Forward Pion Production in Hadron Collisions at STAR

In contrast to the case for the nucleon, little is known about the distribution of gluons in nuclei, especially in the region of Björkenx < 0.01, where x is the fraction of the nucleon’s momentum carried by the parton. Studies of deuteron(proton)+nucleus collisions at large center of mass energy ( √ sNN=200 GeV) can provide constraints on the gluon density in heavy nuclei. In the perturbative QCD explanation of particle production at large pseudorapidity, a largex parton scatters from a low-x parton and then fragments into the observed particle(s). Forward charged particle production is found to be suppressed in d+Au collisions [1], consistent with the expectation of gluon saturation [2,3], possibly indicating a different mechanism for particle production. Explanations of the suppression based on leading-twist pQCD calculations using a model of gluon shadowing have also been suggested [4]. Further tests of the possible role played by gluon saturation at RHIC energies are provided by the study of particle correlations [5,6]. At √ s=200 GeV and larger collision energies, there is quantitative agreement between NLO pQCD calculations and measured p+p cross sections at mid-rapidity [7]. This agreement has been found to extend to π production at 〈η〉 = 3.8 [8]. Further tests of the underlying dynamics responsible for forward particle production can be obtained from the study of particle correlations. In particular, strong azimuthal correlations of hadron pairs are expected when particle production arises from 2 → 2 parton scattering. This paper reports cross sections for forward inclusive π production for p+p collisions at √ s=200 GeV. The azimuthal correlations between a forward π (〈ηπ〉=4.0) and midrapidity charged hadrons were studied. In addition, exploratory studies with d+Au collisions at √ sNN=200 GeV are reported, and azimuthal correlations of hadron pairs are compared to those for p+p collisions. The inclusive yields of π mesons in p+p and d+Au collisions will be forthcoming, and details pertinent to this analysis are presented here. Forward π production in d+Au collisions refers to observation of the π in the direction of the incident deuteron.