Incoherent Particle Production in Ultraperipheral Heavy Ion Collisions

In this paper, we present the first complete treatment of incoherent photon-initiated production in ultraperipheral heavy ion collisions, focussing on the dilepton and diphoton final states. In the former case we compare to the ATLAS measurement of dielectron production and find that our predictions match the data very well. In the latter case we show that this contribution is too small to explain the observed broad in diphoton acoplanarity background to the purely coherent light-by-light scattering signal. We in addition consider a new `quark emission' topology, which while naively might be expected to dominate the incoherent diphoton production channel, is in fact found to be kinematically suppressed. Finally, we revisit QCD-initiated diphoton production and issue of theoretical uncertainties in this case. We find that this again cannot explain the observed size of the higher acoplanarity background. The production mechanism leading to this background to light-by-light production, and the reason for its observed enhancement in comparison to the dilepton case, therefore remains unclear.