Photon-Exchange Dominance over Pomeron in Forward Rapidity Gap Events in p-Pb Collisions at LHC

The forward rapidity gap spectra from proton-lead collisions for the topologies with lead and proton dissociation. The analysis was performed at a nucleon-nucleon center-of-mass energy of 8.16 TeV with the CMS detector at LHC. The previous measurements was done by HELIOS Collaboration at \(\sqrt {{{s}_{{NN}}}} = 29.1\) GeV i.e. almost 300 times lower. For the topology with the lead dissociation the EPOS-LHC and QGSJET II generator predictions are a factor of two and four, respectively, below the CMS data, but the models give a reasonable description of the shape of the spectrum. The shape of HIJING generator differs by both, energy and shape, to the CMS data. For the proton dissociation topology the electromagnetic contribution exceeds strong interaction diffraction. This measurement changes relation between the sources of the diffraction processes with nuclei: strong interaction via Pomeron and electromagnetic one with photon-exchange. Rapidity gap observable in pA measured by CMS shows for the first time that electromagnetic contribution on lead nucleus at the LHC energy exceeds strong interaction diffraction. The obtained data may be of significant help in understanding the high energy limit of QCD and modeling cosmic ray air showers.