Double Longitudinal Spin Asymmetries of Inclusive Charged Pion Production in Polarized p+p Collisions at 200 GeV

TL;DR

This paper reports the first measurements of double longitudinal spin asymmetries in inclusive charged pion production in polarized proton-proton collisions at 200 GeV, providing insights into the polarized gluon distribution function.

Contribution

It presents novel experimental measurements of A_{LL} for charged pions at RHIC, aiding the understanding of gluon polarization in protons.

Findings

Measured A_{LL} for charged pions over 2<p_T<10 GeV/c

Compared results with various gluon polarization models

Identified and estimated systematic biases from event selection

Abstract

A primary goal of the STAR Spin program at RHIC is the measurement of the polarized gluon distribution function $\Delta G$, which can be obtained from a global analysis incorporating measurements of the double spin asymmetry A_{LL} in various final state channels of polarized p+p collisions. Final states with large production cross sections such as inclusive jet and hadron production are analyzed as the program moves towards the measurement of A_{LL} in the theoretically clean channel of prompt photon production. The channels p+p -> pi^{+/-} + X are unique in that the ordering of the measurements of A_{LL} in these two channels is sensitive to the sign of Delta G. Moreover, STAR has already established the procedure for the identification of charged pions and the calculation of their production cross-sections over a broad kinematic range. This contribution will present first measurements of double longitudinal spin asymmetries for inclusive charged pion production extracted from 3 pb^{-1} of data at sqrt{s}=200 GeV and 50% beam polarizations. The asymmetries are calculated over the transverse momentum region 2<p_T<10 GeV/c and compared with theoretical predictions incorporating several gluon polarization scenarios. A systematic bias introduced by the selection of charged pions from events satisfying electromagnetic energy triggers will be discussed and estimated using Monte Carlo.