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

TL;DR

This study measured the energy resolution of PbWO4 crystals and simulated Lambda hyperon polarization in the PANDA experiment, revealing detailed detector performance and hyperon polarization characteristics.

Contribution

It provides new measurements of PbWO4 crystal energy resolution and light yield uniformity, and presents simulation results of Lambda hyperon polarization for PANDA.

Findings

Energy resolution decreases from 12% to 7% between 20 and 55 MeV.

Reconstructed hyperon polarization was approximately 99%.

Detector efficiency was non-uniform, affecting slow pion reconstruction.

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.