  Thermal suppression of quarkonium production in heavy-ion collisions, due to
Debye screening of the quark-antiquark potential, has been proposed as a clear
signature of Quark-Gluon Plasma (QGP) formation. At RHIC energies, the
$\Upsilon$ meson is a clean probe of the early system due to negligible levels
of enhancement from \bbbar recombination and non-thermal suppression from
co-mover absorption. We report on our measurement of the $\Upsilon \to e^+ e^-$
cross section in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. We compute the
Nuclear Modification Factor by comparing these results to new p+p measurements
from 2009 (21 pb$^{-1}$ in 2009 compared to 7.9 pb$^{-1}$ in 2006). In order to
have a complete assessment of both hot and cold nuclear matter effects on
Upsilon production we also report on results from d+Au collisions.

Nuclear Experiment