Speaker
Description
The production of quarkonia in high-energy heavy-ion collisions is
expected to be sensitive to the energy density of the medium due to
color screening of the quark-antiquark binding potential. Sequential
suppression of different quarkonium states may therefore serve as a
thermometer of the medium. Although the suppression of charmonia was
anticipated as a key signature of the quark-gluon plasma formation, the
recombination of uncorrelated $c\bar{c}$ pairs in the medium complicates
the picture. Bottomonia, on the other hand, are less affected by
recombination and thus provide a cleaner probe of the strongly
interacting medium. Recent STAR results show that in central Au+Au
collisions at $\sqrt{s_{NN}}=200$ GeV, the Upsilon 1S state is
suppressed beyond the mere cold nuclear matter effects, and the yields
of the excited states are consistent with a complete suppression.
STAR collected data in U+U collisions at $\sqrt{s_{NN}}=193$ GeV
triggered by high-energy electrons with an integrated luminosity of
263.4 $\mu b^{-1}$. The energy density in central U+U collisions is
estimated to be about 20% higher than that in $\sqrt{s_{NN}}=200$ GeV
central Au+Au data. Studies of quarkonium production in U+U collisions
can therefore serve as further tests of the sequential suppression
hypothesis. The latest results for the nuclear modification factors of
the Upsilon 1S state, as well as all three states together, will be
presented as a function of number of participants in U+U and Au+Au
collisions, and compared to theoretical calculations.
The Muon Telescope Detector (MTD), completed in 2014, allows for
measurements of Upsilon's with better precision through the muon decay
channel. Recent measurements of Upsilon production using the MTD will
also be presented.
