Centrality and jet performance in p+Pb collisions at 5.02 TeV with the ATLAS detector at the LHC

Controlling the geometry of proton-nucleus collisions is an essential aspect of testing whether the observed production of hard and soft particles deviates from a simple scaling with the number of collisions. Traditionally, a wounded nucleon model in which the distribution of a bulk observable scales with the number of participant nucleons has been used to describe the correlation between the number of collisions and the centrality of the event. We describe the centrality determination in proton-lead collisions at sqrt(s_NN) = 5.02 TeV with the ATLAS experiment at the LHC. The class of minimum bias events seen by the ATLAS detector include contributions from diffractive and photonuclear processes, which cannot be modeled through the standard Glauber approach. These are rejected through the identification of large forward pseudorapidity gaps, which are a strong indicator of an event topology that is consistent with single and double diffractive excitation.