Observation of long-range, near-side angular correlations in pPb collisions at the LHC

Chatrchyan, S. and Khachatryan, V. and Sirunyan, A. M. and Tumasyan, A. and Adam, W. and Aguilo, E. and Bergauer, T. and Dragicevic, M. and Erö, J. and Fabjan, C. and Friedl, M. and Frühwirth, R. and Ghete, V. M. and Hörmann, N. and Hrubec, J. and Jeitler, M. and Kiesenhofer, W. and Knünz, V. and Krammer, M. and Krätschmer, I. and Liko, D. and Mikulec, I. and Pernicka, M. and Rabady, D. and Rahbaran, B. and Rohringer, C. and Rohringer, H. and Schöfbeck, R. and Strauss, J. and Taurok, A. and Bencze, György and Hajdu, Csaba and Hidas, Pál and Horváth, Dezső and Siklér, Ferenc and Veszprémi, Viktor and Vesztergombi, György and Zsigmond, Anna Júlia and Béni, Noémi Tekla and Czellár, Sándor and Molnár, József and Pálinkás, József and Szillási, Zoltán and Karancsi, János and Raics, Péter and Trócsányi, Zoltán and Ujvári, Balázs and Veres, Gábor and Krajczár, Krisztián (2013) Observation of long-range, near-side angular correlations in pPb collisions at the LHC. PHYSICS LETTERS B, 718 (3). pp. 795-814. ISSN 0370-2693

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Abstract

Results on two-particle angular correlations for charged particles emitted in pPb col- lisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, η, and full azimuth, φ, as a function of charged particle multiplicity and particle transverse momentum, pT. In high-multiplicity events, a long-range (2 < |∆η| < 4), near-side (∆φ ≈ 0) structure emerges in the two-particle ∆η-∆φ correlation functions. This is the first ob- servation of such correlations in proton–nucleus collisions, resembling the ridge-like correlations seen in high-multiplicity pp collisions at √s = 7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of pT = 1–1.5 GeV/c and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These ob- servations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is signifi- cantly larger in pPb collisions.

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