Model study on Υ(nS) modification in small collision systems

Abstract

Quarkonium production has been studied extensively in relativistic heavy-ion collision experiments to understand the properties of the quark-gluon plasma. The experimental results on the yield modification in heavy-ion collisions relative to that in $p+p$ collisions can be described by several models considering dissociation and regeneration effects. A yield modification beyond initial-state effects has also been observed in small collision systems such as $p+\mathrm{Au}$ and $p+\mathrm{Pb}$ collisions, but it is still premature to claim any hot medium effect. A model study in various small collision systems such as $p+p$, $p+\mathrm{Pb}$, $p+\mathrm{O}$, and $\mathrm{O}+\mathrm{O}$ collisions will help quantitatively the understanding of nuclear effects on the $\mathrm{\ensuremath{\Upsilon}}(nS)$ production. A theoretical calculation considering the gluo-dissociation and inelastic parton scattering for dissociation, and their inverse reaction for regeneration, reasonably describes the modification of $\mathrm{\ensuremath{\Upsilon}}(1S)$ in $\mathrm{Pb}+\mathrm{Pb}$ collisions. Based on this calculation, a Monte Carlo simulation considering the dissociation effect is developed to more realistically incorporate the medium produced in heavy-ion collisions with event-by-event initial collision geometry and hydrodynamic evolution. We extend this framework to small systems to study the medium effects. In this work, we quantify the nuclear modification factor of $\mathrm{\ensuremath{\Upsilon}}(nS)$ as a function of charged particle multiplicity ($d{N}_{ch}/d\ensuremath{\eta}$) and transverse momentum. We also calculate the elliptic flow of $\mathrm{\ensuremath{\Upsilon}}(nS)$ in small collision systems.