Non-flow effects in correlation between harmonic flow and transverse momentum in nuclear collisions

Abstract

A large anti-correlation signal between elliptic flow v2 and average transverse momentum [pT] was recently measured in small collision systems, consistent with a final-state hydrodynamic response to the initial geometry. This negative v2–[pT] correlation was predicted to change to positive correlation for events with very small charged particle multiplicity Nch due to initial-state momentum anisotropies of the gluon saturation effects. However, the role of non-flow correlations is expected to be important in these systems, which is not yet studied. We estimate the non-flow effects in pp, pPb and peripheral PbPb collisions using Pythia and Hijing models, and compare them with the experimental data. We show that the non-flow effects are largely suppressed using the rapidity-separated subevent cumulant method (details of the cumulant framework are also provided). The magnitude of the residual non-flow is much less than the experimental observation in the higher Nch region, supporting the final-state response interpretation. In the very low Nch region, however, the sign and magnitude of the residual non-flow depend on the model details. Therefore, it is unclear at this moment whether the sign change of v2–[pT] can serve as evidence for initial state momentum anisotropies predicted by the gluon saturation.