arXiv subjects

Subikash Choudhury

Publications and source records attributed to Subikash Choudhury.

18 recordsLinked to original sources

arXiv2026

Intrinsic coupling between transverse spherocity and elliptic flow in heavy-ion collisions

Transverse spherocity ($S_{0}$) is an event-shape observable widely used to classify collision events according to their topology, particularly to distinguish jet-like from isotropic events. Low-spherocity events are generally interpreted as being associated with enhanced jet activity. This event classification has also been applied to heavy-ion collisions to investigate the influence of event topology on several observables, including elliptic flow and constituent-quark-number scaling. In this work, we demonstrate that such an interpretation requires careful reconsideration. Using toy Monte Carlo simulations, A Multiphase Transport (AMPT) model calculations, and an analytical formulation of the spherocity observable, we show that transverse spherocity is intrinsically related to the elliptic flow coefficient, $v_{2}$. This connection arises because the axis that minimizes the spherocity aligns with the event symmetry plane, causing events with larger elliptic anisotropy to naturally exhibit smaller spherocity values even in the absence of genuine jet-like topologies. We further show that this intrinsic relation gives rise to an inherent anti-correlation between transverse spherocity and $v_{2}$, implying that several characteristics previously attributed to the enhanced jet-like nature of low-spherocity events in heavy-ion collisions can instead be understood as consequences of collective anisotropic flow. Our results indicate that, in heavy-ion collisions, transverse spherocity should be interpreted primarily as a probe of the collective momentum-space anisotropy rather than as a direct measure of jetty event topology. Consequently, physics conclusions drawn from spherocity-selected events should explicitly account for its intrinsic correlation with elliptic flow.

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arXiv2026

Analysis of quarkonium polarization in proton-proton (p-p) collisions at LHC using PYTHIA model

The measurement of polarization serves as an important probe to investigate the production mechanism of quarkonia, the bound state of heavy quark anti-quark (charm or bottom) pairs, in hadronic collisions. In experimental invesigations, the polarization is usually measured by analyzing the anisotropies in the angular distribution of the muons originating from the decay of the quarkonium state. In the present article, we study the charmonia ($J/\psi$) and bottomonia ($\Upsilon(1S)$) polarization at $\sqrt{s} =7 $ and 13 TeV in proton-proton(p-p) collisions at LHC using Monte Carlo (MC) event generator model PYTHIA8, which is based on perturbative QCD. The transverse momentum ($p_{T}$) differential distribution has been calculated at forward rapidity ($2.5 < y_{\mu\mu} < 4.0$) and the polarization parameters are estimated in Helicity and Collins-Sooper reference frames. In addition, to mimic realistic experimental conditions, we have incorporated, in PYTHIA simulations, effects like detector inefficiencies and muon momentum smearing. These contributions alter the polarization parameters, introducing an artificial degree of polarization, if not properly corrected for. The simulation results have been compared with the recent ALICE measurements for quarkonia polarization in p-p collisions at LHC energy regime.

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arXiv2026

EPOS4 Model Predictions for Global Observables in Pb-Pb Collisions at $\sqrt{s_{NN}}$ = 5.36 TeV

The study of the Quark-Gluon Plasma (QGP), a deconfined state of nuclear matter, remains a central focus of high-energy heavy-ion collision experiments. The recent operation of the Large Hadron Collider (LHC) in Run 3 at the new center-of-mass energy of $\sqrt{s_{NN}}=5.36$ TeV necessitates theoretical predictions to characterize the energy dependence and bulk properties of the medium. In this study, we present comprehensive EPOS4 model predictions for key global observables in Pb-Pb collisions at $5.36$ TeV. We focus on the centrality dependence of the charged-particle pseudorapidity density ($dN_{ch}/d\eta$), integrated yields ($dN/dy$), mean transverse momentum ($\langle p_{T}\rangle$) for light-flavor hadrons ($\pi^{\pm}, K^{\pm}, p(\bar{p})$), and the charged particle nuclear modification factor ($R_{AA}$). The EPOS4 framework successfully captures the strong mass-dependent rise of $\langle p_{T}\rangle$ with multiplicity, a definitive signature of collective radial flow. Furthermore, the predicted charged hadron $R_{AA}$ demonstrates a clear suppression, consistent with energy loss mechanisms incorporated into the model. By comparing these predictions to existing $5.02$ TeV data, we demonstrate that the EPOS4 model offers a consistent and robust description of heavy-ion dynamics, projecting minimal energy evolution for these bulk and hard-probe observables between the two energies.

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arXiv2025

Radiation Tolerance Characterisation of an Indigenously Developed p-type Silicon Pad Sensor for Forward Calorimetry Applications

We report on the radiation tolerance characterisation of a p-type silicon pad sensor indigenously designed by BARC-VECC and fabricated at Bharat Electronics Limited (BEL), India -- representing a significant step toward establishing a domestic silicon sensor manufacturing capability for high-energy physics applications. Single-pad test structures were irradiated with neutrons over a range of fluences from $\sim10^{7}$ to $\sim2.5\times10^{14}$~1~MeV~$n_{\mathrm{eq}}$/cm$^{2}$, spanning the operational regime relevant to forward calorimetry in high-luminosity heavy-ion collider experiments. Post-irradiation performance was characterised through systematic measurement of leakage current evolution and calorimetric response as functions of accumulated neutron fluence. A single-exponential annealing model is introduced to describe the time dependence of leakage current within the observation window, and the current-related damage constant $\alpha$ is extracted and compared with the RD48 reference value. The results demonstrate that the sensors survive the target fluence with measurable but recoverable degradation, validating the fabrication process and providing a baseline for future qualification of this indigenous sensor production chain.

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arXiv2025

Exploring Strangeness Enhancement and Particle Production in Small Collision Systems with EPOS4 at $\sqrt{s_\rm{NN}}$ = 5.02 TeV

The observation of collectivity and strangeness enhancement in small collision systems, such as proton-proton (pp) and proton-lead (p-Pb) collisions, challenges traditional assumptions regarding thermalization and particle production mechanisms. In this study, we investigate particle yields and transverse momentum distributions in pp and p-Pb collisions at $\sqrt{s_\rm{NN}}$ = 5.02 TeV using the EPOS4 event generator, which employs a core-corona framework to model particle production across a variety of system sizes. EPOS4 successfully reproduces many qualitative trends observed in experimental data, including the hardening of $p_{\rm{T}}$-spectra with multiplicity, the hierarchical strangeness enhancement in strange-to-pion ratios, and characteristic modifications of particle yield ratios as a function of $p_{\rm{T}}$ and multiplicity. The microcanonical approach to core hadronization used in EPOS4 seems to provide a more realistic description of small systems compared to grand-canonical treatments. Nonetheless, quantitative discrepancies still persist in describing several physical observables. Future model refinements, including improved core-corona balancing, differential freeze-out conditions for multi-strange hadrons, and incorporation of finite strangeness correlation volumes, may be taken into account for enhancing EPOS4's predictive power and deepening our understanding of the complex dynamics governing the particle production in high-energy collisions.

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arXiv2023

Quarkonia as probes of initial and final states in small systems with ALICE

The multiple parton$-$parton interactions (MPIs) are an important element to describe the observed collective flow and strangeness enhancement in high multiplicity pp and p$-$Pb collisions, the so-called small systems. At LHC energy, MPIs affect both soft and hard scales of the event. Since quarkonium production involves both, it can be an excellent tool to understand the role of MPIs in small systems. Study of multiplicity dependent quarkonium production provides an indirect probe of MPIs in hadronic collisions. Also, relative production of excited-to-ground quarkonium states as a function of multiplicity are sensitive to final state effects. Cross section measurements of different quarkonium states are also important to understand their production mechanisms. In these proceedings, first preliminary results of $ψ$(2S)-over-J/$ψ$ cross section measurements at mid and forward rapidity ($\textit{y}$) from Run 2 and Run 3 pp collisions at $\sqrt{s}$ = 13 and 13.6 TeV, respectively will be presented. Preliminary results on $Υ$(nS) states (n = 1,2) cross sections and final multiplicity dependent measurements of excited-to-ground states quarkonium yields, at forward $\textit{y}$, in pp collisions at $\sqrt{s} =$ 13 TeV will be shown. In addition, few performance plots from Run 3 in the quarkonium sector will be presented and discussed.

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arXiv2022

Investigating the role of partonic and hadronic dynamics in mass splitting of elliptic anisotropy in p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV

The mass ordering of $v_{2}^{hadron}$ is regarded as one of the key signatures of collective behaviour in ultra relativistic heavy ion collisions. This observation has been found to be in compliance with the hydrodynamical response of a strongly interacting system to the initial spatial anisotropy. Flow co-efficients measured with identified particles in p-Pb/d-Au collisions have shown similar mass-splitting of $v_{2}^{hadron}$ indicating towards the presence of collective dynamics in small collision systems. Arguably, small size in the overlap geometry of such colliding systems may not be suitable for hydrodynamical treatment that demands an early thermalization. Studies based on a multi phase transport model suggests that elliptic or triangular anisotropy is primarily due to escape mechanism of partons rather than hydro like collectivity and mass ordering of $v_{2}^{hadron}$ can be generated from coalescence dynamics as implemented in string melting version of AMPT even when parton azimuthal directions are randomized. In this work, studies have been performed on p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV using AMPT model which has been found to explain the elliptic and traingular flow in such a system where escape mechanism is the dominant source of flow generation. We report that the mass splitting of $v_{2}^{hadron}$ can originate independently both at the partonic and hadronic level in the string melting version of the AMPT model.

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arXiv2022

Effect of radial flow on two particle correlations with identified triggers at intermediate $p_{T}$ in p-Pb collisions at $\sqrt{s_{NN}} = $ 5.02 TeV

Results from the two-particle correlation between identified triggers (pions ($π^{\pm}$) , protons (p/$\bar{p}$))) and un-identified charged particles at intermediate transverse momentum ($p_{T}$) in p-Pb collisions at $\sqrt{s_{NN}} = $ 5.02 TeV have been presented. The events generated from a hybrid Monte-Carlo event generator, EPOS 3.107 that implements a flux-tube initial conditions followed by event by event 3+1D viscous hydrodynamical evolution, have been analysed to calculate two-dimensional correlation functions in $Δη$-$Δϕ$. The strength of angular correlations at small relative angles (jet-like correlations), quantified in terms of near-side jet-like per-trigger yield has been calculated as a function of the event multiplicity. The yield associated with pion triggers exhibit negligible multiplicity dependence, while the proton-triggered yield shows a gradual suppression from low to high multiplicity events. In small collision systems like p-Pb where jet modification is expected to be less dominant, the observed suppression may be associated with the hydrodynamical evolution of the bulk system that generates an outward radial flow. Analogous measurements in Au-Au collisions at RHIC energy have shown a hint of dilution in proton-triggered correlation at its highest multiplicity suggesting that the soft physics processes in p-Pb and heavy ion collisions may have qualitative similarity.

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arXiv2021

Investigation of Experimental Observables in Search of the Chiral Magnetic Effect in Heavy-ion Collisions in the STAR experiment

The chiral magnetic effect (CME) is a novel transport phenomenon, arising from the interplay between quantum anomalies and strong magnetic fields in chiral systems. In high-energy nuclear collisions, the CME may survive the expansion of the quark-gluon plasma fireball and be detected in experiments. Over the past decade, the experimental searches for the CME have aroused extensive interest at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The main goal of this article is to investigate three pertinent experimental approaches: the $γ$ correlator, the $R$ correlator and the signed balance functions. We will exploit both simple Monte Carlo simulations and a realistic event generator (EBE-AVFD) to verify the equivalence in the kernel-component observables among these methods and to ascertain their sensitivities to the CME signal for the isobaric collisions at RHIC.

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arXiv2020

Examining the event-shape dependent modifications to charged-particle transverse momentum spectra and elliptic flow in p-Pb collisions at energies available at the CERN Large Hadron Collider

Purported signatures of collective dynamics in small systems like proton-proton (pp) or proton-nucleus (p-A) collisions still lack unambiguous understanding. Despite the qualitative and/or quantitative agreement of the data to hydrodynamic models, it has remained unclear whether the harmonic flows in small systems relate to the common physical picture of hydrodynamic collectivity driven by the initial geometry. In the present work, we aim to address this issue by invoking a novel concept of Event Shape Engineering (ESE), which has been leveraged to get some control of the initial geometry in high-energy heavy-ion collisions. We utilise ESE by constructing a reference flow vector, $q_{2}$ that allows to characterise an event based on it's ellipticity. Applying this technique on a data set, simulated from a 3+1D viscous hydrodynamic model EPOS3, we study the event-shape dependent modifications to some of the bulk properties like, inclusive transverse momentum ($p_{T}$) spectra and $p_{T}$-differential $v_{2}$ for p-Pb collisions at 5.02 TeV. Selecting events on the basis of different magnitudes of reference flow vector $q_{2}$, we observe a hint of event-shape induced modifications of $v_{2}$ as a function of $p_{T}$ but, the inclusive $p_{T}$-spectra of charged particles seem to be insensitive to this event-shape selection.

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arXiv2019

Background evaluations for the chiral magnetic effect with normalized correlators using a multiphase transport model

The chiral magnetic effect (CME) induces an electric charge separation in a chiral medium along the magnetic field that is mostly produced by spectator protons in heavy-ion collisions. The experimental searches for the CME, based on the charge-dependent angular correlations ($γ$), however, have remained inconclusive, because the non-CME background contributions are not well understood. Experimentally, the $γ$ correlators have been measured with respect to the second-order ($Ψ_{2}$) and the third-order ($Ψ_{3}$) symmetry planes, defined as $γ_{112}$ and $γ_{123}$, respectively. The expectation was that with a proper normalization, $γ_{123}$ would provide a data-driven estimate for the background contributions in $γ_{112}$. In this work, we calculate different harmonics of the $γ$ correlators using a charge-conserving version of a multiphase transport (AMPT) model to examine the validity of the said assumption. We find that the pure-background AMPT simulations do not yield an equality in the normalized $γ_{112}$ and $γ_{123}$, quantified by $κ_{112}$ and $κ_{123}$, respectively. Furthermore, we test another correlator, $γ_{132}$, within AMPT, and discuss the relation between different $γ$ correlators.

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arXiv2019

Study of jet-medium interactions using jet shape observables in heavy ion collisions at LHC energies with JEWEL

Based on a pQCD inspired dynamical model of jet-medium interactions, Jewel, we have studied possible modifications to inclusive jet yields and a set of jet shape observables, namely, the fragmentation functions and radial momentum distributions when jets propagate through a deconfined partonic medium created in collisions of heavy nuclei at Large Hadron Collider (LHC) energies. Jets are reconstructed with anti-k T algorithm in the pseudorapidity range $|η_{\rm jet} | < 2.1$ for resolution parameter R= 0.2, 0.3 and 0.4. For background subtraction, a Jewel-compatible 4-Momenta subtraction technique (4MomSub) have been used. The modification of inclusive jet-yields in Pb-Pb collisions relative to proton-proton interactions, quantified by $R^{\rm jet}_{AA}$, are seen to be in reasonable agreement with ALICE, ATLAS and CMS data over a broad transverse momentum range. Jewel is able to capture the qualitative features of the modifications to the fragmentation functions and radial momentum distributions in data but not always quantitatively. This quantitative discrepancy may be related to the simplified treatment of recoil partons in the background model and partly due to background subtraction procedure itself. Nevertheless, observed modification of jet shape variables in Jewel corroborates the fact that in-medium fragmentation is harder and more collimated than the fragmentation in vacuum. We further observe that these modifications depend on the transverse momentum of jets and it seems that medium resolves the core structure of low momentum jets below 100 GeV/c at LHC energies.

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arXiv2018

Centrality dependent long-range angular correlations of intermediate-pT D-mesons and charged particles in pPb collisions at the LHC energy

The high-multiplicity events of pPb collisions at $\sqrt s_{NN}$ = 5.02 TeV at the LHC exhibit unforeseen collective behaviour. One of the possible explanations to the collectivity could be the formation of thermalized partonic matter, like the one formed in relativistic nucleus-nucleus collisions and is described by the hydrodynamic models. This article presents a study on the centrality dependent long-range 2 < $|Δη|$ < 4 two-particle azimuthal correlations of D-mesons and charged particles in pPb collisions at $\sqrt s_{NN}$ = 5.02 TeV. The study has been conducted on the events, generated with the EPOS3 hydrodynamic code that reproduces most of the features of the pPb data at the LHC energy. There appears a ridge-like structure in the long-range two-particle angular correlations of D-mesons, in the intermediate pT -range, and charged particles in the simulated high-multiplicity pPb events.

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arXiv2017

Elliptic flow of $ϕ$-meson at intermediate $p_{T}$: Influence of mass versus quark number

We have studied elliptic flow ($v_{2}$) of $ϕ$-mesons in the framework of a multi phase transport (AMPT) model at LHC energy. In the realms of AMPT model we observe $ϕ$-mesons at intermediate transverse momentum ($p_{T}$) deviate from the previously observed (at RHIC) particle type grouping of $v_{2}$ according to the number of quark content i.e, baryons and mesons. Recent results from the ALICE Collaboration have shown that $ϕ$-meson and proton $v_{2}$ has a similar trend, possibly indicating that particle type grouping might be due to the mass of the particles and not the quark content. A stronger radial boost at LHC compared to RHIC seems to offer a consistent explanation to such observation. However, recalling that $ϕ$-mesons decouple from the hadronic medium before additional radial flow is build-up in the hadronic phase, similar pattern in $ϕ$-meson and proton $v_{2}$ may not be due to radial flow alone. Our study reveals that models incorporating $ϕ$-meson production from $K\bar{K}$ fusion in the hadronic rescattering phase also predict a comparable magnitude of $ϕ$-meson and proton $v_{2}$ particularly in the intermediate region of $p_{T}$. Whereas, $v_{2}$ of $ϕ$-mesons created in the partonic phase is in agreement with quark-coalescence motivated baryon-meson grouping of hadron $v_{2}$. This observation seems to provide a plausible alternative interpretation for the apparent mass-like behaviour of $ϕ$-meson $v_{2}$. We have also observed a violation of hydrodynamical mass ordering between proton and $ϕ$-meson $v_{2}$ further supporting that $ϕ$-mesons are negligibly affected by the collective radial flow in the hadronic phase due to the small in-medium hadronic interaction cross sections.

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arXiv2016

Testing of coalescence mechanism in high energy heavy ion collisions using two-particle correlations with identified particle trigger

In central Au-Au collisions at top RHIC energy, two particle correlation measurements with identified hadron trigger have shown attenuation of near side proton triggered jet-like yield at intermediate transverse momentum ($p{_T}$), 2$< p{_T} <$ 6 GeV/$\it{c}$. The attenuation has been attributed to the anomalous baryon enhancement observed in the single inclusive measurements at the same $p{_T}$ range. The enhancement has been found to be in agreement with the models invoking coalescence of quarks as a mechanism of hadronization. Baryon enhancement has also been observed at LHC in the single inclusive spectra. We study the consequence of such an enhancement on two particle correlations at LHC energy within the framework of A Multi Phase Transport (AMPT) model that implements quark coalescence as a mode of hadronization. In this paper we have calculated the proton over pion ratio and the near side per trigger yield associated to pion and proton triggers at intermediate $p{_T}$ from String Melting (SM) version of AMPT. Results obtained are contrasted with the AMPT Default (Def.) which does not include coalescence. Baryon enhancement has been observed in AMPT SM at intermediate $p{_T}$. Near side jet-like correlated yield associated to baryon (proton) trigger in the momentum region where baryon generation is enhanced is found to be suppressed as compared to the corresponding yields for the meson (pion) trigger in most central Pb-Pb events. No such effect has been found in the Default version of AMPT.

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arXiv2016

Fluctuations in Charged Particle Multiplicities in Relativistic Heavy-Ion Collisions

Multiplicity distributions of charged particles and their event-by-event fluctuations have been compiled for relativistic heavy-ion collisions from the available experimental data at Brookhaven National Laboratory and CERN and also by the use of an event generator. Multiplicity fluctuations are sensitive to QCD phase transition and to the presence of critical point in the QCD phase diagram. In addition, multiplicity fluctuations provide baselines for other event-by-event measurements. Multiplicity fluctuation expressed in terms of the scaled variance of the multiplicity distribution is an intensive quantity, but is sensitive to the volume fluctuation of the system. The importance of the choice of narrow centrality bins and the corrections of centrality bin width effect for controlling volume fluctuations have been discussed. It is observed that the mean and width of the multiplicity distributions monotonically increase as a function of increasing centrality at all collision energies, whereas the multiplicity fluctuations show minimal variations with centrality. The beam energy dependence shows that the multiplicity fluctuations have a slow rise at lower collision energies and remain constant at higher energies.

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arXiv2016

Fabrication and Characterisation of Oil-Free Large High Pressure Laminate Resistive Plate Chamber

A large (240 cm $\times$ 120 cm $\times$ 0.2 cm) oil-free High Pressure Laminate (HPL), commonly referred as "bakelite", Resistive Plate Chamber (RPC) has been developed at VECC-Kolkata using locally available P-302 OLTC grade HPL. The chamber has been operated in streamer mode using Argon, Freon(R134a) and Iso-butane in a ratio of 34:57:9 by volume. The electrodes and glue samples have been characterised by measuring their electrical parameters like bulk resistivity and surface resistivity. The performance of the chamber has been studied by measuring the efficiency, its uniformity and stability in detection of cosmic muons. Timing measurement has been performed at a central location of the chamber. The chamber showed an efficiency $>$95$\%$ and time resolution ($σ$), at the point of measurement, $\sim$0.83 ns at 9000V. Details of the material characterisation, fabrication procedure and performance studies have been discussed.

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arXiv2016

Long Term Performance Studies of Large Oil-Free Bakelite Resistive Plate Chamber

Several high energy physics and neutrino physics experiments worldwide require large-size RPCs to cover wide acceptances. The muon tracking systems in the Iron calorimeter (ICAL) in the INO experiment, India and the near detector in DUNE at Fermilab are two such examples. A (240 cm $\times$ 120 cm $\times$ 0.2 cm) bakelite RPC has been built and tested at Variable Energy Cyclotron Centre, Kolkata, using indigenous materials procured from the local market. No additional lubricant, like oil has been used on the electrode surfaces for smoothening. The chamber is in operation for $>$ 365 days. We have tested the chamber for its long term operation. The leakage current, bulk resistivity, efficiency, noise rate and time resolution of the chamber have been found to be quite stable during the testing peroid. It showed an efficiency $>$ 95$\%$ with an average time resolution of $\sim$0.83 ns at the point of measurement at 9000 V throughout the testing period. Details of the long term performance of the chamber have been discussed.

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