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Two-photon production of dilepton pairs in peripheral heavy ion collisions

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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 pT<150MeV/c and are most prominent in peripheral gold-gold and uranium-uranium collisions. The pairs exhibit a peak at the J/ψ mass, but include a wide continuum, with pair invariant masses from 400MeV/c2 up to 2.6GeV/c2. The ALICE Collaboration observes a similar excess in peripheral lead-lead collisions, but only at the J/ψ 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 pT be greater than 1 GeV/c; this eliminated almost all of the pairs from two-photon interactions, while leaving most of the J/ψ decays.

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