Charmonia Production in Hot QCD Matter and Electromagnetic Fields

Abstract

Both hot QCD matter and extremely strong electromagnetic fields are generated in relativistic heavy-ion collisions. We employ the transport model and the equivalent photon approximation (EPA) to study charmonium hadroproduction and photoproduction in nucleus-nucleus collisions, respectively. In photoproduction, quasi-real photons may interact with the whole nucleus or individual nucleons, which is called the coherent and incoherent processes, respectively. The typical momentum of charmonium produced in two processes is located in $p_T\lesssim 1/R_A$ and $p_T\lesssim 1/R_N$, where $R_A$ and $R_N$ are the radii of nucleus and the nucleon. Both kinds of photoproduction and also hadroproduction are considered to calculate charmonium production in different transverse momentum bins, rapidity bins, and collision centralities, incorporating modifications from hot QCD matter and initial cold nuclear matter effects. Our calculations explain experimental data about charmonium nuclear modification factors and the production cross-section in ultra-peripheral collisions. Charmonium nuclear modification is far above the unit at extremely low $p_T$ ($p_T < 0.1$ GeV/c) in peripheral collisions with centrality 70-90\%, attributed to coherent photoproduction.