Eccentric protons? Sensitivity of flow to system size and shape in p+p, p+Pb and Pb+Pb collisions

TL;DR

This study investigates how the initial system size and shape influence flow observables in small and large collision systems at the LHC, finding good agreement with data in Pb+Pb but discrepancies in p+Pb.

Contribution

It combines Glasma and viscous hydrodynamics to model initial conditions and flow, providing new insights into system size effects and initial shape constraints across different collision systems.

Findings

Hydrodynamic models match Pb+Pb flow data across centralities.

Model underestimates flow coefficients in p+Pb collisions.

Initial shape constraints are derived from HBT and flow data.

Abstract

We determine the transverse system size of the initial non-equilibrium Glasma state and of the hydrodynamically evolving fireball as a function of produced charged particles in p+p, p+Pb and Pb+Pb collisions at the Large Hadron Collider. Our results are consistent with recent measurements of Hanbury-Brown-Twiss (HBT) radii by the ALICE collaboration. Azimuthal anisotropy coefficients v_n generated by combining the early time Glasma dynamics with viscous fluid dynamics in Pb+Pb collisions are in excellent agreement with experimental data for a wide range of centralities. In particular, event-by-event distributions of the v_n agree with the experimental data out to fairly peripheral centrality bins. In striking contrast, our results for p+Pb collisions significantly underestimate the magnitude and do not reproduce the centrality dependence of data for v_2 and v_3 coefficients. We argue that the measured v_n data and HBT radii strongly constrain the shapes of initial parton distributions across system sizes that would be compatible with a flow interpretation in p+Pb collisions. Alternately, additional sources of correlations may be required to describe the systematics of long range rapidity correlations in p+p and p+Pb collisions.