Andronic, A.

Gronefeld, J.

Knichel, M. L.

Lüttig, P.

Marquard, M.

Miskowiec, D.

Otwinowski, J.

Schweda, K.

GSI Repository

Multiplicity dependence of the average transverse momentum in pp, p–Pb, andPb-Pb collisions at the LHC∗A. Andronic1, J. Gronefeld1,2, M.L. Knichel1, P. Lu¨ttig3, M. Marquard3, D. Mis´kowiec1,J. Otwinowski1, K. Schweda1 for the ALICE Collaboration1EMMI & GSI Darmstadt; 2Technische Universita¨t Darmstadt; 3Universita¨t FrankfurtOne of the key ways of investigating particle produc-tion in high-energy hadron collisions is to measure the re-lationship between the number, or multiplicity, of parti-cles produced and their momentum transverse to the direc-tion of the colliding beams. The results cast light on pro-cesses ranging from the interactions of individual partons(quarks and gluons) to the collective motion of hot, densematter containing hundreds of partons. We have investi-gated, using data collected with ALICE in proton-proton(pp), proton-lead (p–Pb) and lead-lead collisions (Pb–Pb)at the LHC, the first moment, 〈pT〉, of the charged-particletransverse momentum spectrum and its correlation with thecharged-particle multiplicity Nch [1].chN0 20 40 60 80 100)c (GeV/〉 Tp〈0.450.50.550.60.650.70.750.80.850.9 ALICE, charged particlesc<10.0 GeV/Tp|<0.3, 0.15< η| = 7 TeVspp  = 5.02 TeVNNsp-Pb  = 2.76 TeVNNsPb-Pb Figure 1: Average transverse momentum 〈pT〉 versuscharged-particle multiplicity Nch in pp, p–Pb, and Pb–Pbcollisions for |η| < 0.3. The boxes represent the systematicuncertainties on 〈pT〉. The statistical errors are negligible.Figure 1 shows the average transverse momentum 〈pT〉of charged particles versus the charged-particle multiplic-ity Nch measured in pp collisions at√s = 7 TeV, in p–Pbcollisions at √sNN = 5.02 TeV, and in Pb–Pb collisions at√sNN = 2.76 TeV. We note that the same Nch value corre-sponds to a very different collision regime in the three sys-tems. The p–Pb data exhibit features of both pp and Pb–Pbcollisions, at low and high multiplicities, respectively. Thesaturation trend of 〈pT〉 versus Nch is less pronounced inp–Pb than in Pb–Pb collisions and leads to a much highervalue of 〈pT〉 at high multiplicities than in Pb–Pb.∗Work supported by GSI, BMBF, Helmholtz Alliance HA216/EMMI,H-QM, and HGS-HIRe0 20 40 60 80 100)c (GeV/〉 Tp〈0.50.60.70.80.91DataPYTHIA 8, tune 4Cwithout CRwith CRALICE, charged particlesc<10.0 GeV/Tp|<0.3, 0.15<η| = 7 TeVspp  0 20 40 60 80 100)c (GeV/〉 Tp〈0.450.50.550.60.650.70.750.80.85DataEPOSDPMJETHIJINGAMPTGlauber MC = 5.02 TeVNNsp-Pb  chN0 20 40 60 80 100)c (GeV/〉 Tp〈0.450.50.550.60.650.7 = 2.76 TeVNNsPb-Pb  DataFigure 2: Average transverse momentum 〈pT〉 as a functionof charged-particle multiplicity Nch measured in pp (upperpanel), p–Pb (middle panel), and Pb–Pb (lower panel) col-lisions in comparison to model calculations.Figure 2 shows a comparison of the data to model pre-dictions (see references in [1]). The strong correlation of〈pT〉 with Nch is reproduced in the PYTHIA event gener-ator with the mechanism of color reconnections betweenhadronizing strings. This can be interpreted as a collectivefinal-state effect, in which strings from independent par-ton interactions do not independently produce hadrons, butfuse prior to hadronization. This leads to fewer, but moreenergetic, hadrons. The EPOS model, which implementscollective behavior in pp collisions, also describes the ppdata. Remarkably, also p–Pb collisions are well described,while for Pb–Pb collisions there is a significant discrepancybetween model and data. A Glauber Monte Carlo model forp–Pb and Pb-Pb collisions, with inputs from pp data, failsto describe the data. These data are an essential input toimprove our understanding of particle production as wellas the role of initial and final state effects in these systems.References[1] ALICE Collaboration, B. B. Abelev et al., Phys. Lett. B727,371 (2013).NQM-ALICE-02 GSI SCIENTIFIC REPORT 201366 doi:10.15120/GR-2014-1-NQM-ALICE-02

Multiplicity dependence of the average transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC

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Multiplicity dependence of the average transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC

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