Speaker
Robert Vertesi
(Nuclear Physics Institute of the Czech Academy of Sciences)
Description
Due to color screening, the production of quarkonia in high energy heavy ion collisions is expected to be sensitive to the energy density of the medium. 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 QGP, the observed energy dependence of $J/\psi$ suppression is rather weak. This phenomenon is explained by the recombination (coalescence) of $c\bar{c}$ pairs. Bottomonia, on the other hand, are less affected by recombination and can provide a cleaner probe of the strongly interacting medium. 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.
The energy density in U+U $\sqrt{s_{NN}}=193$ GeV central collisions is estimated to be about 20% higher than that of $\sqrt{s_{NN}}=200$ GeV central Au+Au. Therefore U+U collisions provide a further test of the sequential suppression hypothesis. The STAR detector collected a total of 17.2 million high-tower triggered events in $\sqrt{s_{NN}}=193$ GeV U+U collisions in the year 2012, corresponding to an integrated luminosity of 263.4 $\mu b^{-1}$. Upsilon yields at mid-rapidity, measured in central and mid-central collisions through the dielectron channel, as well as the nuclear modification factor with respect to the number of participants, will be reported in this presentation.
| On behalf of collaboration: | STAR |
|---|
Primary author
Robert Vertesi
(Nuclear Physics Institute of the Czech Academy of Sciences)
