Research on Cluster Reconstruction Algorithm for the STAR Forward Electromagnetic Calorimeter

The Forward Meson Spectrometer, FMS, is a forward electromagnetic calorimeter at the STAR detector at RHIC, covering pseudorapidity from 2.6 to 4.0. It is an electromagnetic calorimeter comprised of two different types of 1264 lead-glass cells where the two types vary in both composition and size. The FMS was built primarily to unravel the novel spin effects seen in transversely polarized proton collisions. These effects relate to the spin-orbit correlation of the partons in the proton to the spin of the proton, which is a consequence of the confined motion of partons in nucleons. The reconstruction of neutral pions in the FMS is essential to study these effects. The gain calibration of the FMS is solely based on the reconstruction of the invariant mass of neutral pions at a fixed energy - because there is no tracker or hadronic calorimeter around - which poses the complication that the calibration gets intertwined with the reconstruction algorithm. Also, in order to cover the kinematic region of interest, the FMS needs to measure pion energies as high as 80 GeV at pp collisions with center-of-mass energies of 200 and 500 GeV. It was found that there was a strong correlation between the reconstructed neutral pion mass and its energy due to a combination of biases in the photon finding, namely the energy and the opening angle of the decay photons. The impact of the non-linear response of the lead-glass on the energy-scale was verified in detailed simulations of the light attenuation in the lead-glass and it was found that a correction function could be established. The fitting of the electromagnetic shower shape was modified to include non-zero incident angles and non-zero vertex positions. The reconstruction algorithm was optimized for clusters with two showers, especially when the separation of the two photons nears the physical limit in terms of the cell sizes. This article will present details of all these improvements and their impact on the reconstruction of neutral pions with the FMS.