Measurement of Direct-Photon Cross Section and Double-Helicity Asymmetry at $\sqrt{s}=510$ GeV in $\vec{p}+\vec{p}$ Collisions

TL;DR

This paper reports measurements of the direct-photon production cross section and double-helicity asymmetry in polarized proton-proton collisions at 510 GeV, providing insights into the gluon helicity distribution within protons.

Contribution

It presents the first measurements of direct-photon cross section and asymmetry at this energy, offering a clean probe of gluon polarization in a specific momentum range.

Findings

Measured direct-photon cross section at 510 GeV

Determined double-helicity asymmetry A_{LL} for direct photons

Provided constraints on gluon helicity distribution

Abstract

We present measurements of the cross section and double-helicity asymmetry $A_{LL}$ of direct-photon production in $\vec{p}+\vec{p}$ collisions at $\sqrt{s}=510$ GeV. The measurements have been performed at midrapidity ($|\eta|<0.25$) 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 strong force at leading order. Therefore, at $\sqrt{s}=510$ 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.