Elliptic flow splitting of charged pions in relativistic heavy-ion collisions
Authors: Liu He , Chu Peng-Cheng ,   Science School, Qingdao University of Technology, Qingdao 266000, China,   Research Center of Theoretical Physics, Qingdao University of Technology, Qingdao 266033, China
Published: May 2023

Relativistic heavy-ion collisions are an important experimental way of studying the new state of matter as well as the phase diagram of the quantum chromodynamics (QCD) under extremely high temperatures and high densities. In recent years, the beam-energy scan program has been carried out on the relativistic heavy-ion collider at Brookhaven National Laboratory in USA, and the STAR collaboration at relativistic heavy ion collision (RHIC) has measured the difference in the elliptic flow <inline-formula><tex-math id="M5">\begin{document}$v_2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M5.png"/></alternatives></inline-formula> between <inline-formula><tex-math id="M6">\begin{document}$\pi^-$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M6.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M7">\begin{document}$\pi^+$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M7.png"/></alternatives></inline-formula> on an event-by-event basis, and found a linear dependence on the charge asymmetry <i>A</i><sub>ch</sub> of the collision system, which is considered as a possible signal of the chiral magnetic wave. Based on the extended multi-phase transport model (AMPT), this paper uses a 3 flavor Nambu-Jona-Lasinio (NJL) model to study the quark isospin Mean-field potential, which provides a new idea for explaining the experimental phenomenon of the linear relationship between the pion elliptic flow splitting <inline-formula><tex-math id="M8">\begin{document}$\Delta v_2=v_2(\pi^-)- $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M8.png"/></alternatives></inline-formula><inline-formula><tex-math id="M8-1">\begin{document}$ v_2(\pi^+)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M8-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M8-1.png"/></alternatives></inline-formula> and charge asymmetry <i>A</i><sub>ch</sub>. Our results show that isovector interaction can cause a splitting of the isospin asymmetric quark matter mean-field potential, manifesting as <inline-formula><tex-math id="M9">\begin{document}$d(\bar{u})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M9.png"/></alternatives></inline-formula> quarks experiencing a mean-field potential greater than <inline-formula><tex-math id="M10">\begin{document}$u(\bar{d})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M10.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M10.png"/></alternatives></inline-formula> quarks. Therefore, <inline-formula><tex-math id="M11">\begin{document}$d(\bar{u})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M11.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M11.png"/></alternatives></inline-formula> quarks experience more repulsion than <inline-formula><tex-math id="M12">\begin{document}$u(\bar{d})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M12.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M12.png"/></alternatives></inline-formula> quarks in collision processes, resulting in a small increase in the elliptic flow of the partial subflow <inline-formula><tex-math id="M13">\begin{document}$v_2(d)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M13.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M13.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M14">\begin{document}$v_2(\bar{u})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M14.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M14.png"/></alternatives></inline-formula>, while <inline-formula><tex-math id="M15">\begin{document}$v_2(u)$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M15.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M15.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M16">\begin{document}$v_2(\bar{d})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M16.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M16.png"/></alternatives></inline-formula> decrease slightly. In the hadronization process, the <inline-formula><tex-math id="M17">\begin{document}$\pi$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M17.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M17.png"/></alternatives></inline-formula> elliptic flow splitting occurs due to the split of <inline-formula><tex-math id="M18">\begin{document}$d$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M18.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M18.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M19">\begin{document}$u$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M19.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M19.png"/></alternatives></inline-formula> elliptic flows combined with the split of <inline-formula><tex-math id="M20">\begin{document}$\bar{u}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M20.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M20.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M21">\begin{document}$\bar{d}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M21.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M21.png"/></alternatives></inline-formula> elliptic flows. We also found a linear correlation between charge asymmetry and isospin asymmetry at mid-rapidity region from our transport model, and thus explained the experimental phenomenon of the linear relationship between the pion elliptic flow splitting <inline-formula><tex-math id="M22">\begin{document}$\Delta v_2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M22.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230454_M22.png"/></alternatives></inline-formula> and charge asymmetry <i>A</i><sub>ch</sub> by using the isospin mean-field potential of quark matter. Further, isospin properties of quark matter also provide a theoretical basis for isobar collisions and the equation of state of compact star matter.