arXiv subjects

Ashik Ikbal Sheikh

Publications and source records attributed to Ashik Ikbal Sheikh.

12 recordsLinked to original sources

arXiv2025

Projective Imaging of High-Energy Nuclei via Coherent Exclusive Vector Meson Production in Electron-Nucleus Collisions

One of the major goals of modern nuclear experiments is to study the distributions of gluons inside nuclei at high energy. A key measurement is the coherent exclusive vector meson (VM) production in diffractive electron-nucleus collisions, where the gluon spatial distribution inside the nucleus can be obtained through a Fourier transform of the squared nuclear momentum transfer ($|t|$) distribution. This research aims to overcome the two main obstacles of the $|t|$ measurement: limited precision in measuring $|t|$ arising from the momentum resolution of the outgoing electron and the overwhelming incoherent background. We demonstrate that by measuring the projected $|t|$ distribution along the direction perpendicular to the electron scattering plane, the effect of the outgoing electron's momentum resolution can be effectively mitigated, and the diffractive pattern is largely restored. Furthermore, we propose to measure the angular distribution of the VM's decay daughters to statistically remove the incoherent background.

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arXiv2022

Investigating the coalescence-inspired sum rule for light nuclei and hypernuclei in heavy-ion collisions

A data-driven idea is presented to test if light nuclei and hypernuclei obey the coalescence-inspired sum rule, i.e., to test if the flow of a light nucleus or hypernucleus is the summed flow of each of its constituents. Here, the mass difference and charge difference among the constituents of light nuclei and hypernuclei are treated appropriately. The idea is applied to the available data for $\sqrt{s_{NN}} = 3$ GeV fixed-target Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC), published by the STAR collaboration. It is found that the sum rule for light nuclei is approximately valid near mid-rapidity ($-0.3 < y < 0$), but there is a clear violation of the sum rule at large rapidity ($y < -0.3$). The Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, generates a similar pattern as obtained from the experimental data. In the present approach, the rapidity dependence of directed flow of the hypernuclei $\mathrm{_Λ^{3}H}$ and $\mathrm{_Λ^{4}H}$ is predicted in a model-independent way for $\sqrt{s_{NN}} = 3$ GeV Au+Au collisions, which will be explored by ongoing and future measurements from STAR.

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arXiv2022

The splitting of directed flow for identified light hadrons ($K$ and $p$) and strange baryons ($Ξ$ and $Ω$) in Au+Au collisions at STAR

The first measurements for rapidity-odd directed flow of $Ξ$ and $Ω$ in Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}=$ 27 and 200 GeV are reported. The coalescence sum rule is examined with various combinations of hadrons where all constituent quarks are produced, such as $K^{\!^-}\!\!({ \bar{u}}{s})$, $\bar{p}({\bar{u}\bar{u}}{\bar{d}})$, $\barΛ(\bar{u}\bar{d}\bar{s})$, $ϕ({s}{\bar{s}})$, $\barΞ^{+}(\bar{d}\bar{s}\bar{s})$, $Ω^{-}(sss)$, and $\barΩ^{+}(\bar{s}\bar{s}\bar{s})$. For such combinations, a systematic violation of the sum rule is observed with increasing difference in the electric charge and the strangeness content of the combinations. Measurements are compared with the calculations of A Multi-Phase Transport (AMPT) model and Parton-Hadron String Dynamics (PHSD) model with electromagnetic (EM) field. The PHSD model with EM field agrees with the measurements within uncertainties.

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arXiv2022

Effect of thermal gluon absorption and medium fluctuations on heavy flavour nuclear modification factor at RHIC and LHC energies

The presence of thermal gluons reduces the stimulated gluon emission off a heavy quark propagating in the quark-gluon plasma (QGP) while absorption causes reduction of the radiative energy loss. On the other hand, the chromo-electromagnetic field fluctuations present in the QGP lead to collisional energy gain of the heavy quark. The net effect of the thermal gluon absorption and field fluctuations is a reduction of the total energy loss of the heavy quark, prominent at the lower momenta. We consider both kind of the energy gains along with the usual losses, and compute the nuclear modification factor ($R_{AA}$) of heavy mesons, viz., $D$ and $B$ mesons. The calculations have been compared with the experimental measurements in Au-Au collisions at $\sqrt{s_{NN}} = 200$ GeV from STAR and PHENIX experiments at the RHIC and Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV and $5.02$ TeV from CMS and ALICE experiments at the LHC. We find a significant effect of the total energy gain due to thermal gluon absorption and field fluctuations on heavy flavour suppression, especially at the lower transverse momenta.

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arXiv2022

Testing the impact of electromagnetic fields on the directed flow of constituent quarks in heavy-ion collisions

It has been proposed that strong electromagnetic fields produced in the early stages of heavy-ion collisions can lead to splitting of the rapidity-odd directed flow of positive and negative hadrons. For light hadrons, the interpretation of such measurements is complicated by the low magnitude of directed flow as well as by ambiguities arising from transported quarks. To overcome these complications, we propose measurements using only hadrons carrying produced quarks ($\bar{u},\bar{d},s,\bar{s}$). We discuss how to identify the kinematics where such hadrons are produced via the coalescence mechanism and therefore their flow is the sum of the flow of their constituent quarks. With this sum rule verified for certain combinations of hadrons, the expected systematic violation of this rule with increasing electric charge can be measured, which could be a consequence of the electromagnetic fields produced in the collisions. Our approach can be tested with the high statistics data from Phase II of the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC).

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arXiv2021

Study of medium modified jet shape observables in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV using EPOS and JEWEL event generators

The jet-medium interaction in high energy heavy ion collisions is an important phenomena to characterize the hot and dense medium produced in such collisions. The study of medium-induced modifications to the substructure of inclusive charged jets indicates a redistribution of energy inside the jet cone and provides insight into the energy loss mechanisms of jets in the medium. We investigate the in-medium modification to two jet shape observables i.e., the differential jet shape ($ρ$(r)) and the angularity (g) in the most central $Pb-Pb$ collisions at $\sqrt{s_{NN}} ~=~ 2.76 $ TeV using two commonly used event generators i.e., JEWEL (recoil OFF) and EPOS-3 in the jet-p$_T$ range of 20-40 GeV/c. JEWEL with recoil OFF has been used primarily as a reference system as that has been found to explain the global jet observables satisfactorily but lacks in jet-shape variables at higher jet-radii. EPOS-3 that explains the bulk properties in such collisions quite well takes into account a hydrodynamically evolving bulk matter, jets and hard-soft interactions. A comparison between the results from these models shows that while JEWEL (recoil OFF) does not explain the distribution of lost energy at higher radii with respect to the jet-axis, EPOS-3 explains the effect quite well. However, in EPOS-3, the partonic energy loss mechanism and secondary hard-soft interactions during hadronization and hadronic cascade phase are different from the conventional jet energy loss models. The current study can, therefore, provide important new insights on mechanisms regarding the modeling of the medium and hard-soft interactions in heavy ion collisions.

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arXiv2020

Measurements of heavy-flavor jets with ALICE at the LHC

Heavy quarks created in ultra-relativistic heavy-ion collisions are mostly produced in hard QCD processes during the early stages of the reaction. They interact with the hot and cold nuclear matter throughout the evolution of the medium via semi-hard and soft processes such as energy loss via gluon radiations and collisions. Nuclear modification of heavy flavors in p-A systems provides insight into cold nuclear matter effects such as (anti)shadowing and $k_{T}$-broadening, and serves as a baseline for A-A studies. In addition to that the fully reconstructed heavy-flavor jets provide additional information on the flavor (or mass) dependence of fragmentation, color charge effects as well as insight into the contribution of late gluon splitting. In this contribution, we present the measurements of $b$-jet production in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV and $c$-jet production in pp, p-Pb and Pb-Pb collisions measured by the ALICE experiment at the LHC. The measurements of the nuclear modification factors for $c$-jet in p-Pb and Pb-Pb collisions are also presented. The experimental measurements are compared with the results from Monte Carlo event generators (PYTHIA 6, PYTHIA 8 and Herwig 7) and the NLO pQCD calculations (POWHEG+PYTHIA6). We find good agreement of the measurements with the results from Monte Carlo event generators and from NLO pQCD calculations.

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arXiv2019

Impact of the chromo-electromagnetic field fluctuations on transport coefficients of heavy quarks and shear viscosity to entropy density ratio of quark-gluon plasma

The chromo-electromagnetic field fluctuations in the quark-gluon plasma (QGP) play an important role as these field fluctuations result energy gain of heavy quarks. We consider these fluctuations and evaluate the transport coefficients, e.g., drag and diffusion coefficients of charm quarks and shear viscosity to entropy density ratio ($η/s$) of the QGP. We find a significant effect of such fluctuations on the transport coefficients. These fluctuations cause a reduction of the drag and diffusion coefficients. We also observe that the shear viscosity to entropy density ratio of the QGP is closer to the value obtained in Lattice QCD (LQCD) and functional renormalization group calculations when the effects of such fluctuations are included.

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arXiv2019

The role of the stochastic color field fluctuations on $J/ψ$ suppression in ultra-relativistic heavy-ion collisions

We consider the effect of the stochastic color field fluctuations in addition to the collisional as well as the radiative energy losses in the propagation of charm quarks in the hot and dense deconfined medium of quarks and gluons created in ultra-relativistic heavy-ion collisions. These fluctuations lead to energy gain of the propagating charm quarks. We construct and solve Langevin transport equations for charm quarks under the evolving background matter described by the ($3+1$)-D relativistic viscous hydrodynamics. Considering the energy gain of charm quarks, the nuclear modification factor $R_{AA}$ of $J/ψ$ is calculated. Interestingly, the experimental measurements for $J/ψ$ suppression in $Au-Au$ collisions at $\sqrt{s_{NN}} = 200$ GeV by PHENIX Collaboration and $Pb-Pb$ collisions at $\sqrt{s_{NN}} = 2.76$ TeV by ALICE and CMS Collaborations, can be nicely described with the effect of these field fluctuations without invoking the regeneration phenomena.

hep-ph↗

arXiv2019

Heavy flavour Langevin diffusion with the chromo-electromagnetic field fluctuations in the quark-gluon plasma

The chromo-electromagnetic field is produced due to the motion of partons in a quark-gluon plasma created by relativistic heavy-ion collisions. The fluctuations in the produced chromo-electromagnetic field are important, since they cause heavy quarks to gain energy in the low velocity limit. We study the effect of such fluctuations on heavy quark diffusion in quark-gluon plasma within the framework of Langevin dynamics under the background matter described by the ($3+1$)-dimensional relativistic viscous hydrodynamics. Theoretical calculations of the nuclear modification factor ($R_{AA}$) of heavy mesons ($D$ and $B$ mesons), with the effect of these fluctuations, are compared with experimental measurements in $Au-Au$ collisions at $\sqrt{s_{NN}} = 200$ GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider (RHIC) and $Pb-Pb$ collisions at $\sqrt{s_{NN}} = 2.76$ and $5.02$ TeV by the ALICE and CMS experiments at the CERN Large Hadron Collider (LHC). We find a significant effect of these fluctuations in describing the the measured $R_{AA}$ of $D$ and $B$ mesons in both RHIC and LHC energies.

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arXiv2018

Characterization of quark gluon plasma as seen through the energy loss of heavy quarks

The shear viscosity to entropy density ratio (η/s) of quark gluon plasma produced in ultra- relativistic heavy-ion collisions has been studied from the energy loss of heavy quarks in QGP medium. We have also studied the bulk viscosity to entropy density ratio (ζ/s) and fluidity measure(F) of the medium using the obtained η/s values. In addition to that, we have estimated the heavy quark bound state potential (V) inside this medium. Our finding of η/s agrees well with the results obtained by Lattice QCD (LQCD) and functional renormalization group technique.

hep-ph↗

arXiv2017

Effect of the chromo-electromagnetic field fluctuations on heavy quark propagation at the LHC energies

We consider the effect of the chromo-electromagnetic field fluctuations in addition to the collisional as well as the radiative energy loss suffered by heavy quarks while propagating through the hot and densed deconfined medium of quarks and gluons created in relativistic heavy ion collisions. The chromo-electromagnetic field fluctuations play an important role as it leads to an energy gain of heavy quarks of all momentum, significantly effective at the lower momentum region. With this, we have computed, for the first time, the nuclear modification factor (R_{AA}) of heavy mesons, viz., D-mesons and B-mesons and compared with the those experimental measurements in Pb-Pb collisions at \sqrt{s_{NN}} = 2.76 TeV and \sqrt{s_{NN}} = 5.02 TeV by the CMS and ALICE experiments at the LHC. Our results are found to be in very good agreement with those available data measured by CMS and ALICE experiments.

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