Initial state geometry and the role of hydrodynamics in proton-proton, proton-nucleus and deuteron-nucleus collisions

TL;DR

This paper investigates initial state geometries and hydrodynamic effects in small collision systems like proton-proton, proton-nucleus, and deuteron-nucleus, revealing sensitivities and similarities to larger nucleus-nucleus collisions.

Contribution

It applies Monte Carlo Glauber and IP-Glasma models to small systems, highlighting differences in initial geometry sensitivity and flow development compared to heavy ion collisions.

Findings

Initial geometries are highly sensitive to model details.

System sizes in p+A and p+p are similar, with comparable HBT radii.

Flow coefficients v_2 and v_3 show smaller model-dependent differences.

Abstract

We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.