Measurement of Direct-Photon Cross Section and Double-Helicity Asymmetry at SQUARE ROOTs=510 GeV in (p)over-right-arrow + (p)over-right-arrow Collisions

Abstract

We present measurements of the cross section and double-helicity asymmetry ALL of direct-photon production in p ⃗ + p ⃗ collisions at ffis ffi p = 510 GeV. The measurements have been performed at midrapidity (I & eta;I < 0.25) with the PHENIX detector at the Relativistic Heavy Ion Collider. At relativistic energies, direct photons are dominantly produced from the initial quark-gluon hard scattering and do not interact via the ffiffistrong force at leading order. Therefore, at p s = 510 GeV, where leading-order-effects dominate, these measurements provide clean and direct access to the gluon helicity in the polarized proton in the gluon-momentum-fraction range 0.02 < x < 0.08, with direct sensitivity to the sign of the gluon contribution.