Azimuthal asymmetries and the emergence of “collectivity” from multi-particle correlations in high-energy pA collisions

Adrian Dumitru,Larry Mclerran,Vladimir Skokov

doi:10.1016/j.physletb.2015.02.046

Abstract

We show how angular asymmetries ∼cos⁡2ϕ can arise in dipole scattering at high energies. We illustrate the effects due to anisotropic fluctuations of the saturation momentum of the target with a finite correlation length in the transverse impact parameter plane, i.e. from a domain-like structure. We compute the two-particle azimuthal cumulant in this model including both one-particle factorizable as well as genuine two-particle non-factorizable contributions to the two-particle cross section. We also compute the full BBGKY hierarchy for the four-particle azimuthal cumulant and find that only the fully factorizable contribution to c2{4} is negative while all contributions from genuine two, three and four-particle correlations are positive. Our results may provide some qualitative insight into the origin of azimuthal asymmetries in p+Pb collisions at the LHC which reveal a change of sign of c2{4} in high-multiplicity events.

Highlights

Large azimuthal asymmetries have been observed in p+Pb collisions at the LHC [1,2,3,4] and in d+Au collisions at RHIC [5]
These asymmetries are usually measured via multi-particle angular correlations and were found to extend over a long range in rapidity
The data shows that the asymmetries persist up to rather high transverse momenta, well beyond asymmetries p⊥ in

Summary

INTRODUCTION

Large azimuthal asymmetries have been observed in p+Pb collisions at the LHC [1,2,3,4] and in d+Au collisions at RHIC [5] These asymmetries are usually measured via multi-particle angular correlations (see below) and were found to extend over a long range in rapidity. On the other hand the four-particle cumulant, c2{4}, decreases monotonically and changes sign to become negative in high multiplicity events, an effect seen by the CMS collaboration (see second paper in [4]). As shown below, this requires an anisotropy of the single-particle angular distribution. In this paper we perform a first computation of all connected and disconnected contributions to the cumulants in the short distance regime using a model that allows for anisotropic “domains” of the color-electric fields E of the target [10, 18]

CALCULATION

DISCUSSION