Direct virtual photon production in Au+Au collision at 27 and 54.4 GeV

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

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Published online: 5 November 2025

Abstract

As electromagnetic probes, photons have the advantage of escaping unimpeded from their emission source, allowing them to carry valuable information about the properties and dynamics of the hot quantum chromodynamics (QCD) medium created in heavy-ion collisions. Particularly, the transverse momentum distribution of direct virtual photons emitted from the hot QCD medium exhibits sensitivity to the system temperature. Direction photons from thermal radiation in the low transverse momentum region are expected to be a quark-gluon plasma (QGP) thermometer. However, the photon from hadronic decays make the extraction of the thermal radiation contribution very challenging.

The STAR experiment has recorded large datasets of Au+Au collisions in the Beam Energy Scan Phase-II (BES-II) program, spanning center-of-mass energies of √s_NN = 3 - 54.4 GeV. In these proceedings, we present the latest direct virtual photon measurement in Au+Au collisions at √s_NN = 27 and 54.4 GeV, including p_T differential invariant yields and total yields in different centrality bin. The total yields increase rapidly with increasing dN_ch/dη. Notably, it scales with (dN_ch/dη)^α, where α = 1.46 ± 0.07.