Incoherent Particle Production in Ultraperipheral Heavy Ion Collisions

TL;DR

This work provides the first complete treatment of incoherent photon-initiated production in ultraperipheral heavy ion collisions, focusing on dilepton and diphoton final states. It develops a mixed-ion framework with a detailed treatment of initial photon flux, final-state kinematics, Pauli blocking, and the ion survival factor, implemented in the SuperChic MC and validated against ATLAS dielectron data. The analysis shows the incoherent PI contribution to light-by-light scattering is sub-percent after typical cuts and cannot account for the observed broad high-acoplanarity background, while QCD-initiated diphoton production remains an insufficient explanation. The results clarify the origin of the higher-acoplanarity tail and provide a more reliable modeling framework for UPCs, with implications for ongoing LbL measurements and future cross-checks in exclusive processes.

Abstract

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.

Figures (12)

• Figure 1: Representative Feynman diagrams for light--by--light scattering in AA ultraperipheral collisions in the case of (a) coherent and (b) incoherent production. The lower double line corresponds to the ion beam that undergoes coherent photon emission, while the upper line corresponds to the incoherent interaction. The latter can be interpreted as either a nucleon or quark/anti--quark line depending on whether elastic or inelastic photon emission from the individual nucleons is considered.
• Figure 2: Diagram indicating the definitions of the impact parameter vectors defined in \ref{['eq:AA']}.
• Figure 3: Ratio of incoherent to coherent luminosities \ref{['eq:lumis']} as a function of (left) the diphoton invariant mass, and (right) the absolute diphoton rapidity, for $m_{\gamma\gamma} = 10$ GeV. Results are also shown in the inelastic case with additional cuts placed on the final--state quark in $q\to q\gamma$ emission generating the corresponding photon PDF, assuming LO kinematics.
• Figure 4: Predicted survival factors for coherent and incoherent electron pair production, within the ATLAS ATLAS:2022srr event selection, for $|y_{ee}|<2.4$ and as a function of the dielectron invariant mass.
• Figure 5: Predicted coherent and incoherent electron pair production cross sections, differential with respect to the dielectron acoplanarity, within the ATLAS ATLAS:2022srr event selection, for $|y_{ee}|<0.8$. Results are given for different ZDC selections, as indicated.