Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at sqrt(s_NN)=5.02 TeV

TL;DR

This paper reports on measurements of azimuthal anisotropy in p+Pb collisions at 5.02 TeV using the ATLAS detector, revealing collective flow features similar to larger systems, and supporting the role of final-state effects.

Contribution

First measurement of v_2 using multi-particle cumulants in p+Pb collisions at the LHC, demonstrating collective behavior in small systems.

Findings

v_2 shows features of collective flow

Results align with hydrodynamic model predictions

Large v_2 magnitude indicates significant final-state effects

Abstract

In order to study further the long-range correlations ("ridge") observed recently in p+Pb collisions at sqrt(s_NN) =5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v_2, 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 microb^(-1), the parameter v_2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |eta|<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<eta<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 hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v_2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.