Determining the longitudinal double-spin asymmetry (A_LL) for π⁰ production from STAR

The Solenoidal Tracker at RHIC (STAR) located at Brookhaven National Laboratory uses longitudinally polarized proton-proton collisions to study the gluon spin contribution to the known proton spin of ½ h-bar. The relative contributions of the quarks and gluons to the spin of the proton remain uncertain. Using data from the 2013 longitudinally polarized proton-proton collisions we study the asymmetry of proton spin-dependent production of neutral pions (π⁰s) from these collisions. π⁰s rapidly (8.5*10-17 s) decay into 2 photons that are detected by the Endcap Electromagnetic Calorimeter. By comparing the number of π⁰s produced when protons collide with different helicities, the asymmetry of π⁰ production (A_LL), which can be related to the contribution of the gluon spin to the spin of the proton, can be measured. The two-photon invariant mass spectrum is reconstructed and then fit using a skewed Gaussian function to represent the π⁰ signal and a Chebyshev function to characterize the background. Various checks must be made to assure the quality of the data being analyzed. The status of this analysis will be presented.