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

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 multiparticle 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 among the generated flow coefficients v₂ and v₃ are smaller. For a large number of participants in proton-lead collisions, the v₂ 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.