First results for the two-spin parameter ALL in pi0 production by 200 GeV polarized protons and antiprotons

The two-spin parameter A(LL) in inclusive pi-0 production by longitudinally-polarized protons and antiprotons on a longitudinally-polarized proton target has been measured at the 200 GeV Fermilab spin physics facility, for pi-0,s at X(F) = 0 with 1 less-than-or-equal-to p(t) less-than-or-equal-to 3 GeV/c. The results exclude, at the 95% confidence level, values of A(LL) (pp) > 0.1 and < -0.1, for pi-0,s produced by protons, and values of A(LL) (antiproton-antiproton) > 0.1 and < -0.2 for incident antiprotons. The relevance of A(LL) (pp) for the gluon spin density is discussed. The data are in good agreement with "conventional", small or zero, gluon polarization.