arXiv subjects

Subhash Singha

Publications and source records attributed to Subhash Singha.

At least 19 recordsLinked to original sources

arXiv2026

A practical methodology for $\Lambda$ global polarization extraction in fixed-target experiments

Non-central heavy-ion collisions generate large orbital angular momentum in the created medium, which leads to polarization of final-state particles via spin-orbit coupling, known as global spin polarization. The observation of significant global polarization of $\Lambda$ hyperon in heavy-ion collisions indicates that the quark-gluon plasma is the most vortical fluid known in nature. Exploring $\Lambda$ global polarization at lower energies is important for understanding spin dynamics across different regions of the quantum chromodynamics (QCD) phase diagram. Low-energy nuclear experiments are typically conducted with asymmetric detector acceptance, as in fixed-target collisions at RHIC-STAR, and at facilities such as FAIR, NICA, HIAF and HIRFL-CSR. The asymmetric rapidity coverage in these experiments enhances the coupling between directed flow and detector inefficiencies, creating significant bias in $\Lambda$ global polarization measurements. In this paper, we propose a methodology to eliminate such bias arising from asymmetric detector acceptance. The method is validated using realistic detector simulations based on the STAR fixed-target configuration.

physics.data-an↗

arXiv2026

Probing $\alpha$ clustering in $^{12}\mathrm{C}$ at CSR energies using the Jet AA Microscopic Transport Model

We investigate the sensitivity of low-energy nuclear collisions to intrinsic nuclear structure by studying the interplay between initial-state geometry and final-state observables in C+C and C+Pb collisions at $\sqrt{s_{NN}}=2.36$~GeV, relevant for experiments at the Cooling Storage Ring (CSR) facility in Lanzhou and forthcoming experiments at the High Intensity heavy-ion Accelerator Facility (HIAF) in Huizhou. Calculations are performed within the Jet AA Microscopic Transport Model (JAM) using Woods--Saxon and triangular $\alpha$-clustered configurations for the $^{12}C$ nucleus. The initial geometry is characterized in terms of transverse size, compactness, eccentricities, and their ensemble-averaged fluctuations. We find that $\alpha$ clustering leads to a more compact participant configuration than the Woods--Saxon case, while transverse-size and eccentricity fluctuations show only weak sensitivity to clustering. At this beam energy, radial observables remain sensitive to geometric compactness, with the ensemble-averaged proton mean transverse momentum $\langle p_T \rangle$ enhanced for $\alpha$-clustered configurations, whereas pions show little sensitivity. The anisotropic response is examined using flow harmonic coefficients. We find an enhancement of the root-mean-square flow magnitudes, $v_n\{2\} = \sqrt{\langle v_n^2\rangle}$, for $\alpha$-clustered configurations at large $N_{part}$, while the ensemble-averaged fluctuation strength of individual harmonics remains small. Symmetric cumulants of the initial-state eccentricities show sensitivity to clustering, whereas the corresponding ensemble-averaged correlations among final-state flow harmonics do not exhibit a comparably strong separation. These results indicate that radial observables and correlation-based flow measurements provide complementary probes of $\alpha$ clustering in low-energy nuclear collisions.

nucl-th↗

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 $\rho$(770), $K^{\star}$(892), $\phi$(1020), $\Delta$(1232), $\Lambda^{\star}$(1520), $\Sigma^{\star}$(1385) and $\Xi^\star$(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

Impact of particle production mechanisms on pseudorapidity distribution and directed flow in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 19.6 GeV using AMPT model

The STAR experiment at the top RHIC energy has observed that the directed flow ($v_1$) of inclusive light hadrons is independent of the collision system size at a given centrality~\cite{STAR:2008jgm}. However, recent STAR measurements indicate a system-size dependence in the $v_1(y)$-slope ($dv_{1}/dy$) of protons, antiprotons, and their differences ($\Delta dv_{1}/dy$) at a given centrality, suggesting a potential influence of baryon production and transport mechanisms~\cite{Taseer:SQM2024talk}. We have studied pseudorapidity ($dN/dy$) distributions and directed flow ($v_1$ and $dv_{1}/dy$) for pions, kaons, and protons in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}} = 19.6$ GeV using the A Multi-Phase Transport (AMPT) model. Specifically, we investigated the influence of string junction parameters in AMPT via the PYTHIA/JETSET routines, focusing on the popcorn mechanism and string-splitting parameters, on $dN/dy$, $dv_{1}/dy$, and their charge-dependent splittings ($\Delta dN/dy$ and $\Delta dv_{1}/dy$). We observe that string junction parameters can affect $dN/dy$, $dv_{1}/dy$, $\Delta dN/dy$, and $\Delta dv_{1}/dy$ for $\pi$, K, and p, and influence their system-size dependence. The effect is most prominent on the $v_1$ of protons, non-trivial for kaons, while the pions $v_1$ remain largely unchanged. These findings provide insights into the interplay between particle production mechanisms, baryon transport, and directed flow in heavy-ion collisions.

nucl-th↗

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, $\Delta dv_1/dy$, 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 $\Delta dv_1/dy (p-\bar{p})$ 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 $\Delta dv_1/dy (p-\bar{p})$ 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 $\Delta dv_1/dy$ for oppositely charged kaons and pions, particularly by incorporating the diffusion of other conserved charges.

nucl-th↗

arXiv2023

Study of bulk properties of the system formed in U+U collisions at $\sqrt{s_{\mathrm NN}}$ =~2.12~GeV using JAM model

The Lanzhou Cooling-Storage-Ring facility is set to conduct experiments involving Uranium-Uranium collisions at the center of mass energies ranging from 2.12 to 2.4 GeV. Our investigation is focused on various bulk observables, which include charged particle multiplicity ($N_{\text{ch}}$), average transverse momentum ($\langle p_{\text{T}}\rangle$), initial eccentricity ($\epsilon_{n}$), and flow harmonics ($v_{n}$), for different orientations of U+U collisions within the range of $0^{\circ} < \theta < 120 ^{\circ}$ at $\sqrt{s_{\mathrm NN}} = 2.12$ GeV ($p_{\mathrm lab}$ = 500 MeV). Among the various collision configurations at this energy, the tip-tip scenario emerged with the highest average charged particle multiplicity, denoted as $\langle N_{\text{ch}} \rangle$. Notably, both the second and third-order eccentricities, $\epsilon_{2,3}$, revealed intricate patterns as they varied with impact parameter across distinct configurations. The tip-tip configuration displayed the most pronounced magnitude of rapidity-odd directed flow ($v_{1}$), whereas the body-body configuration exhibited the least pronounced magnitude. Concerning elliptic flow ($v_{2}$) near mid-rapidity ($\eta < 1.0$), a negative sign is observed for all configurations except for the side-side exhibited a distinctly positive sign. Within the spectrum of configurations, the body-body scenario displayed the highest magnitude of $v_{2}$. For reaction plane correlated triangular flow ($v_{3}$), the body-body configuration emerged with the largest magnitude while the side-side exhibited the smallest magnitude. Our study seeks to establish a fundamental understanding of various U+U collision configurations in preparation for the forthcoming CEE experiment.

nucl-th↗

arXiv2023

Examining the influence of hadronic interactions on the directed flow of identified particles in RHIC Beam Energy Scan energies using UrQMD model

The directed flow of identified particles can serve as a sensitive tool for investigating the interactions during initial and final states in heavy ion collisions. This study examines the rapidity-odd directed flow ($v_{1}$) and its slope ($dv_{1}/dy$) for $\pi^{\pm}$, $K^{\pm}$, p, and $\bar{\mathrm p}$ in Au+Au collisions at different collision centralities and beam energies ($\sqrt{s_{\mathrm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV) using the UrQMD model. We investigate the impact of late-stage hadronic interactions on charge dependent $v_{1}(y)$ and its slope by modifying the duration of the hadronic cascade lifetime ($\tau$). The energy dependence of $dv_{1}/dy$ for p ($\bar{\mathrm p}$) exhibits distinct pattern compared to $\pi^{\pm}$ and $K^{\pm}$. Notably, we observe a change in the sign reversal position of proton $dv_{1}/dy$ at different beam energies with varying $\tau$ in central and mid-central collisions. Moreover, the difference in $dv_{1}/dy$ between positively and negatively charged hadrons ($\Delta dv_{1}/dy$) demonstrates a stark centrality dependence for different particle species. The deuteron displays a significant increase in $dv_{1}/dy$ with increasing $\tau$ compared to p and n. This investigation underscores the importance of considering the temporal evolution and duration of the hadronic phase when interpreting the sign reversal, charge splitting of $v_{1}$ and light nuclei formation at lower RHIC energies.

nucl-th↗

arXiv2023

The study of hadronic rescattering on $K^{*0}$ resonance yield in baryon-rich QCD matter

The effect of hadronic rescattering on $K^{*0}$ resonance yield can be studied by measuring $K^{*0}/K$ ratio as a function of centrality or multiplicity. This study investigates how the size of the system and the chemical composition (meson-meson versus meson-baryon interaction) of the matter formed in heavy-ion collisions impact the process of hadronic rescattering. It is shown that existing calculation of $K^{*0}/K$ ratio, which considers the interaction of $K^{*0}$ and $K$ mesons with only light mesons in the hadronic medium (neglecting interactions with baryons), fails to explain the measured $K^{*0}/K$ ratio at RHIC BES energies. To understand the multiplicity dependence of the $K^{*0}/K$ ratio at RHIC BES and SPS energies ($\sqrt{s_{NN}}$ $<$ 20 GeV), the Ultra Relativistic Quantum Molecular Dynamics (UrQMD) model is employed. UrQMD calculations suggest that for a given multiplicity, $K^{*0}/K$ is more suppressed in Au+Au collision at $\sqrt{s_{NN}}$ =7.7 GeV, compared to that in $\sqrt{s_{NN}}$ =200 GeV. This is possibly because of formation different type of QCD medium at 200 GeV and 7.7 GeV, and change in interaction cross-section between hadrons with change in particle type and energy.

nucl-th↗

arXiv2023

$K^{*0}$ meson production using a transport and a statistical hadronization model at energies covered by the RHIC beam energy scan

In this paper, we discuss the centrality and energy dependence of $K^{*0}$ resonance production using ultrarelativistic quantum molecular dynamics (UrQMD) and thermal models. The $K^{*0}/K$ ratios obtained from the UrQMD and thermal models are compared with measurements done by the STAR experiment in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The $K^{*0}/K$ ratio from the thermal model is consistent with data in most-peripheral collisions, however it overpredicts the ratio in central Au+Au collisions. This could be due to the fact that the thermal model does not have a hadronic rescattering phase, which is expected to be dominant in more central collisions. Furthermore, we have studied the $K^{*0}/K$ ratio from UrQMD by varying the hadron propagation time ($\tau$) within the range 5 to 50 fm/c. It was found that the $K^{*0}/K$ ratio decreases with increasing $\tau$. Comparison between data and UrQMD suggest, one needs to consider a $\tau$ $\approx$ 10-50 fm/c to explain data at $\sqrt{s_{NN}}$ = 7.7-39 GeV in Au+Au collisions. We also predict the rapidity distribution of $K^{*0}$ from UrQMD which could be measured in the STAR beam energy scan phase II (BES-II) program.

nucl-th↗

arXiv2022

Spin polarization and alignment in heavy-ion collisions

We report the measurements of global spin polarization of hyperons ($P_{\Lambda}$) from FAIR, RHIC and LHC energies. The $P_{\Lambda}$ continue to follow an increasing trend at $\sqrt{s_{\mathrm NN}}$ < 7.7 GeV indicating that enormous vorticity prevail even in hadronic dominant region. New measurements of local spin polarization ($P_{z,2}$) from isobar collisions follow the same pattern observed in previous RHIC and LHC energies. An observable motivated by predicted baryonic Spin Hall Effect, the net local polarization ($P_{z,2}^{\mathrm{net}}$), shows negative value in Au+Au collisions at $\sqrt{s_{\mathrm NN}}$ = 19.6 and 27 GeV. The first $P_{z}$ measurement with respect to third order harmonic in 200 GeV RHIC isobar collisions show a non-zero sine modulation, indicating the presence of complex vortical structure in the medium. Surprising new patterns of spin alignment ($\rho_{00}$) for various vector mesons ($K^{*0}$, $K^{*\pm}$, $\phi$ and $J/\psi$) emerged at both RHIC and LHC energies. The large deviation in $\rho_{00}$ of several species poses challenge to the theory.

nucl-ex↗

arXiv2022

Measurement of global spin alignment of vector mesons at RHIC

We report the measurements of spin alignment ($\rho_{00}$) for $K^{*0}$, $\overline{K^{*0}}$, $K^{*+}$, and $K^{*-}$ vector mesons in RHIC isobar collisions (Zr+Zr and Ru+Ru) at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV. We observe the first non-zero spin alignment for $K^{*\pm}$ in heavy-ion collisions. The $K^{*\pm}$ $\rho_{00}$ is about 3.9$\sigma$ larger than that of $K^{*0}$. The observed difference and the ordering between $K^{*\pm}$ and $K^{*0}$ are surprising, and require further inputs from theory. When comparing between the isobar and Au+Au collisions, no significant system size dependence in $K^{*0}$ $\rho_{00}$ is observed within uncertainties.

nucl-ex↗

arXiv2021

Spin alignment measurement of vector mesons produced in high energy collisions

This review covers the recent experimental development on spin alignment measurements of $K^{*0}$ and $\phi$ vector mesons in heavy-ion and pp collisions at RHIC and LHC energies. Measurements in $e^+e^-$ collisions at LEP energies are also discussed. Spin alignment of vector mesons are studied by measuring the second diagonal element $\rho_{00}$ of spin density matrix. The $\rho_{00}$ is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured $\rho_{00}$ values for vector mesons are found to be larger than 1/3 at high momentum in $e^+e^-$ collisions at LEP energies, suggesting the preferential production of vector meson with helicity zero state from the fragmentation process. The $\rho_{00}$ values are found to be smaller than 1/3 ($\rho_{00}$ = 1/3 implies no spin alignment) for $K^{*0}$ and $\phi$ vector mesons at low transverse momentum in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. This observations are qualitatively consistent with the expectation from models which attribute the spin alignment effect due to polarization of quarks in the presence of large initial angular momentum in non-central heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for $K^0_S$ (spin = 0) in mid-central Pb--Pb collisions and for vector mesons in pp collisions. However, the preliminary results of $\rho_{00}$ for $\phi$ mesons are larger than 1/3 at intermediate $p_{\mathrm{T}}$ in Au--Au collisions at RHIC energies and can be attributed to the presence of $\phi$ meson field. Although there is evidence of spin alignment effect of vector mesons in heavy-ion collisions but the measured effect is surprisingly larger in context of hyperon polarization. Therefore these results will trigger further theoretical study.

nucl-ex↗

arXiv2021

Probing the profile of bulk matter in p+Pb collisions via directed flow of heavy quarks

The asymmetric initial geometry in p+Pb collisions provide an opportunity to probe the initial matter distribution in these collisions. Using A Multi-Phase Transport (AMPT) model, we have studied the production of light and heavy quark particles in p+Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The pseudo-rapidity density (dN/d$\eta$) and elliptic flow ($v_{2}$) of light-quark particles from AMPT reasonably agrees with the measurements by ALICE. We predicted the directed flow of light and heavy quarks in p+Pb collisions. The $v_{1}$ of both light and heavy-quark mesons show a non-trivial $p_{T}$ dependence in p- and Pb-going directions. When integrated over the transverse momentum range 0 $ < p_{T} < $5 GeV/c, the magnitude of heavy quark directed flow ($v_{1}$) is found to be 15 times larger than the light quark species in the Pb-going direction while the same in the p-going direction are comparable. The light and heavy quarks $v_{1}$ may offer the possibility to probe the initial matter density as well as the collective dynamics in p+Pb collisions

hep-ph↗

arXiv2020

Measurement of global spin alignment of $K^{*0}$and $\phi$ vector mesons using the STAR detector at RHIC

We report the transverse momentum ($p_{\mathrm{T}}$) and centrality dependence of global spin alignment ($\rho_{00}$) of $K^{*0}$ vector meson at midrapidity ($|y|<0.5$) in Au + Au collisions at $\sqrt{s_{NN}}$ = 54.4 and 200~GeV with the STAR experiment at RHIC. The $K^{*0}$ results are compared to that of $\phi$ meson. At low-$p_{\mathrm{T}}$ region and midcentral collisions, the $K^{*0}$ $\rho_{00}$ is found to be smaller than 1/3 with about 4$\sigma$ significance, while that of $\phi$ meson is observed to be larger than 1/3 with about 3$\sigma$ significance. The $\rho_{00}$ results are compared between RHIC and LHC energies. The physics implication of our results is also discussed.

nucl-ex↗

arXiv2018

Measurements of directed and elliptic flow for $D^{0}$ and $\overline{D^{0}}$ mesons using the STAR detector at RHIC

We report on the first evidence for a non-zero rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $\overline{D^{0}}$ mesons in 10-80\% centrality Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200~GeV measured with the STAR detector at RHIC. The slope of the $v_1$ rapidity dependence ($dv_{1}/dy$) averaged over $D^{0}$ and $\overline{D^{0}}$ mesons is -0.081 $\pm$ 0.021 $\pm$ 0.017, while that of charged kaons is -0.0030 $\pm$ 0.0001 $\pm$ 0.0002, suggesting significantly larger slope for the $D^{0}$ mesons. Models indicate that the large $dv_{1}/dy$ of $D^{0}$ mesons is sensitive to the initially tilted source. We also present a new measurement of the $D^{0}$ meson elliptic flow ($v_2$) as a function of transverse momentum ($p_{T}$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200~GeV with an improved precision with respect to the previously published results.The $D^{0}$ $v_{2}$ results are compared to those of light-flavor hadrons to test the number-of-constituent-quark (NCQ) scaling. Both the $v_{1}$ and $v_{2}$ results are compared to recent hydrodynamic and transport model calculations.

nucl-ex↗

arXiv2018

Directed flow of open charm in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using a quark coalescence model

The directed flow ($v_{1}$) of open charm meson ($D^{0}$) is studied in Au+Au collisions at $\sqrt{s_{NN}}$ = 200~GeV using A Multi-Phase Transport (AMPT) model framework with partonic interactions (string melting version). Within this framework, it is found that although the initial spatial eccentricity ($\epsilon_{1}$) of charm quark is smaller than light quarks, the charm quark $v_{1}$ magnitude is found to be approximately 7 times larger than that of the light u quark at large rapidity. This indicates that the charm quarks can retain more information from initial condition than the light quarks. We have studied the directed flow of $D^{0}$ as a function of rapidity and transverse momentum using quark coalescence as the mechanism for hadron production. Like charm quark, the $D^{0}$ $v_{1}$ magnitude is found to be about 7 times larger than that of the light ($\pi$) hadrons at large rapidity.

nucl-th↗

arXiv2017

Directed flow in Au+Au collisions from the RHIC Beam Energy Scan at the STAR experiment

We report results of $v_1(y)$ and $dv_1/dy$ near mid-rapidity for $\pi^{\pm}$, $K^{\pm}$, $K_s^0$, $p$, $\overline{p}$, $\Lambda$, $\overline{\Lambda}$ and $\phi$ from Beam Energy Scan Au+Au collisions at $\sqrt{s_{NN}} = $ 7.7 - 200 GeV using the STAR detector at RHIC. The $dv_{1}/dy$ of $\pi^{\pm}$, $K^{\pm}$ and $K_s^0$ mesons remains negative over all beam energies. The $dv_1/dy$ of $p$ and $\Lambda$ baryons shows a sign change around 10 - 15 GeV, while net baryons (net p and net $\Lambda$) indicate a double sign change. The $dv_1/dy$ of $\overline{p}$, $\overline{\Lambda}$ and $\phi$ show a similar trend for $\sqrt{s_{NN}}>$ 14.5 GeV. For the first time, $v_{1}$ measurements are used to test a quark coalescence hypothesis. Many measurements are found to be consistent with the particles being formed via coalescence of constituent quarks. The observed deviations from that consistency offer a new approach for probing the collision process at the quark level.

nucl-ex↗

arXiv2016

Directed flow of $Λ$, $\barΛ$, $K^{\pm}$, $K_S^0$ and $ϕ$ mesons from Beam Energy Scan Au+Au collisions using the STAR experiment

We report the results of $v_1$ and $dv_1/dy$ near mid-rapidity for $Λ$, $\barΛ$, $K^{\pm}$, $K_s^0$ and $ϕ$ in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 14.5, 19.6, 27 and 39~GeV using the STAR detector at RHIC. The $dv_1/dy$ of $Λ$ is found to be consistent with that of the proton and shows a change in sign near $\sqrt{s_{\rm NN}} = 11.5$~GeV. The $v_{1}$ slope for $\barΛ$, $\bar{p}$ and $ϕ$ shows a similar trend for $\sqrt{s_{\rm NN}}$ $>$ 14.5~GeV, while below 14.5~GeV, $ϕ$ $v_{1}$ is consistent with zero but with a large uncertainty. The $dv_{1}/dy$ for net protons and net kaons is similar for $\sqrt{s_{\rm NN}} >$ 14.5~GeV, but they deviate at lower beam energies.

nucl-ex↗