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Inclusive cross section and double-helicity asymmetry for $$pi^0$$ production at midrapidity in $$p+p$$ collisions at $$sqrt{s}=510$$ GeV

Adare, A.*; Hasegawa, Shoichi   ; Imai, Kenichi; Nagamiya, Shoji; Sako, Hiroyuki   ; Sato, Susumu  ; Tanida, Kiyoshi   ; PHENIX Collaboration*; 406 of others*

PHENIX measurements are presented for the cross section and double-helicity asymmetry ($$A_{LL}$$) in inclusive $$pi^0$$ production at midrapidity from $$p+p$$ collisions at $$sqrt{s}=510$$ 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 $$pi^0; A_{LL}$$ results follow an increasingly positive asymmetry trend with $$p_T$$ and $$sqrt{s}$$ with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on $$pi^0$$ and jet ALL and suggested a positive contribution of gluon polarization to the spin of the proton $$Delta G$$ for the gluon momentum fraction range $$x>0.05$$. The data presented here extend to a currently unexplored region, down to $$x sim 0.01$$, and thus provide additional constraints on the value of $$Delta G$$.

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