arXiv subjects

Niseem Magdy

Publications and source records attributed to Niseem Magdy.

At least 37 records · Page 2Linked to original sources

arXiv2022

Rebuttal to: "Comment on Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt{s}_{\rm NN}=200$ GeV"

Recently, F. Wang commented on our work "Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt{s}_{\rm NN} = 200$ GeV" and made several claims to support his conclusion that our results are fallacious. His conclusion and claims are not only incorrect; they show a fundamental disconnect with the rudiments of the $R_{Ψ_2}(ΔS)$ correlator. This rebuttal addresses the root misconception responsible for Wang's false claims.

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arXiv2022

Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV

The Anomalous Viscous Fluid Dynamics model, AVFD, is used in concert with the charge-sensitive correlator $R_{Ψ_2}(ΔS)$ to investigate the scaling properties of background- and chiral-magnetically-driven (CME) charge separation ($ΔS$), characterized by the inverse variance $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ of the $R_{Ψ_{2}}(ΔS)$ distributions obtained in collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV. The $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ values for the background are observed to be event-shape-independent. However, they scale with the reciprocal charged-particle multiplicity $(1/\left< N_{\rm ch} \right>)$, indicating an essential constraint for discerning background from the signal and a robust estimate of the difference between the backgrounds in Ru+Ru and Zr+Zr collisions. By contrast, the $\mathrm{σ^{-2}_{R_{Ψ_2}}}$values for signal + background show characteristic $1/\left< N_{\rm ch} \right>$ scaling violations that characterize the CME-driven contributions. Corrections to recent ${R_{Ψ_2}(ΔS)}$ measurements \cite{STAR:2021mii} that account for the background difference in Ru+Ru and Zr+Zr collisions indicate a charge separation difference compatible with the CME. The results further suggest that $\mathrm{σ^{-2}_{R_{Ψ_2}}}$ measurements for peripheral and central collisions in concert with $1/\left< N_{\rm ch} \right>$ scaling, provides a robust constraint to quantify the background and aid characterization of the CME.

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arXiv2021

Model study of the energy dependence of the correlation between anisotropic flow and the mean transverse momentum in Au+Au collisions

A hybrid model that employs the hadron-string transport model UrQMD and the (3+1)D relativistic viscous hydrodynamic code vHLLE, is used to investigate the beam energy dependence of the correlation coefficient $ρ(v^{2}_{2},[p_{T}])$ between the average transverse momentum $[p_{T}]$ of hadrons emitted in an event and the square of the anisotropic flow coefficient $v_2^2$. For Au+Au collisions, the model predicts characteristic patterns for the energy and event-shape dependence of the variances for $[p_{T}]$ and $v_n^2$ (${\rm Var}([p_T])$ and ${\rm Var}(v_2^2)$), and the covariance of $v_n^2$ and $[p_{T}]$ (${\rm cov}(v_{2}^{2},[p_T])$), consistent with the attenuation effects of the specific shear viscosity $η/s$. In contrast, $ρ(v^{2}_{2},[p_{T}])$ is predicted to be insensitive to the beam energy but sensitive to the initial-state geometry of the collisions. These observations suggest that a precise set of measurements for ${\rm Var}([p_T])$, ${\rm Var}(v_2^2)$, ${\rm cov}(v_{2}^{2},[p_T])$ and $ρ(v^{2}_{2},[p_{T}])$ as a function of beam-energy and event-shape, could aid precision extraction of the temperature and baryon chemical-potential dependence of $η/s$ from the wealth of Au+Au data obtained in the RHIC beam energy scan.

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arXiv2021

Investigations of the linear and non-linear flow harmonics using the A Multi-Phase Transport model

The Multi-Phase Transport model (AMPT) is used to study the effects of the parton-scattering cross-sections ($σ_{pp}$) and hadronic re-scattering on the linear contributions to the flow harmonic $\textit{v}_{4}$, the non-linear response coefficients, and the correlations between different order flow symmetry planes in Au+Au collisions at 200~GeV. The model results, which agree with current experimental measurements, indicate that the higher-order flow harmonics are sensitive to the $σ_{pp}$ variations. However, the non-linear response coefficients and the correlations between different order flow symmetry planes are $σ_{pp}$ independent. These results suggest that further detailed experimental measurements which span a broad range of collision systems and beam energies could serve as an additional constraint for the theoretical models' calculations.

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arXiv2021

Model investigations of the correlation between the mean transverse momentum and anisotropic flow in shape-engineered events

The correlation between the event mean-transverse momentum $[p_{\mathrm{T}}]$, and the anisotropic flow magnitude $v_n$, $ρ(v^{2}_{n},[p_{T}])$, has been argued to be sensitive to the initial conditions in heavy-ion collisions. We use simulated events generated with the AMPT and EPOS models for Au+Au at $\sqrt{\textit{s}_{NN}}$ = 200 GeV, to investigate the model dependence and the response and sensitivity of the $ρ(v^{2}_{2},[p_{T}])$ correlator to collision-system size and shape, and the viscosity of the matter produced in the collisions. We find good qualitative agreement between the correlators for the string melting version of the AMPT model and the EPOS model. The model investigations for shape-engineered events as well as events with different viscosity ($η/s$), indicate that $ρ(v^{2}_{2},[p_{T}])$ is sensitive to the initial-state geometry of the collision system but is insensitive to sizable changes in $η/s$ for the medium produced in the collisions. These findings suggest that precise differential measurements of $ρ(v^{2}_{2},[p_{T}])$ as a function of system size, shape, and beam-energy could provide more stringent constraints to discern between initial-state models and hence, more reliable extractions of $η/s$.

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arXiv2021

A model investigation of the longitudinal broadening of the transverse momentum two-particle correlator

The Multi-Phase Transport model (AMPT) is used to investigate the longitudinal broadening of the transverse momentum two-particle correlator $C_{2}\left(Δη,Δφ\right)$, and its utility to extract the specific shear viscosity, $η/s$, of the quark-gluon plasma formed in ultra-relativistic heavy ion collisions. The results from these model studies indicate that the longitudinal broadening of $C_{2}\left(Δη,Δφ\right)$ is sensitive to the magnitude of $η/s$. However, reliable extraction of the longitudinal broadening of the correlator requires the suppression of possible self-correlations associated with the definition of the collision centrality.

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arXiv2021

Quantification of the Chiral Magnetic Effect in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV

The Multi-Phase Transport model, AMPT, and the Anomalous Viscous Fluid Dynamics model, AVFD, are used to assess a possible chiral-magnetically-driven charge separation ($ΔS$) recently measured with the ${R_{Ψ_2}(ΔS)}$ correlator in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. The Comparison of the experimental and simulated ${R_{Ψ_2}(ΔS)}$ distributions indicates that background-driven charge separation is insufficient to account for the measurements. The AVFD model calculations, which explicitly account for CME-driven anomalous transport in the presence of background, indicate a CME signal quantified by the $P$-odd Fourier dipole coefficient ${a_1'}\approx 0.5\%$ in mid-central collisions. A similar evaluation for the $Δγ$ correlator suggests that only a small fraction of this signal ($f_{\rm CME}=Δγ_{\rm CME}/Δγ\approx 25\%$) is measurable with this correlator in the same collisions. The related prediction for signal detection in isobaric collisions of Ru+Ru and Zr+Zr are also presented.

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arXiv2021

Azimuthal dependence of two-particle transverse momentum current correlations

Two-particle transverse momentum correlation functions are a powerful technique for understanding the dynamics of relativistic heavy-ion collisions. Among these, the transverse momentum correlator $G_{2}\left(Δη,Δφ\right)$ is of particular interest for its potential sensitivity to the shear viscosity per unit of entropy density $η/s$ of the quark-gluon plasma formed in heavy-ion collisions. We use the UrQMD, AMPT, and EPOS models for Au--Au at $\sqrt{s_{\rm NN}}$ = 200~GeV and Pb--Pb at $\sqrt{s_{\rm NN}}$ = 2760~GeV to investigate the long range azimuthal dependence of $G_{2}\left(Δη,Δφ\right)$, and explore its utility to constrain $η/s$ based on charged particle correlations. We find that the three models yield quantitatively distinct transverse momentum Fourier harmonics coefficients $a^{p_{\rm T}}_{\rm n}$. We also observe these coefficients exhibit a significant dependence on $η/s$ in the context of the AMPT model. These observations suggest that exhaustive measurements of the dependence of $G_{2}\left(Δφ\right)$ with collision energy, system size, collision centrality, in particular, offer the potential to distinguish between different theoretical models and their underlying assumptions. Exhaustive analyses of $G_{2}\left(Δφ\right)$ obtained in large and small systems should also be instrumental in establishing new constraints for precise extraction of $η/s$.

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arXiv2020

Investigation of the elliptic flow fluctuations of the identified particles using the A Multi-Phase Transport model

A Multi-Phase Transport (AMPT) model is used to study the elliptic flow fluctuations of identified particles using participant and spectator event planes. The elliptic flow measured using the first order spectator event plane is expected to give the elliptic flow relative to the true reaction plane which suppresses the flow fluctuations. However, the elliptic flow measured using the second-order participant plane is expected to capture the elliptic flow fluctuations. Our study shows that the first order spectator event plane could be used to study the elliptic flow fluctuations of the identified particles in the AMPT model. The elliptic flow fluctuations magnitude shows weak particle species dependence and transverse momentum dependence. Such observation will have important implications for understanding the source of the elliptic flow fluctuations.

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arXiv2020

An extended $R^{(2)}_{Ψ_m}(ΔS_2)$ correlator for detecting and characterizing the Chiral Magnetic Wave

The extended $R^{(2)}_{Ψ_{m}}(ΔS_{2})$ correlator is presented and examined for its efficacy to detect and characterize the quadrupole charge separation ($ΔS_{2}$) associated with the purported Chiral Magnetic Wave (CMW) produced in heavy-ion collisions. Sensitivity tests involving varying degrees of proxy CMW signals injected into events simulated with the Multi-Phase Transport Model (AMPT), show that the $R^{(2)}_{Ψ_{m}}(ΔS_{2})$ correlator provides discernible responses for the background- and CMW-driven charge separation. This distinction could aid identification of the CMW via measurements of the $R^{(2)}_{Ψ_{2}}(ΔS_{2})$ and $R^{(2)}_{Ψ_{3}}(ΔS_{2})$ correlators, relative to the second- ($Ψ_2$) and third-order ($Ψ_3$) event planes. The tests also indicate a level of sensitivity that would allow for robust experimental characterization of the CMW signal.

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arXiv2020

Beam-energy and collision-system dependence of the linear and mode-coupled flow harmonics from STAR

Recent measurements and hydrodynamic model calculations suggest that the higher-order flow coefficients $v_{4}$ and $v_{5}$ have two contributions: a linear contribution driven by the initial-state eccentricities, $\varepsilon_{n}$, and a mode-coupled contribution derived from the lower-order eccentricity coefficients $\varepsilon_{2}$ and $\varepsilon_{3}$. Measurements of these two contributions to $v_{4}$ and $v_{5}$ provide crucial insights to discern initial-state models and to constrain the temperature-dependent specific shear viscosity, $η/s$, of the plasma produced in heavy-ion collisions. In this work, we have employed the two-subevents cumulant technique to provide the first beam-energy and collision-system dependence of the linear and mode-coupled contributions to the higher-order flow harmonics. Our results are shown and discussed for several centrality intervals for U+U collisions at $\mathrm{\sqrt{s_{NN}}}$= 193 GeV, Au+Au collisions at $\mathrm{\sqrt{s_{NN}}}$=200, and 54.4 GeV and Cu+Au collisions at $\mathrm{\sqrt{s_{NN}}}$=200 GeV. The results are compared with similar studies performed by the ALICE experiment at LHC.

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arXiv2020

A sensitivity study of the primary correlators used to characterize chiral-magnetically-driven charge separation

A Multi-Phase Transport (AMPT) model is used to study the detection sensitivity of two of the primary correlators -- $Δγ$ and $R_{Ψ_{2}}$ -- employed to characterize charge separation induced by the Chiral Magnetic Effect (CME). The study, performed relative to several event planes for different input "CME signals", indicates a detection threshold for the fraction $f_{\rm CME}=Δγ_{\rm CME}/Δγ$, which renders the $Δγ$-correlator insensitive to values of the Fourier dipole coefficient $a_1 \lesssim 2.5\%$, that is larger than the purported signal(signal difference) for ion-ion(isobaric) collisions. By contrast, the $R_{Ψ_{2}}$ correlator indicates concave-shaped distributions with inverse widths ($\mathrm{σ^{-1}_{R_{Ψ_2}}}$) that are linearly proportional to $a_1$, and independent of the character of the event plane used for their extraction. The sensitivity of the $R_{Ψ_{2}}$ correlator to minimal CME-driven charge separation in the presence of realistic backgrounds, could aid better characterization of the CME in heavy-ion collisions.

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arXiv2020

A method to test the coupling strength of the linear and nonlinear contributions to higher-order flow harmonics via Event Shape Engineering

A Multi-Phase Transport (AMPT) model is used to study the efficacy of shape-engineered events to delineate the degree of coupling between the linear and nonlinear contributions to the higher-order flow harmonics $v_{4}$ and $v_{5}$. The study shows that the nonlinear contributions are strongly shape-dependent while the linear contributions are shape-independent, indicating little if any, coupling between the linear and nonlinear flow coefficients. The experimental verification of such patterns could be an invaluable tool for robust extraction of the linear and mode-coupled flow coefficients, especially for beam energies where the charged particle multiplicity and the event statistics precludes the use of current methods to establish the coupling strength.

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arXiv2019

Beam-energy dependence of the azimuthal anisotropic flow from RHIC

Recent STAR measurements of the anisotropic flow coefficients (v$_{n}$) are presented for Au+Au collisions spanning the beam energy range $\sqrt{s_{NN}} = 7.7 - 200$ GeV. The measurements indicate dependences on the harmonic number ($n$), transverse momentum ($p_T$), pseudorapidity ($η$), collision centrality and beam energy ($\sqrt{s_{NN}}$) which could serve as important constraints to test different initial-state models and to aid precision extraction of the temperature dependence of the specific shear viscosity.

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arXiv2019

Influence of finite volume effect on the Polyakov Quark-Meson model

In the current work, we study the influence of a finite volume on $2+1$ $SU(3)$ Polyakov Quark-Meson model (PQM) order parameters, (fluctuations) correlations of conserved charges and the quark-hadron phase boundary. Our study of the PQM model order parameters and the (fluctuations) correlations of conserved charges indicates a sizable shift of the quark-hadron phase boundary to higher values of baryon chemical potential ($μ_{B}$) and temperature ($T$) for decreasing the system volume. The detailed study of such effect could have important implications for the extraction of the (fluctuations) correlations of conserved charges of the QCD phase diagram from heavy ion data.

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arXiv2018

Examination of the observability of a chiral magnetically-driven charge-separation difference in collisions of the $\mathrm{^{96}_{44}Ru +\, ^{96}_{44}Ru}$ and $\mathrm{^{96}_{40}Zr +\, ^{96}_{40}Zr}$ isobars at energies available at RHIC

Anomalous Viscous Fluid Dynamics (AVFD) model calculations for $\mathrm{^{96}_{44}Ru +\, ^{96}_{44}Ru}$ and $\mathrm{^{96}_{40}Zr +\, ^{96}_{40}Zr}$ collisions ($\sqrt{s_{\rm NN}} = 200$ GeV) are used in concert with a charge-sensitive correlator, to test its ability to detect and characterize the charge separation difference expected from the Chiral Magnetic Effect (CME) in these isobaric collisions. The tests indicate a larger charge separation for $\mathrm{^{96}_{44}Ru +\, ^{96}_{44}Ru}$ than for $\mathrm{^{96}_{40}Zr +\, ^{96}_{40}Zr}$ collisions, and a discernible CME-driven difference of $\sim 10$\% in the presence of realistic non-CME backgrounds. They also indicate a strategy for evaluating the relative influence of the background correlations, present for each isobar. These results suggest that charge separation measurements for these isobaric species could serve to further constrain unambiguous identification and characterization of the CME in upcoming measurements at RHIC.

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arXiv2018

Collision system and beam energy dependence of anisotropic flow fluctuations

New measurements of two- and four-particle elliptic flow are used to investigate flow fluctuations in collisions of U+U at $\sqrt{s_{NN}}$ = 193~GeV, Cu+Au at $\sqrt{s_{NN}}$ = 200~GeV and Au+Au at several beam energies. These measurements highlight the dependence of these fluctuations on the event-shape, system-size and beam energy and indicate a dominant role for initial-state-driven fluctuations. These measurements could provide further constraints for initial-state models, as well as for precision extraction of the temperature-dependent specific shear viscosity $\fracη{s}(T)$.

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arXiv2018

A New Correlator to Detect and Characterize the Chiral Magnetic Effect

A charge-sensitive in-event correlator is proposed and tested for its efficacy to detect and characterize charge separation associated with the Chiral Magnetic Effect (CME) in heavy ion collisions. Tests, performed with the aid of two reaction models, indicate discernible responses for background- and CME-driven charge separation, relative to the second- ($Ψ_{2}$) and third-order ($Ψ_{3}$) event planes, which could serve to identify the CME. The tests also indicate a degree of sensitivity which would enable robust characterization of the CME via Anomalous Viscous Fluid Dynamics (AVFD) model comparisons.

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