Abstract
The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 upmu b^{-1} of proton–lead collisions at a nucleon–nucleon centre-of-mass energy of sqrt{s_{_text {NN}}} = 5.02 text {TeV} using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity 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 estimations of the number of nucleons participating in the collision have been carried out using the Glauber model as well as two Glauber–Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon–nucleon collisions in the modelling of the initial state of collisions.
Highlights
The left panel shows the dNch/dη distribution measured in the fiducial acceptance of the ATLAS detector, detecting particles with pT > 0.1 GeV
This paper presents a measurement of the centrality dependence of the charged-particle pseudorapidity distribution, dlisNiochn/sdaηt,amneuacslueroend–innuacplepornoxciemnatrtee-loyf1-mμabs−s 1enoefrpgy+oPf b√csoNlN= 5.02 TeV collected by the ATLAS detector at the LHC
The fully corrected measurements are presented for the fiducial acceptance of the ATLAS detector and in the full acceptance
Summary
C (2016) 76:199 particle multiplicity distributions as a function of pseudorapidity measured in central d + Au collisions are asymmetric and peaked in the Au-going direction [7] This observation has been explained using well-known phenomenology of soft hadron production [13]. Measurements of the centrality dependence of dNch/dη distributions in p + Pb collisions provide an essential test of soft hadron production mechanisms at the LHC Such tests have become of greater importance given the observation of two-particle [18,19,20,21] and multi-particle [21,22,23] correlations in the final state of p + Pb collisions at the LHC. That analysis compared different methods for characterising centrality and suggested that the method used to define centrality may have a significant impact on the centrality dependence of the measured dNch/dη distribution
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