Determining the Longitudinal Double-spin Asymmetry (A_LL) for η Production from STAR 2013 Endcap Calorimeter Data

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. One method is to measure the longitudinal double-spin asymmetry (A_LL) in the production of η mesons from the longitudinally polarized proton-proton collisions at √s = 510 GeV. The η mesons that are produced from the collision quickly decay (in about 5.0×10^-19 s) into two photons that are detected using the Endcap Electromagnetic Calorimeter (EEMC, 1.09 < η < 2.00). The EEMC determines the energies and positions of the incoming photons. With the data from the EEMC, we can calculate the invariant mass of the photon pairs and produce a two-photon invariant mass spectrum. This spectrum is fitted with a Gaussian function to represent the η mesons plus a third-order polynomial function to describe the background photon pairs. The total number of η mesons is then obtained by integrating the fitted Gaussian function. The asymmetry is calculated using the number of η mesons resulting from the collisions of protons with different spin alignments. The status of the analysis of the 2013 dataset to measure the η meson A_LL will be presented.