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

G. Aad (1) , Z. Barnovska (2) , N. Berger (2) , M. Delmastro (2) , L. Di Ciaccio (2) , T.K.O. Doan (2) , S. Elles (2) , C. Goy (2) , T. Hryn’ova (2) , S. Jézéquel (2) , I. Koletsou (2) , R. Lafaye (2) , J. Leveque (2) , N. Massol (2) , G. Sauvage (2) , E. Sauvan (2) , O. Simard (2) , T. Todorov (2) , I. Wingerter-Seez (2) , G . Rahal (3) , L. Alio (1) , M. Barbero (1) , C. Bertella (1) , N. Bousson (1) , J.-C. Clémens (1) , Y. Coadou (1) , F. Djama (1) , L. Feligioni (1) , J. Gao (1) , D. Hoffmann (1) , F. Hubaut (1) , E. B. F. G. Knoops (1) , E. Le Guirriec (1) , B. Li (1) , J. Maurer , E. Monnier (1) , S. Muanza (1) , Y. Nagai (1) , P. Pralavorio (1) , A. Rozanov (1) , T. Serre (1) , M. Talby (1) , N. Tannoury (1) , E. Tiouchichine (1) , S. Tisserant (1) , J. Toth (1) , F. Touchard (1) , M. Ughetto (1) , L. Vacavant (1) , S. Albrand (4) , J. Brown (4) , B. Clement (4) , J. Collot (4) , S. Crépé-Renaudin (4) , B. Dechenaux (4) , T. Delemontex (4) , P.A. Delsart (4) , C. Gabaldon (4) , M.H. Genest (4) , J.Y. Hostachy (4) , E. Laisne (4) , B.T. Le (4) , F. Ledroit-Guillon (4) , A. Lleres (4) , A. Lucotte (4) , F. Malek (4) , C. Monini (4) , J. Stark (4) , X. Sun (4) , B. Trocmé (4) , D. Boumediene (5) , Emmanuel Busato (5) , D. Calvet (5) , S. Calvet (5) , J. Donini (5) , E. Dubreuil (5) , N. Ghodbane (5) , Ph. Gris (5) , C. Guicheney (5) , H. Liao (5) , D. Pallin (5) , F. Podlyski (5) , C. Santoni (5) , Timothée Theveneaux-Pelzer (5) , Loïc Valery (5) , F. Vazeille (5) , M.K. Ayoub (6) , A. Bassalat (6) , C. Bécot (6) , S. Binet (6) , C. Bourdarios (6) , D. Delgove (6) , J-B. de Vivie De Regie (6) , L. Duflot (6) , M. Escalier (6) , L. Fayard (6) , D. Fournier (6) , E.L. Gkougkousis (6) , J-F. Grivaz (6) , T. Guillemin (6) , F. Hariri (6) , S. Henrot-Versille (6) , J. Hrivnac (6) , L. Iconomidou-Fayard (6) , M. Kado (6) , A. Lounis (6) , N. Makovec (6) , N. Morange (6) , C. Nellist (6) , P. Petroff (6) , L. Poggioli (6) , P. Puzo (6) , A. Renaud (6) , D. Rousseau (6) , G. Rybkin (6) , A.C. Schaffer (6) , E. Scifo (6) , L. Serin (6) , S. Simion (6) , R. Tanaka (6) , D. Zerwas (6) , Zhongkai Zhang (6) , Y. Zhao (6)

1 CPPM - Centre de Physique des Particules de Marseille
2 LAPP - Laboratoire d'Annecy de Physique des Particules
3 CC-IN2P3 - Centre de Calcul de l'IN2P3
4 LPSC - Laboratoire de Physique Subatomique et de Cosmologie
5 LPC - Laboratoire de Physique Corpusculaire - Clermont-Ferrand
6 LAL - Laboratoire de l'Accélérateur Linéaire

The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 $\mu$b$^{-1}$ of proton--lead collisions at a nucleon--nucleon centre-of-mass energy of $\sqrt{s_{_{\rm{NN}}}} = 5.02$ 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 $p$+Pb 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 $p$+Pb 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 $p$+Pb collisions.

Domaines: Physique des Hautes Energies - Expérience [hep-ex]; Physique Nucléaire Expérimentale [nucl-ex]

G. Aad, Z. Barnovska, N. Berger, M. Delmastro, L. Di Ciaccio, et al.. Measurement of the centrality dependence of the charged-particle pseudorapidity distribution in proton--lead collisions at $\sqrt{s_{_{\rm{NN}}}} = 5.02$ TeV with the ATLAS detector. European Physical Journal C: Particles and Fields, 2016, 76, pp.199. ⟨10.1140/epjc/s10052-016-4002-3⟩. ⟨in2p3-01185504⟩