Studies of heavy-ion collisions using PYTHIA Angantyr and UrQMD

Abstract

In this paper, we show predictions from a new QGP-free, no-equilibration, improved baseline model for heavy-ion collisions. It is comprised of the PYTHIA Angantyr event generator coupled to UrQMD, as a hadronic cascade simulator, and compared to ALICE and CMS data from Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV. This coupling is made possible due to a new implementation of the hadron vertex model in PYTHIA Angantyr. Hadronic rescattering in UrQMD is shown to lead to a significant suppression of mid- to high-$p_\perp$ particle yields that is qualitatively consistent with measurements of nuclear modification factors. We further study the effect of hadronic rescatterings on high-$p_\perp$ particles by using two-particle correlations and show that some suppression of away-side jets occurs in the hadronic phase even without any partonic energy loss. Finally, the decorrelation of dijet structures at high momenta also leads to a reduction of the elliptic flow coefficient $v_{2}\{2\}$. These findings suggest that significant jet-quenching-like effects may still originate in the hadronic, as opposed to the partonic phase and prove that the usual Pb-Pb baseline, composed of a superposition of incoherent pp collisions, ignores coherent phenomena that are not strictly related to the QGP but may still be highly relevant.