Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at √sNN = 5.02 TeV

Aad, G. and Abajyan, T. and Abbott, B. and Abdallah, J. and Khalek, S. A. and Abdelalim, A. A. and Abdinov, O. and Aben, R. and Abi, B. and Abolins, M. and AbouZeid, O. S. and Abramowicz, H. and Abreu, H. and Abulaiti, Y. and Acharya, B. S. and Adamczyk, L. and Adams, D. L. and Addy, T. N. and Adelman, J. and Adomeit, S. and Adye, T. and Aefsky, S. and Aguilar-Saavedra, J. A. and Agustoni, M. and Ahlen, S. P. and Ahles, F. and Ahmad, A. and Ahsan, M. and Aielli, G. and Akesson, T. P. A. and Krasznahorkay, Attila and Nagai, Yoshikazu and Pásztor, Gabriella and Tóth, József (2013) Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at √sNN = 5.02 TeV. PHYSICS LETTERS B, 725 (1-3). pp. 60-78. ISSN 0370-2693

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Abstract

In order to study further the long-range correlations (“ridge”) observed recently in p+Pb collisions at √sNN = 5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v2, has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 μb−1, the parameter v2 has been ob- tained using two- and four-particle cumulants over the pseudorapidity range |η| < 2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1 < η < 4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrody- namic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.

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