Evidence for Gluon Energy Loss as the Mechanism for Heavy Quarkonium   Suppression in $pA$ Collisions
  We study the energy and nuclear A dependence of the hadronic production of heavy quarkonia. We review theoretical ideas which have been put forward, seeking a consistent global picture reconciling the large effects in quarkonia with the small nuclear effects observed in continuum Drell Yan production. The data indicates that shadowing or leading twist modifications of parton distributions can be ruled out as explanations, leaving higher twist energy loss. {}From general principles the maximum allowed energy loss of partons traversing the nuclear medium can be related to the parton transverse momenta. We then show that the experimental data on nuclear suppression of charm- and bottom- onium for large $x_F$ is consistent with this effect: using the observed transverse momenta to bound the $x_F$ dependence in an almost model independent manner generates a relation that practically reproduces the data. Several prediction are discussed; the dependence on $x_F$ as $x_F\to 1$, and large and small $k_T^2$ cuts, can be used to discriminate between quark and gluon induced effects. 
Pankaj Jain and John P. Ralston