Nuclear effects in HERA-B

Abstract

In this contribution we estimate the influence of the shadowing effect and initial state parton energy loss in the quarkonium production at HERA-B. We analyze the xF behavior of the effective exponent a(xF) and present a comparison with the preliminary HERA-B data for J=Y production. Moreover, we present our predictions for i production at HERA-B. The magnitude of these effects in the J=Y production at RHIC is also analyzed. The relativistic collider facilities RHIC and LHC will for the first time provide the opportunity to systematically study the physics of hot and ultradense matter in hadron-nucleus (pA) and nucleus-nucleus (AB) collisions at energies that are orders of magnitude larger than the energies of the current ac- celerators. The systematic study of pA collisions at the same energies is essential to gain insight into the structure of the dense medium effects. Such effects, as the energy loss and shadowing, are absent or small in pp collisions, but become increasingly prominent in pA collisions, and are of major im- portance in AA reactions. By comparing pA and AA reactions involving very heavy nuclei, one may be able to distinguish basic hadronic effects that dominate the dynamics in pA col- lisions, from a quark-gluon formation predicted to occur in heavy ion AA collisions. To gain insight into the underlying hadronic processes, one has to study collisions that are ex- pected to not lead to a QGP formation. Once the physics of at high densities is better understood, the mechanisms of quark-gluon plasma formation and related collective phe- nomena in heavy ion collisions could be disentangled from the basic hadronic effects. In this contribution we study the influence of the nuclear medium effects in the quarkonium production, particularly of the J=y, which is one of the proposed signatures of the QCD phase transition (1). In particular, we will consider the shad- owing effects in the parton distributions and the initial state parton energy loss, which modify the xF (· x1i x2) behavior of the cross section. We will focus our analyzes in the kine- matical range of HERA-B, which recently has obtained data for the quarkonium production at negative values of xF . As we will show, these data could be used to discriminate between the different models for the medium effects.