Chiral magnetic and vortical effects in high-energy nuclear collisions—A status report

Abstract

The interplay of quantum anomalies with magnetic field and vorticity results in a variety of novel non-dissipative transport phenomena in systems with chiral fermions, including the quarkgluon plasma. Among them is the Chiral Magnetic Effect (CME) -the generation of electric current along an external magnetic field induced by chirality imbalance. Because the chirality imbalance is related to the global topology of gauge fields, the CME current is topologically protected and hence non-dissipative even in the presence of strong interactions. As a result, the CME and related quantum phenomena affect the hydrodynamical and transport behavior of strongly coupled quark-gluon plasma, and can be studied in relativistic heavy ion collisions where strong magnetic fields are created by the colliding ions. Evidence for the CME and related phenomena has been reported by the STAR Collaboration at Relativistic Heavy Ion Collider at BNL, and by the ALICE Collaboration at the Large Hadron Collider at CERN. The goal of the present review is to provide an elementary introduction into the physics of anomalous chiral effects, to describe the current status of experimental studies in heavy ion physics, and to outline the future work, both in experiment and theory, needed to eliminate the existing uncertainties in the interpretation of the data. Superconductors also demonstrate the deep link between topology and non-dissipative currents.Since this link is of crucial importance for our discussion, let us elaborate on it by using superconductor as an example. Around the Abrikosov vortex, there exists a supercurrent that screens the magnetic field of the vortex in the bulk. The corresponding physics is captured by the London relation between the electric current and gauge potential (∇ · A = 0):(1)

One-loop QCD thermodynamics in a strong homogeneous and static magnetic field

“…Similarly the unitary matrix needed to thermalize the gluon propagator in the vacuum is given by distribution function, 16) where the distribution function for gluons is given by…”

“…Moreover the magnetic field may be assumed uniform because even though the spatial distribution of the magnetic field is globally inhomogeneous, but in the central region of the overlapping nuclei, the magnetic field in the transverse plane varies very smoothly, which is noticed in the hadron-string simulations [9] for Au-Au collisions at √ s N N = 200 GeV with an impact parameter, b = 10 fm. Therefore, a large number of QCD related phenomena are investigated in the strong and homogeneous magnetic field, such as the chiral magnetic effect related to the generation of electric current parallel to the magnetic field due to the difference in number of right and left-handed quarks [10][11][12], the axial magnetic effect due to the flow of energy by the axial magnetic field [13,14], the chiral vortical effect due to an effective magnetic field in the rotating QGP [15,16], the magnetic catalysis and the inverse magnetic catalysis at finite temperature arising due to the breaking and the restoration of the chiral symmetry [17][18][19][20][21], the thermodynamic properties [22][23][24], the refractive indices and decay constant [25,26] of mesons in a hot magnetized medium, the conformal anomaly and the production of soft photons [27,28] at RHIC and LHC, the dispersion relation in a magnetized thermal QED [29], the synchrotron radiation [30], the dilepton production from both the weakly [31][32][33][34] and the strongly [35] coupled plasma etc.…”

Chiral magnetic effect search in p+Au, d+Au and Au+Au collisions at RHIC

“…In relativistic heavy-ion collisions, this can lead to observable electric charge separation along the direction of the strong magnetic field produced by spectator protons [4][5][6]. This is called the chiral magnetic effect (CME).…”

“…The measurements of the charge separation can provide a means to studying the non-trivial QCD topological structures. Extensive theoretical and experimental efforts have been devoted to the search for CME [6] .…”

“…This indicates significant background contributions in Pb+Pb collisions at LHC energy. It is predicted that the CME would decrease with the collision energy due to the more rapidly decaying B at higher energies [6]. Hence, the similarity between small-system and heavy-ion collisions at the LHC may be expected, and the situation at RHIC could be different [6].…”