Azimuthal correlations within exclusive dijets with large momentum transfer in photon-lead collisions

TL;DR

This paper reports the first measurement of azimuthal correlations in exclusive dijet production in photon-lead collisions, revealing potential gluon polarization effects in nuclei at high momentum transfer.

Contribution

It provides the first experimental measurement of azimuthal correlations in exclusive dijet events in photon-lead interactions at the LHC, highlighting deviations from existing models and indicating gluon polarization.

Findings

Positive and rising second harmonic correlation with increasing dijet momentum

Standard models fail to describe the observed correlations

Evidence suggesting gluon polarization effects in nuclei

Abstract

The structure of nucleons is multidimensional and depends on the transverse momenta, spatial geometry, and polarization of the constituent partons. Such a structure can be studied using high-energy photons produced in ultraperipheral heavy-ion collisions. The first measurement of the azimuthal angular correlations of exclusively produced events with two jets in photon-lead interactions at large momentum transfer is presented, a process that is considered to be sensitive to the underlying nuclear gluon polarization. This study uses a data sample of ultraperipheral lead-lead collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV, corresponding to an integrated luminosity of 0.38 nb$^{-1}$, collected with the CMS experiment at the LHC. The measured second harmonic of the correlation between the sum and difference of the two jet momenta is found to be positive, and rising, as the dijet momentum increases. A well-tuned model that has been successful at describing a wide range of proton scattering data from the HERA experiments fails to describe the observed correlations, suggesting the presence of gluon polarization effects.