First measurement of the forward rapidity gap distribution in pPb collisions at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV

TL;DR

This paper presents the first measurements of forward rapidity gap distributions in proton-lead collisions at 8.16 TeV, providing new insights into diffractive processes at unprecedented energies at the LHC.

Contribution

It introduces the first measurement of rapidity gap spectra in pPb collisions at 8.16 TeV, comparing data with various models and highlighting discrepancies and potential contributions from ultra-peripheral events.

Findings

EPOS-LHC underestimates lead dissociation data by a factor of two.

All models significantly underestimate pomeron-proton topology data.

Ultra-peripheral photoproduction may contribute to diffractive signatures.

Abstract

For the first time at LHC energies, the forward rapidity gap spectra from proton-lead collisions for both proton and lead dissociation processes are presented. The analysis is performed over 10.4 units of pseudorapidity at a center-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV, almost 300 times higher than in previous measurements of diffractive production in proton-nucleus collisions. For lead dissociation processes, which correspond to the pomeron-lead event topology, the EPOS-LHC generator predictions are a factor of two below the data, but the model gives a reasonable description of the rapidity gap spectrum shape. For the pomeron-proton topology, the EPOS-LHC, QGSJET II, and HIJING predictions are all at least a factor of five lower than the data. The latter effect might be explained by a significant contribution of ultra-peripheral photoproduction events mimicking the signature of diffractive processes. These data may be of significant help in understanding the high energy limit of quantum chromodynamics and for modeling cosmic ray air showers.