相对论重离子碰撞中正反粒子椭圆流的劈裂

Elliptic flow splittings between particles and their antiparticles in relativistic heavy-ion collisions

相对论重离子碰撞是研究极端高温高密条件下新物质形态及量子色动力学相图的重要实验手段,而椭圆流是相对论重离子碰撞的重要实验观测量.近年来在美国布鲁克海文国家实验室的相对论重离子对撞机上进行束流能量扫描实验,研究人员发现了一些异于极端相对论重离子碰撞的行为,比如正反粒子椭圆流的劈裂.基于拓展的多相输运模型,可以通过正反粒子的不同平均场势来解释该实验现象,并可据此提取高重子及同位旋化学势下夸克物质的状态方程及相图信息.

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.The elliptic flow is an important experimental observable in relativistic heavy-ion collisions.In recent years,the beam-energy scan program has been carried out on the relativistic heavy-ion collider at Brookhaven National Laboratory in USA,where phenomena different from those in heavy-ion collisions at extremely high energies,such as the elliptic flow splittings between particles and their antiparticles,are observed.Based on an extended multiphase transport model,this phenomenon can be explained through different mean-field potentials for particles and their antiparticles.In the almond-shaped medium produced in these collisions,particles affected by the repulsive potential are more likely to leave the system,while those affected by the attractive potential are more likely to be trapped in the system,enhancing and reducing their respective elliptic flow.The partonic mean-field potentials are extracted from an extended 3-flavor Nambu-Jona-Lasinio(NJL)model.Quarks are affected by more repulsive potentials compared with their antiquarks as a result of the vector-isoscalar coupling,while d quarks are affected by a more repulsive potential compared with u quarks as a result of the vector-isovector coupling,in the baryon-rich and d-quark-rich medium.The hadronic mean-field potentials are also incorporated through the relativistc mean-field model for baryons and antibaryons,and through the chiral effective field theory for kaons and antikaons as well as forπ^+andπ^-.In the baryon-rich and neutron-rich medium,antinucleons,antikaons,andπ^+are affected by more attractive potentials than nucleons,kaons,andπ^-,respectively.It is found that a strong vector-isoscalar and vectorisovector coupling are needed to reproduce the elliptic flow splittings between nucleons and antinucleons,kaons and antikaons,as well asπ^+andπ^-,as observed experimentally by the STAR Collaboration.On the other hand,the NJL model can be used to study the spontaneous chiral symmetry breaking and the phase diagram of the chiral phase transition.In order to describe both the chiral and deconfinement phase transition,one needs to extend the NJL model by incorporating the Polyakov-loop contribution(p NJL).Although the temperature of the crtical point for the chiral phase transition is higher in the p NJL model,it is found that the position or the behavior of the critical point is sensitive to the strength of the vector-isoscalar and the vector-isovector couplings in both the NJL and the p NJL model.In addition,the equation of state of baryon-rich and d-quark-rich quark matter is sensitive to these couplings as well.With these couplings constrainted by the elliptic flow splittings between particles and their antiparticles,information of the quark matter equation of state as well as the QCD phase diagram at finite baryon and isospin chemical potentials can be thus extracted.The study shows that the QCD critical point may be at very low temperatures or does not even exist,while the strong vector-isoscalar and vectorisovector couplings favor a stiff equation of state of quark matter at high net-baryon and isovector densities.In future studies,the transport framework can be further extended to incorporate the Polyakov-loop contribution.