Direct virtual photon production in Au+Au collisions with STAR BES-II data

Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao 266237, China

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Published online: 17 April 2026

Abstract

In relativistic heavy-ion collisions, electromagnetic probes such as direct photons have the advantage of escaping from the emission sources without suffering strong interactions. Consequently, these photons, which are not from hadron decay, carry the information about the properties and dynamics of the hot and dense medium created in heavy-ion collisions. We report the measurements of the direct virtual photons from STAR experiment, based on the data collected in the Beam Energy Scan Phase II (BES-II) Program at RHIC. The p_T-integrated yields exhibit strong dependence on multiplicity and scale with (dN_ch/dη)^α, where α = 1.43 ± 0.04 (stat.) ± 0.04 (sys.). The p_T-dependent virtual photon mass spectra in √s_NN = 54.4 GeV Au+Au collisions are measured for the first time. The results reveal that the observed mass shape of virtual photons varies with p_T, indicating a transition in their production mechanism from an equilibrated system to a non-equilibrated one as p_T increases.