Enhancements of electron-positron pair production at very low transverse momentum in peripheral hadronic A + A collisions

Abstract

The STAR collaboration has observed an excess production of electron-positron pairs which have transverse momenta p⊥< 150 MeV/c in peripheral gold-gold and uranium-uranium collisions. ALICE has also reported on an excess of μ+μ− pairs at low p⊥ in very peripheral (70–90% centrality) lead-lead collisions at a center-of-mass energy of 2.76 TeV/nucleon pair. These excesses cannot be explained by the QGP thermal radiation and ρ in-medium broadening calculations. This is a sign of coherent photon–photon interactions in ultra-peripheral collisions. However, the number of lepton pair production is more than the predictions from hadronic production models. In literature, there are number of explanations about the p⊥ broadening, however none of them has included the multi-pair production processes in their calculations. The aim of this paper is to show that at RHIC and LHC energies and for small impact parameters unitarity is violated and lowest order calculations are not adequate to calculate electromagnetic lepton pair production. Therefore, we have to include higher order effects in the calculations. We have shown that electron-positron multi-pair production cross section is large and it cannot be ignored in explaining the excess production of electron-positron pairs for low transverse momenta.