Inclusive cross section and double helicity asymmetry for π0 production in p+p collisions at s=200GeV: Implications for the polarized gluon distribution in the proton

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider2005 run with polarized proton collisions at sqrt(s)=200 GeV, for inclusive {pi}{sup 0} production at midrapidity. Unpolarized cross section results are given for transverse momenta p{sub T}=0.5 to 20 GeV/c, extending the range of published data to both lower and higher p{sub T}. The cross section is described well for p{sub T} 2 GeV/c, by perturbative QCD. Double helicity asymmetries ALL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluonpolarization in the proton. Using one representative model of gluonpolarization it is demonstrated that the gluonspin contribution to the proton spin is significantly constrained.