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 $p_T<150\mathrm{MeV}/c$ 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 $400{\mathrm{MeV}/c}^2$ up to $2.6{\mathrm{GeV}/c}^2$. 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 Weizsäcker-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.

• Received 12 January 2018

DOI:https://doi.org/10.1103/PhysRevC.97.054903