STAR results and perspectives on transverse spin asymmetries

Abstract

The STAR collaboration reported precision measurements on the transverse single spin asymmetries for the production of forward $π^0$ mesons from polarized proton collisions at $\sqrt{s}=$ 200 GeV. To disentangle contributions to measured forward asymmetries one has to look beyond inclusive $π^0$ production to the production of forward jets or direct photons. In 2006, STAR with the Forward Pion Detector++ (FPD++) in place, collected 6.8 pb$^{-1}$ of forward data with an average polarization of 60%. FPD++ had sufficient acceptance for "jet-like" objects, which are clustered responses of an electromagnetic calorimeter primarily sensitive to incident photons, electrons and positrons. For these objects, the angle of the outgoing leading $π^0$ with respect to the fragmenting parton was reconstructed, thus enabling us to disentangle the contributions to the forward $π^0$ asymmetries. The simulated data set shows that on average there are approximately 2.5 fragmenting mesons per one "jet-like" object, making them reasonably "jetty". Preliminary 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 forward $π^0$ asymmetries. A similar effort was made in the mid-rapidity $(|η|<1)$ region of the STAR detector, where 2.2 pb$^{-1}$ of data was collected. We present progress made by making measurements of the azimuthal asymmetry of leading charged pions in jets produced by transversely polarized proton collisions.