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

We show how angular asymmetries ∼ cos ⁡ 2 ϕ 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 c 2 { 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 c 2 { 4 } in high-multiplicity events.