Bottomonium production in heavy-ion collisions at STAR

Abstract

Bottomonium measurements provide unique insight into hot and cold nuclear matter effects present in the medium that is formed in high-energy heavy-ion collisions. Recent STAR results show that in $\sqrt{s_{NN}}$ = 200 GeV central Au+Au collisions the $\Upsilon$(1S) state is suppressed more than if only cold nuclear matter effects were present, and the excited state yields are consistent with a complete suppression. In 2012, STAR also collected 263.4 $\mu$b$^{-1}$ high-energy-electron triggered data in U+U collisions at $\sqrt{s_{NN}}$= 193 GeV. Central U+U collisions, with an estimated 20% higher energy density than in central Au+Au data, extend the $\Upsilon$(1S+2S+3S) and Upsilon(1S) nuclear modification trends observed in Au+Au towards higher number of participant nucleons, and confirm the suppression of the $\Upsilon$(1S) state. We see a hint with 1.8 sigma significance that the $\Upsilon$(2S+3S) excited states are not completely suppressed in U+U collisions. These data support the sequential in-medium quarkonium dissociation picture and favor models with a strong $q\bar{q}$ binding.