[Paper Review] Partonic mean-field effects on matter and antimatter elliptic flows

Taesoo Song, Salvatore Plumari+3 more|arXiv (Cornell University)|Nov 23, 2012

High-Energy Particle Collisions Research4 citations

TL;DR

This study investigates partonic mean-field effects—specifically scalar and vector interactions—on quark and antiquark elliptic flows in baryon-rich quark matter formed in Au+Au collisions at √sNN = 7.7 GeV. Using a Nambu-Jona-Lasinio (NJL) model-based transport approach, it shows that the vector mean field, which repels quarks and attracts antiquarks, induces a significant splitting in their elliptic flows, leading to enhanced v₂ for protons and suppressed v₂ for antiprotons, with the effect increasing linearly with vector coupling strength. These predictions align qualitatively with STAR Collaboration data on p–p̄ and Λ–Λ̄ differences, suggesting partonic vector interactions as a key probe of the QGP equation of state at finite baryon density.

ABSTRACT

Using a partonic transport model based on the Nambu-Jona-Lasinio model, we study the effect of scalar and vector mean fields on the elliptic flows of quarks and antiquarks in relativistic heavy ion collisions in Au+Au collisions at $\sqrt{s_{ m NN}}=7.7~$GeV and impact parameter $b=8~{ m fm}$ that leads to the production of a baryon-rich matter. Although the scalar mean field, which is attractive for both quarks and antiquarks, reduces both their elliptic flows, the vector mean field, which is repulsive for quarks and attractive for antiquarks, leads to a splitting of their elliptic flows, and this effect increases with the strength of the vector coupling in the baryon-rich quark matter. Converting quarks and antiquarks at hadronization to hadrons via the quark coalescence model, we further study the dependence of the transverse momentum integrated relative elliptic flow differences between protons and antiprotons, lambda and anti-lambdas, and positively and negatively charged kaons on the strength of the quark vector coupling. These results are then compared with the experimental data measured by the STAR Collaboration in the Beam Energy Scan program at the Relativistic Heavy Ion Collider.

Motivation & Objective

• To investigate the impact of scalar and vector mean fields in partonic matter on the elliptic flow (v₂) of quarks and antiquarks in relativistic heavy-ion collisions.
• To determine how partonic mean fields, particularly the vector interaction, influence the splitting of v₂ between particles and antiparticles in baryon-rich quark-gluon plasma.
• To compare model predictions with experimental data from the STAR Collaboration's Beam Energy Scan program at RHIC.
• To assess the potential of elliptic flow differences between particles and antiparticles as a probe of the partonic vector interaction and the equation of state of QGP at finite baryon chemical potential.

Proposed method

• A partonic transport model based on the Nambu-Jona-Lasinio (NJL) model is employed to simulate quark and antiquark dynamics in Au+Au collisions at √sNN = 7.7 GeV and b = 8 fm.
• The model incorporates both scalar mean fields (attractive for both quarks and antiquarks) and vector mean fields (repulsive for quarks, attractive for antiquarks) as effective interactions in the quark matter.
• The time and space components of the vector mean field are treated separately to assess their distinct contributions to v₂ splitting.
• Quark coalescence is applied at hadronization to convert partonic degrees of freedom into hadrons (protons, antiprotons, lambdas, antilambdas, kaons), enabling comparison with measured hadron v₂.
• The relative integrated v₂ differences between particles and antiparticles are computed and compared with STAR experimental data across varying vector coupling strengths.
• The study isolates the role of partonic vector interactions by excluding chemical reactions and hadronic mean-field effects, focusing on initial partonic dynamics.

Experimental results

Research questions

• RQ1 How do scalar and vector mean fields in baryon-rich quark matter affect the elliptic flow (v₂) of quarks and antiquarks in Au+Au collisions at √sNN = 7.7 GeV?
• RQ2 To what extent does the vector mean field—repulsive for quarks and attractive for antiquarks—induce a splitting in the v₂ of quarks and antiquarks?
• RQ3 Can the resulting particle-antiparticle v₂ differences, particularly for protons and antiprotons, be quantitatively matched to experimental data from the STAR Collaboration’s Beam Energy Scan?
• RQ4 How does the strength of the vector coupling influence the relative v₂ differences between protons and antiprotons, as well as between lambdas and antilambdas?
• RQ5 Why does the model fail to reproduce the measured v₂ difference for K⁺ and K⁻, and what additional effects might be needed to resolve this discrepancy?

Key findings

• The scalar mean field, which is attractive for both quarks and antiquarks, reduces their elliptic flows equally, consistent with previous findings in the NJL model framework.
• The vector mean field, being repulsive for quarks and attractive for antiquarks, causes a significant splitting in their v₂: quark v₂ increases while antiquark v₂ decreases, with the time component of the vector field having the dominant effect.
• The relative integrated v₂ difference between protons and antiprotons increases almost linearly with the strength of the vector coupling, reaching up to ~15% for the highest coupling values studied.
• Similarly, the relative v₂ difference between Λ and Λ̄ increases with vector coupling strength, indicating a strong sensitivity of hyperon flows to partonic vector interactions.
• In contrast, the relative v₂ difference between K⁺ and K⁻ decreases with increasing vector coupling, due to the similar vector potential acting on strange and light quarks, which suppresses the effect of differing spatial eccentricities.
• The model successfully reproduces the qualitative trend of increasing p–p̄ v₂ difference with decreasing energy but fails to match the magnitude of the K⁺–K⁻ v₂ difference, suggesting the need to include additional effects such as chemical reactions or hadronic mean fields.