Inclusive cross section and double-helicity asymmetry for π0 production at midrapidity in p+p collisions at s =510 GeV

PHENIX measurements are presented for the cross section and double-helicity asymmetry (ALL) in inclusive π0 production at midrapidity from p+p collisions at s=510 GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider. The next-to-leading-order perturbative-quantum-chromodynamics theory calculation is in excellent agreement with the presented cross section results. The calculation utilized parton-to-pion fragmentation functions from the recent DSS14 global analysis, which prefer a smaller gluon-to-pion fragmentation function. The π0ALL results follow an increasingly positive asymmetry trend with pT and s with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on π0 and jet ALL and suggested a positive contribution of gluon polarization to the spin of the proton ΔG for the gluon momentum fraction range x>0.05. The data presented here extend to a currently unexplored region, down to x∼0.01, and thus provide additional constraints on the value of ΔG.