is found and it becomes negative with nuclear mass number, reflecting the hard-soft asymmetry in particle production. The charge-dependent splitting $Δdv_1/dy$ reveals a striking baryon-meson dichotomy: baryon pairs ($p-\bar{p}$ and $Λ-\barΛ$) exhibit significant splitting that grows with system size, whereas meson pairs ($π^+-π^-$ and $K^+-K^-$) show minimal splitting. The effect of final state hadronic interactions on the $v_1$-slope is found to be negligible confirming that it is primarily generated during the partonic phase and coalescence process. A comparison of the AMPT results with measurements from the STAR experiment at RHIC in Au+Au collisions establish the transported quark contribution as a baseline for the observed charge-dependent $v_1$ splitting, on top of which electromagnetic field effects must be considered.

hep-ph↗

arXiv2026

Light-Flavour Resonance Production in High-Energy Heavy-Ion Collisions: An Experimental Review

Resonances provide sensitivity to the late-stage dynamics of heavy-ion collisions, as their lifetimes are comparable to the duration of the hadronic phase. This review summarizes state-of-the-art measurements of light-flavour mesonic and baryonic resonances, including $ρ$(770), $K^{\star}$(892), $ϕ$(1020), $Δ$(1232), $Λ^{\star}$(1520), $Σ^{\star}$(1385) and $Ξ^\star$(1530), in pp, p-A and A-A collisions at SPS, RHIC and the LHC. Systematic trends in yields, mass and width modifications, transverse-momentum spectra, nuclear modification factors, and particle ratios reveal the interplay of re-scattering and regeneration, medium-induced suppression, and the development of collective dynamics with increasing system size and multiplicity. Anisotropic flow results confirm the coupling of resonances to the expanding medium, while recent vector-meson spin-alignment measurements offer fresh insights into hadronization mechanisms and local fields. Ultra-peripheral collisions provide vacuum-like baselines for isolating in-medium effects. Emerging opportunities for charm-resonance studies in upcoming high-luminosity experiments are also outlined.Together, these advances demonstrate the important role of resonance measurements in constraining the space-time evolution of strongly interacting matter.

nucl-ex↗

arXiv2025

System size dependence of charged hadrons directed flow at $\sqrt{s_{NN}}$ = 200 GeV using a multi-phase transport model

The directed flow ($v_1$) of charged hadrons ($h^{\pm}$) in symmetric collision systems (O+O, Cu+Cu, Zr+Zr, Ru+Ru, Au+Au, and U+U) at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV using string-melting version of A Multiphase Transport (AMPT-SM) model is reported. The $v_1$ as a function of pseudo-rapidity ($η$) is obtained for transverse momentum ($p_{\mathrm{T}}$) ranges of 0.2-2.0 GeV/$c$ and 2.0-5.0 GeV/$c$. The dependence of $v_1$-slope ($dv_1/dη$) at mid-rapidity on $p_{\mathrm{T}}$ range, collision centrality, and system size are discussed particularly in the context of the hard-soft asymmetry in the flow profiles of produced particles. In the AMPT-SM model, a system size independence of the magnitude of $dv_1/dη$ between Cu+Cu and Au+Au collisions at low-$p_{\mathrm{T}}$ is observed, and this finding is similar to the observation from the STAR experiment at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV. In contrast, a strong centrality and system size dependence, with the opposite sign of $dv_1/dη$, is found for the high-$p_{\mathrm{T}}$ charged hadrons. The AMPT-SM model demonstrates a clear violation of the expected scaling of charged hadrons $(dv_1/dη)/A^{1/3}$ across different colliding systems.

hep-ph↗

arXiv2024

Energy dependence of particle production in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 7.7-200 GeV using a multiphase transport model

In this study, we employ a multi-phase transport (AMPT) model to understand the production of $π^{\pm}$, $K^{\pm}$, $p$, $\overline{p}$, $K^{0}_{s}$, $Λ$, $\barΛ$, and $ϕ$ in Au + Au collisions at $\sqrt{s_{NN}} = 7.7$, $27$, $39$, $62.4$, and $200$ GeV. We have studied the energy dependence of various bulk properties of the system such as transverse momentum ($p_T$) spectra, particle yields ($dN/dy$), mean transverse mass ($\langle m_T \rangle$), and anti-particle to particle ratios. Model calculations using both default and string melting versions of the AMPT with three distinct sets of initial conditions are compared to the data from the STAR experiment. In the case of $π^{\pm}$, $K^{\pm}$, $p$, and $\overline{p}$, we observe that the string melting version shows better agreement with data at higher energies, while the default version performs better at lower collision energies. However, for $K^{0}_{s}$, $Λ$, and $ϕ$, it is observed that the default version is able to describe the data better at all energies. In addition, we have used the blast-wave model to extract the kinetic freeze-out properties, like the kinetic freeze-out temperature and the radial flow velocity. We observe that these parameters are comparable with the data.

nucl-th↗

arXiv2023

Coalescence sum rule and the electric charge- and strangeness-dependences of directed flow in heavy ion collisions

The rapidity-odd directed flows ($v_{\rm 1}$) of identified hadrons are expected to follow the coalescence sum rule when the created matter is initially in parton degrees of freedom and then hadronizes through quark coalescence. A recent study has considered the $v_{\rm 1}$ of produced hadrons that do not contain $u$ or $d$ constituent quarks. It has constructed multiple hadron sets with a small mass difference but given difference in electric charge $Δq$ and strangeness $ΔS$ between the two sides, where a nonzero and increasing $Δv_{\rm 1}$ with $Δq$ has been proposed to be a consequence of electromagnetic fields. In this study, we examine the consequence of coalescence sum rule on the $Δv_{\rm 1}$ of the hadron sets in the absence of electromagnetic fields. We find that in general $Δv_{\rm 1} \neq 0$ for a hadron set with nonzero $Δq$ and/or $ΔS$ due to potential $v_{\rm 1}$ differences between $\bar u$ and $\bar d$ and between $s$ and $\bar s$ quarks. We further propose methods to extract the coefficients for the $Δq$- and $ΔS$-dependences of the direct flow difference, where a nonzero constant term would indicate the breaking of the coalescence sum rule. The extraction methods are then demonstrated with transport model results.

nucl-th↗

arXiv2020

Directed and elliptic flow of identified hadrons, high-$p_{\rm T}$ charged hadrons and light nuclei in Au+Au collisions at STAR

In this proceeding, we present the new precise directed flow $v_{1}$, measurement of $π$, $K$, $p$ and $ϕ$ in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 27, 54.4 GeV and deuteron at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5 and 19.6 GeV. The first measurement of pseudorapidity and centrality dependence of the $v_{1}$ of high-$p_{T}$ ($>$5 GeV/$c$) charged hadrons in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV are reported. The elliptic flow $v_{2}$, of identified hadrons ($π$, $K$, $p$, $ϕ$, $K^{0}_{S}$, $Λ$, $Ξ$, $Ω$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 54.4 GeV are also presented. The results are compared with model calculations to discuss the physics implications.

nucl-ex↗

arXiv2019

Energy dependence study of directed flow in Au+Au collisions using an improved coalescence in AMPT model

The rapidity-odd component of directed flow ($v_{1}$) of identified hardons ($π^{\pm}$, $K^{\pm}$, $K^{0}_{S}$, $p$, $\bar{p}$, $ϕ$, $Ξ$, $\barΞ$, $Λ$, $\barΛ$) and partons ($u$, $\bar{u}$, $d$, $\bar{d}$, $s$, $\bar{s}$) in Au+Au collisions at various beam energies ($\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, 200 GeV) using a multi-phase transport model is analyzed. A data driven approach (inspired from the experimental analysis) is performed here to distinguish the transported and produced quarks which are found to have different directed flow values at various collision beam energies. The coalescence sum rule (Number of Constituent Quark scaling) violation is observed at lower energies where hadronic matters dominate. The strange quark ($s$) and $ϕ$ meson slope (d$v_{1}$/d$y$) show a double sign change around 14.5 GeV, unlike other partons and hadrons. It suggests that strange quark is more sensitive to the softening of Equation of State (EoS).

nucl-ex↗