Measurement of jet photoproduction in ultra-peripheral Pb+Pb collisions without nuclear breakup at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with the ATLAS detector

TL;DR

This paper reports the first measurement of photon-pomeron jet production in ultra-peripheral Pb+Pb collisions at 5.02 TeV, using ATLAS data to distinguish different photon-induced processes without nuclear breakup.

Contribution

It introduces a method to statistically separate photon-pomeron, photon-photon, and photonuclear events in ultra-peripheral heavy ion collisions at the LHC.

Findings

First measurement of $oldsymbol{ ext{γ+I extit{P}} ightarrow ext{jets}}$ cross-sections in nuclear collisions.

Supports that events without forward neutrons are more peripheral collisions.

Provides data on electromagnetic dissociation rates in ultra-peripheral collisions.

Abstract

In ultra-relativistic heavy ion collisions at the LHC, each nucleus acts as a source of high-energy quasi-real photons that can participate in scattering processes without causing either participating nucleus to break up and emit forward neutrons. This paper extends recent measurements of $\gamma+A\rightarrow\mathrm{jets}$ production in ultra-peripheral Pb+Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with forward neutron emission on exactly one side of the event. The data presented here was recorded by the ATLAS collaboration at the LHC in 2018, corresponding to a luminosity of $1.72$ nb$^{-1}$. These results examines $5.02$ TeV Pb+Pb collisions where neither nucleus breaks up ($0n0n$), providing a mixture of photon--pomeron ($\gamma+I\!\!P\rightarrow\mathrm{jets}$), photon--photon ($\gamma+\gamma\rightarrow\mathrm{jets}$), and peripheral photonuclear ($\gamma+A\rightarrow\mathrm{jets}$) events. The different processes are statistically separated via a template fit of the minimum rapidity gap distribution. The kinematics of the hard processes are determined from $R = 0.4$ jets reconstructed using the anti-$k_t$ algorithm. The statistical separation of the different processes then allows for the first measurement of $\gamma+I\!\!P\rightarrow\mathrm{jets}$ cross-sections in nuclear collisions at the LHC. The rate for electromagnetic dissociation of $0n0n$ $\gamma+A\rightarrow\mathrm{jets}$ events is also measured and compared to the analogous result from collisions with single-sided neutron emission. These comparisons support the hypothesis that $\gamma+A\rightarrow\mathrm{jets}$ events without forward neutron emission select a more peripheral class of $\gamma+A$ collisions.