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Longitudinal polarized p+p collisions provide a leading-order probe of the gluon spin contributions to the proton spin. The direct-photon production is named the "golden" channel due to its "cleanliness" and sensitivity to the sign of the gluon spin. The PHENIX measurement at the Relativistic Heavy-Ion Collider performed the first published direct-photon double-helicity asymmetry (A₋₋) measurement. Our data indicate the gluon spin has a large probability of aligning with the proton spin. I will show the direct-photon cross sections and A₋₋ measurements at PHENIX.
Z. Ji (2024) studied this question.