Jet Production in Polarized pp Collisions at RHIC

One of the primary goals of the RHIC spin program is to determine the gluon polarization distribution within the proton. At leading order, pp collisions at RHIC involve a mixture of quark-quark (qq), quark-gluon (qg), and gluon-gluon (gg) scattering processes, with qg and gg contributions dominating. This makes RHIC an ideal place to study gluon polarization. For the past several years, the STAR Collaboration has been studying mid-rapidity inclusive jet production in √ s = 200 GeV pp collisions in order to constrain the gluon contribution to the proton spin. We find that the cross section for inclusive jet production is well described by next-to-leading-order (NLO) perturbative QCD predictions for jet transverse momenta (pT ) spanning the range 5 to 50 GeV/c [2]. Our first measurements of the longitudinal double-spin asymmetry, ALL, for inclusive jet production [2], based on data that were recorded during the 2003 and 2004 RHIC runs, disfavored a maximal gluon polarization scenario where the gluon polarization distribution, ∆g(x,Q 0 ), at the input scale, Q 0 = 0.4 GeV, is assumed to be equal to the unpolarized gluon distribution, g(x,Q 0 ). Here, we describe measurements of ALL for inclusive jet production that were performed by STAR during the 2005 [3] and 2006 RHIC runs. The increased precision and kinematic coverage of these data provide valuable new constraints on the gluon polarization in the proton when included in a NLO global analysis. We also describe the STAR plans to study di-jet production in upcoming RHIC runs. This will provide direct leading-order access to the momentum dependence of the gluon polarization, ∆g(x,Q).