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Phenomenology developments in UPC: $γγ\to γγ$ scattering

Paweł Jucha, Antoni Szczurek

Abstract

We discuss several possible extensions of present studies of $γγ\to γγ$ scattering in ultraperipheral heavy ion collisions. One of the possible extensions are studies for lower diphoton invariant masses (smaller transverse momenta). There new mechanisms may show up. This includes possible studies with future FOCAL and ALICE 3 detectors. So far only coherent $γγ\to γγ$ processes, when initial photons couple to nuclei, were sudied theoretically. Recently we proposed to study also inelastic processes, when initial photons couple to individual nucleons. Without special cuts the corresponding processes are of the order of 20 - 30 \%. A study to which extent the inelastic processes survive with the present cuts used for the $γγ\to γγ$ studies is an interesting question. They can be of interest by itself and dedicated studies are suggested. We calculate also cross section for associated neutron production. Deviation from our predictions could signal presence of the inelastic contribution. Finally we discuss production of single photons in UPC ($A A \to A A γ$). Corresponding cross sections for selected mechanisms are presented.

Phenomenology developments in UPC: $γγ\to γγ$ scattering

Abstract

We discuss several possible extensions of present studies of scattering in ultraperipheral heavy ion collisions. One of the possible extensions are studies for lower diphoton invariant masses (smaller transverse momenta). There new mechanisms may show up. This includes possible studies with future FOCAL and ALICE 3 detectors. So far only coherent processes, when initial photons couple to nuclei, were sudied theoretically. Recently we proposed to study also inelastic processes, when initial photons couple to individual nucleons. Without special cuts the corresponding processes are of the order of 20 - 30 \%. A study to which extent the inelastic processes survive with the present cuts used for the studies is an interesting question. They can be of interest by itself and dedicated studies are suggested. We calculate also cross section for associated neutron production. Deviation from our predictions could signal presence of the inelastic contribution. Finally we discuss production of single photons in UPC (). Corresponding cross sections for selected mechanisms are presented.

Paper Structure

This paper contains 7 sections, 17 equations, 14 figures, 1 table.

Table of Contents

  1. Introduction
  2. Theoretical calculation of nuclear cross sections
  3. Double vector meson fluctuations
  4. Inelastic contributions
  5. $\gamma \gamma \to \gamma \gamma$ scattering and neutron emission
  6. Single photon production
  7. Summary

Figures (14)

  • Figure 1: Possible mechanisms for $\gamma \gamma \to \gamma \gamma$ scattering: a) fermionic box, b) light-meson resonances c) VDM-Regge vector mesons fluctuation, d) 2-gluon exchange.
  • Figure 2: Diagram of 2$\pi^0$ photo-production - the dominant background contribution in the region of small di-photon masses.
  • Figure 3: A ratio of each contribution to a coherent sum of them. The red solid line represents leptonic cross section divided by the total cross section, the green dotted line refers to quarks, magenta dash-dotted line to electrons and dashed dark-red line applies to a sum of electron and muon contributions.
  • Figure 4: $cos(\theta)$ distributions for (a) boxes, (b) double hadronic fluctuations and (c) $t/u$-channel $\pi^0$ exchange for different energies $W$ = 1, 2, 5, 10, 20, 50$~$GeV.
  • Figure 5: Differential cross section as a function of two-photon invariant mass at $\sqrt{s_{NN}}=5.02$ TeV. The ATLAS data ATLAS are shown for comparison.
  • ...and 9 more figures