International Journal of Modern Physics E

The chiral magnetic effect (CME) is a collective quantum phenomenon that arises from the interplay between gauge field topology and fermion chiral anomaly, encompassing a wide range of physical systems from semimetals to quark-gluon plasma. This paper, with a focus on CME and related effects in heavy ion collisions, aims to provide an introductory discussion on its conceptual foundation and measurement methodology, a timely update on the present status in terms of experimental findings and theoretical progress, as well as an outlook into the open problems and future developments.

The strongly interacting system created in ultrarelativistic nuclear collisions behaves almost as an ideal fluid with rich patterns of the velocity field exhibiting strong vortical structure. Vorticity of the fluid, via spin-orbit coupling, leads to particle spin polarization. Due to the finite orbital momentum of the system, the polarization on average is not zero. It depends on the particle momenta reflecting the spatial variation of the local vorticity. In the last few years, this field experienced a rapid growth due to experimental discoveries of the global and local polarizations. Recent measurements triggered further development of the theoretical description of the spin dynamics and suggestions of several new mechanisms for particle polarization. In this review, we focus mostly on the experimental results. We compare the measurements with the existing theoretical calculations but try to keep the discussion of possible underlying physics at the qualitative level. Future measurements and how they can help to answer open theoretical questions are also discussed. We pay a special attention to the employed experimental methods, as well as to the detector effects and associated corrections to the measurements.

The Large Hadron Collider (LHC) at CERN has a plan to have Oxygen–Oxygen (O+O) collisions at $\sqrt{s_{\mathrm{\text{NN}}}}=7$TeV collision energy in the forthcoming run. As the system size of $\mathrm{\text{O+O}}$ collisions has the final state multiplicity overlap with those produced in PbPb, p+Pb, and pp collisions, it becomes exciting to study thermodynamic quantities using the Color String Percolation Model (CSPM). The thermodynamic quantities like temperature, shear viscosity to entropy density ratio, and trace anomaly are obtained from the soft region of the transverse momentum spectra of O+O collisions at $\sqrt{s_{\mathrm{\text{NN}}}}=7$TeV. The percolation approach within CSPM can be successfully used to describe the initial stages in high-energy heavy ion collisions in the soft region in high-energy heavy ion collisions. The obtained results for O+O collisions are compared with the published results from pp and AA collisions using ALICE data.

The possible appearance of the effects of local parity breaking in the QCD medium formed in heavy ion collisions due to violation of chiral symmetry can be quantified by corresponding chiral chemical potential ${\mu }_5$. The experimental observables sensitive to the effects of local parity violation in strong interaction include search for polarization splitting of the ${\rho }_0$ and ${\omega }_0$ mesons via angular dependence of spectral functions in their decay to leptons. In this paper, we study the space-time evolution and fluctuations of ${\mu }_5$ using relativistic hydrodynamics and estimate their effect on the light vector meson polarization splitting in Pb–Pb collisions at LHC energy.