Seminar in Hadronic Physics

The ultra-strong electromagnetic field in heavy-ion collisions and its impact on the directed flow of charged particles in the STAR experiment

Diyu Shen

Fudan University

Non-central heavy-ion collisions can generate an ultra-strong magnetic field, on the order of 10¹⁸ Gauss, the evolution of which in the quark gluon plasma (QGP) is critical to some novel phenomena related to the restoration of fundamental symmetries of quantum chromodynamics (QCD), such as the chiral magnetic effect (CME). However, direct evidence of the electromagnetic field in the QGP is elusive because the magnetic field drops rapidly with time in the vacuum until the QGP is formed. Since the collective motion of oppositely charged quarks can be affected by the electromagnetic force, it was proposed that the directed flow of charged particles could serve as the probe to detect the evolution of the electromagnetic field in the QGP. In this report, I will present recent experimental measurements of the charge-dependent directed flow in heavy-ion collisions. While the observed charge splitting of directed flow of pions, kaons and protons in central collisions can be explained by the u and d quarks transported from initial-state nuclei, results in peripheral collisions reveal the impacts of the electromagnetic field on the QGP. The comparison between experimental data and theoretical calculations indicates that the electrical conductivity of the QGP given by lattice QCD falls within a reasonable range.