Longitudinal double-spin asymmetries for $π^0s$ in the forward direction for 510 GeV polarized $pp$ collisions

Abstract

The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass energy of 510 GeV. Results are given for transverse momenta in the range 2<$p_T$<10 GeV/$c$ within two regions of pseudorapidity that span 2.65<η<3.9. These results are sensitive to the polarized gluon parton distribution function, Δg(x), down to the region of Bjorken x∼10$^{−3}$. The asymmetries observed are less than ±5⋅10$^{−3}$ in magnitude, and will help constrain the contribution to the spin of the proton from polarized gluons at low x, when combined with other measurements as part of a global analysis.