Longitudinal Spin Asymmetry and Cross Section of Inclusive pi0 Production in Polarized p+p Collisions at RHIC

TL;DR

This paper reports the first measurements of the cross section and double longitudinal spin asymmetry of inclusive pi0 production in polarized proton-proton collisions at RHIC, providing insights into gluon polarization and fragmentation functions.

Contribution

It presents novel experimental data on pi0 production and spin asymmetry at RHIC, constraining gluon polarization and pion fragmentation models.

Findings

Measured cross section agrees with NLO pQCD calculations.

Asymmetry measurements disfavor large positive gluon polarization.

Results help refine understanding of gluon contribution to proton spin.

Abstract

We present the first measurement of the cross section and the double longitudinal spin asymmetry of inclusive pi0 production in polarized p+p collisions at Sqrt(s) = 200 GeV at mid-rapidity with the STAR detector, using the barrel electromagnetic calorimeter. The measured cross section is compared to NLO pQCD calculations and can provide constraints on the pion fragmentation functions. Fragmentation is studied directly by measuring the momentum fraction of pi0 in jets, a quantity that is affected by the fragmentation process and jet reconstruction effects. The double longitudinal spin asymmetry is compared to NLO pQCD calculations based on different assumptions for the gluon polarization in the nucleon to provide constraints on delta g/g. At the present level of statistics the measured asymmetry disfavors a large positive gluon polarization, but can not yet distinguish between other scenarios.