Out-of-equilibrium chiral magnetic effect from chiral kinetic theory

Abstract

Recently there has been significant interest in the macroscopic manifestation of chiral anomaly in many-body systems of chiral fermions. A notable example is the Chiral Magnetic Effect (CME). Enthusiastic efforts have been made to search for the CME in the quark-gluon plasma created in heavy ion collisions. A crucial challenge is that the extremely strong magnetic field in such collisions may last only for a brief moment and the CME current may have to occur at so early a stage that the quark-gluon matter is still far from thermal equilibrium. This thus requires modeling of the CME in an out-of-equilibrium setting. With the recently developed theoretical tool of chiral kinetic theory, we make a first phenomenological study of the CME-induced charge separation during the pre-thermal stage in heavy ion collisions. The effect is found to be very sensitive to the time dependence of the magnetic field and also influenced by the initial quark momentum spectrum as well as the relaxation time of the system evolution toward thermal equilibrium. Within the present approach, such pre-thermal charge separation is found to be modest.

Mapping the phases of quantum chromodynamics with beam energy scan

“…Also, it allows the study of such anomalous transport even far from equilibrium, see e.g. recent examples in [151,152,156]. Currently the exploration of the transport theory for chiral fermions is under intensive development, aiming to fully develop its conceptual foundation as well as various applications.…”

“…[174], MS-1 and MS-100 from [175], and GKR from [176]. The middle panel is adapted from [177] and the right panel is adapted from [152].…”

“…The EBE-AVFD includes fluctuating initial conditions as well as a hadronic cascade stage via UrQMD after the hydrodynamic evolution which accounts for the influence of hadronic re-scatterings and resonance decays. Further improvements of the framework shall address a number of issues, including more accurate determination of axial charge initial conditions [62,63], the fluctuation and dissipation of axial charge during the hydrodynamic evolution [347][348][349][350], as well as the possible generation of the CME current in the pre-equilibrium stage [152].…”

Probing the topological charge in QCD matter via multiplicity up–down asymmetry

“…This so-called chiral magnetic effect (CME) [5][6][7], which has been observed in condensed matter systems such as the Weyl semimetals in external magnetic fields [8] and studied in other areas of physics [9], has been suggested as a possible explanation for the observed charge separation in experiments [10][11][12][13]. Realistic studies of this effect based on anomalous hydrodynamics [14][15][16][17] and chiral kinetic approach [18,19] require a magnetic field of lifetime of at least τ B = 0.6 fm/c to account for the experimental data. Such a long-lived magnetic field does not seem to be supported by the small electric conductivity of QGP from lattice QCD calculations [20,21].…”

Out-of-equilibrium UA(1) symmetry breaking in electromagnetic fields

“…Considering that the parton system in the initial stage is in nonequilibrium state, the interaction between the partons and the electromagnetic fields should be described in the frame of a kinetic theory. Recently, a quantum kinetic equation to describe the chiral fermion motion in electromagnetic fields is developed via different methods and applied to heavy ion collisions [27][28][29][30][31]. In this paper, we study the out-ofequilibrium U A ð1Þ symmetry breaking in electromagnetic fields and its relation to the chiral symmetry breaking in the frame of Wigner function formalism [32][33][34][35].…”