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

TL;DR

This study measures the double helicity asymmetry in neutral pion production in polarized proton-proton collisions to estimate the gluon spin contribution to the proton's overall spin, finding it consistent with small or zero contribution within uncertainties.

Contribution

It provides the first experimental constraints on the gluon spin contribution in the proton using high-energy collision data and explores theoretical uncertainties affecting these measurements.

Findings

Measured asymmetries are consistent with zero.

Estimated gluon spin contribution Delta-G is between 0.1 and 0.2.

Constraints on Delta-G range from -0.7 to 0.5 at 3 sigma.

Abstract

The double helicity asymmetry in neutral pion production for p_T = 1 to 12 GeV/c has been measured with the PHENIX experiment in order to access the gluon spin contribution, Delta-G, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of \mu^2 = 4 GeV^2 give Delta-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 measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta-G constraint on the choice of theoretical scale, a dominant uncertainty in these predictions.