First Observation of Diffractive Processes in Proton-Lead Collisions at the LHC with the CMS Detector

We present the first measurements of diffraction in $\sqrt {{{s}_{{NN}}}} = 8.16$ proton-lead collisions at LHC with CMS. The very large angular coverage of CMS is used to tag rapidity gaps on both the proton-going and lead-going sides and to identify both pomeron-lead and pomeron-proton topologies. Since the previous highest energy measurement of these processes was at $\sqrt {{{s}_{{NN}}}} = 29.1$, the obtained data provide essentially unique information. The rapidity gap distributions are not only sensitive to the diffractive parton distributions of nuclei but also provide important information for modeling cosmic ray collisions. The results are compared to the predictions from the EPOS-LHC, QGSJET II and HIJING event generators.