The STAR collaboration has observed an excess production of $e^+e^-$ pairs in relativistic heavy ion collisions, over the expectations from hadronic production models. The excess pairs have transverse momenta $p_T < 150\ { \rm MeV}/c$ and are most prominent in peripheral gold-gold and uranium-uranium collisions. The pairs exhibit a peak at the $J/\psi$ mass, but include a wide continuum, with pair invariant masses from 400 MeV/c$^2$ up to 2.6 GeV/c$^2$. The ALICE Collaboration observes a similar excess in peripheral lead-lead collisions, but only at the $J/\psi$ mass, without a corresponding continuum. This paper presents a calculation of the cross-section and kinematic for two-photon production of $e^+e^-$ pairs, and find general agreement with the STAR data. The calculation is based on the STARlight simulation code, which is based on the Weizs\"acker-Williams virtual photon approach. The STAR continuum observations are compatible with two-photon production of $e^+e^-$ pairs. The ALICE analysis required individual muon $p_T$ be greater than 1 GeV/c; this eliminated almost all of the pairs from two-photon interactions, while leaving most of the $J/\psi$ decays.
Categories: nucl-th hep-ph