Forward Jet-like Event Spin-dependent Properties in Polarized p+p Collisions at $\sqrt{s}=200$ GeV

Abstract

The STAR collaboration has reported precision measurements on the transverse single spin asymmetries for the production of forward $\pi^0$ mesons from polarized proton collisions at the center of mass energy of 200 GeV. Disentangling contributions to forward asymmetries requires one to look beyond inclusive neutral pion production to the production of forward jets or direct photons. During the RHIC running in the year 2006, STAR with the Forward Pion Detector++ (FPD++) in place collected 6.8 inverse picobarn of data with an average polarization of 60%. FPD++ had sufficient acceptance for "jetlike" objects, which are clustered responses of an electromagnetic calorimeter primarily sensitive to incident photons, electrons and positrons. For the "jet-like" objects, we reconstructed the angle of the outgoing leading neutral pion with respect to the fragmenting parton, thus enabling us to disentangle the contributions to the forward $\pi^0$ asymmetries. The simulated data set shows that on average there are approximately 2.5 fragmenting mesons per one "jet-like" object. Preliminary FPD++ results provide no evidence of measured contributions to the asymmetry from jet fragmentation, implying the Sivers distribution functions play a substantial role in producing the large inclusive $\pi^0$ asymmetries previously measured by STAR.