PWO Crystal Measurements and Simulation Studies of Lambdabar Hyperon Polarisation for PANDA

Abstract

Two measurements of the energy resolution were performed at MAX-Lab in Lund, Sweden, with an array of 3$\times$3 PbWO4 crystals using a tagged photon beam with energies between 19 and 56 MeV. The crystals were cooled down to -15 and -25 degrees C. The measured relative energy resolution, sigma/E, is decreasing from approximately 12% at 20 MeV to 7% at 55 MeV. The value of a was therefore fixed to that one would expect from Poisson statistics of the light collection yield (50 phe/MeV) and the results from fits were sigma/E= 0.45%/sqrt(E) + 0.18%/E + 8.63% and sigma/E= 0.45%/sqrt(E) + 0.21%/E + 6.12% for the April and September measurements, respectively. Light yield uniformity studies of five PbWO4 crystals, three tapered and two non-tapered ones, have also been performed. The tapered crystals delivered a light output which increased with increasing distance from the Photo Multiplier Tube (PM tube). For the pbar p -> Lambdabar Lambda simulations, events were generated using a modified generator from the PS185 experiment at LEAR. With a 100% polarisation perpendicular to the scattering plane, a polarisation of (99+-1.8)% was reconstructed. Slight non-zero polarisations along the axis determined by the outgoing hyperon as well as the axis in the scattering plane, were also reconstructed. These were (4.1+-2.1)% and (2.6+-2.0)% respectively. From this investigation it was shown that the detector efficiency was not homogeneous and that slow pions are difficult to reconstruct.