Measurement of the centrality dependence of the charged particle pseudorapidity distribution in proton-lead collisions at $\sqrt{s_{NN}} = 5.02$ TeV with the ATLAS detector

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions, $dN_{ch}/d\eta$, in $p$+Pb collisions at a nucleon-nucleon centre-of-mass energy of $\sqrt{s_{NN}}=5.02$~TeV. Charged particles were reconstructed over $|\eta|<2.7$ using the ATLAS pixel detector. The proton-lead collision centrality was characterized by the total transverse energy measured over the pseudorapidity interval $3.2 <\eta<4.9$ in the direction of the lead beam. The $dN_{ch}/d\eta$ distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different calculations of the number of participants, $N_{part}$, have been carried out using a standard Glauber model as well as two Glauber-Gribov extensions. Charged particle multiplicities per participant pair, $dN_{ch}/d\eta$/$(\langle N_{part}\rangle/2)$, are found to vary differently with $N_{part}$ for these three models, pointing to the importance of the fluctuating nature of nucleon-nucleon collisions in the modeling of the initial state of p+Pb collisions.