Evidence for sequential $\Upsilon$(nS) suppression in light ion collisions

Abstract

Bound states of heavy quark-antiquark pairs, known as quarkonia, have long been regarded as particularly sensitive probes of the quark-gluon plasma (QGP). Comparing quarkonium yields in collisions of heavy nuclei, such as gold or lead, with a proton-proton (pp) reference reveals a characteristic pattern of sequential suppression, in which weakly-bound excited states are more strongly suppressed than the ground states. We report the first measurements of the three lowest mass $\mathrm{S}$-wave vector bottomonium resonances, the ground state $\Upsilon$(1S) and the excited states $\Upsilon$(2S) and $\Upsilon$(3S), in oxygen-oxygen collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV. Measurements of the yields of the $\Upsilon$(1S) and $\Upsilon$(2S) resonances in neon-neon collisions, at the same $\sqrt{s_\mathrm{NN}}$, are also presented. The $\Upsilon$(2S)/$\Upsilon$(1S) ratio is found to be significantly below the measured pp reference value, and the $\Upsilon$(3S)/$\Upsilon$(1S) ratio shows an even larger reduction. The significance of the relative $\Upsilon$(3S) to $\Upsilon$(2S) suppression exceeds three standard deviations. These results provide evidence for the sequential suppression of $\Upsilon$(nS) states in light ion collisions, similar to observations made in lead-lead and gold-gold collisions, generally attributed to the presence of a QGP medium.