Probing the production mechanisms of virtual photons at RHIC

Abstract

We report the measurement of direct virtual photon ($\gamma^*_\text{dir} \rightarrow e^+e^-$) in four Au+Au collision energies at $\sqrt{s_\text{NN}}$ = 14.6--54.4 GeV. The $\gamma^*_\text{dir}$ yields scale with charged-particle multiplicity ($(dN_{\mathrm{ch}}/d\eta)^{\alpha}$) as $\alpha=1.43 \pm 0.04 \text{(stat.)} \pm 0.03\text{(sys.)}$, independent of collision energies and centralities, exceeding hadronic expectations and indicating notable contribution from QGP phase. At $\sqrt{s_{\rm NN}}=$ 54.4 GeV, the direct virtual photon mass distributions are further studied over $0.1<M_{ee}<1.1$ GeV/$c^2$, and are found to evolve from exponential to power-law form with increasing $p_\text{T}$, signaling a shift from"blackbody"radiation to emission governed by Kroll-Wada formalism.