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The double helicity asymmetry in neutral pion production for p_T = 1 to 12GeV/c has been measured with the PHENIX experiment in order to access the gluonspin contribution, Delta-G, to the proton spin. Measured asymmetries areconsistent with zero, and at a theory scale of \mu^2 = 4 GeV^2 giveDelta-G^[0.02,0.3] = 0.1 to 0.2, with a constraint of -0.7 < Delta-G^[0.02,0.3]< 0.5 at Delta-chi^2 = 9 (~3 sigma) for our sampled gluon momentum fraction (x)range, 0.02 to 0.3. The results are obtained using predictions for our measuredasymmetries generated from four representative fits to polarized deep inelasticscattering data. We also consider the dependence of the Delta-G constraint onthe choice of theoretical scale, a dominant uncertainty in these predictions.

physical review letters

The gluon spin contribution to the proton spin from the double helicity  asymmetry in inclusive pi^0 production in polarized p+p collisions at  sqrt(s)=200 GeV