Proton emission in ultraperipheral Pb-Pb collisions at $\sqrt{\textit{s}_{\mathrm{\textbf{NN}}}}=5.02$ TeV

TL;DR

This paper reports the first measurements of proton emission in electromagnetic dissociation of lead nuclei at the LHC, comparing experimental results with the RELDIS model and revealing discrepancies in certain proton-neutron emission channels.

Contribution

It provides novel measurements of proton and neutron emission cross sections in ultraperipheral Pb-Pb collisions at 5.02 TeV, and evaluates the RELDIS model's accuracy in describing these processes.

Findings

0p and 3p cross sections agree with RELDIS model within uncertainties

1p and 2p cross sections are underestimated by the model by 17-25%

Experimental data for proton-neutron emission channels are significantly overestimated by the model

Abstract

The first measurements of proton emission accompanied by neutron emission in the electromagnetic dissociation (EMD) of $^{208}$Pb nuclei in the ALICE experiment at the LHC are presented. The EMD protons and neutrons emitted at very forward rapidities are detected by the proton and neutron Zero Degree Calorimeters of the ALICE experiment. The emission cross sections of zero, one, two, and three protons accompanied by at least one neutron were measured in ultraperipheral $^{208}$Pb--$^{208}$Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.02$ TeV. The 0p and 3p cross sections are described by the RELDIS model within their measurement uncertainties, while the 1p and 2p cross sections are underestimated by the model by 17-25%. According to this model, these 0p, 1p, 2p, and 3p cross sections are associated, respectively, with the production of various isotopes of Pb, Tl, Hg, and Au in the EMD of $^{208}$Pb. The cross sections of the emission of a single proton accompanied by the emission of one, two, or three neutrons in EMD were also measured. The data are significantly overestimated by the RELDIS model, which predicts that the (1p,1n), (1p,2n), and (1p,3n) cross sections are very similar to the cross sections for the production of the thallium isotopes $^{206,205,204}$Tl.