Performance tests of LYSO crystals for the new PIONEER experiment

PIONEER is a new and approved rare-pion decay experiment which will be mounted at the Paul Scherrer Institute (PSI) in Switzerland. The first goal of PIONEER will be to measure the branching ratio π→e/π→μ to the precision of 10^-4 which would represent the world’s most sensitive test of Lepton Flavor Universality. Such a measurement requires a calorimeter with more than 20 radiation lengths, fast response time and an energy resolution approaching 2-3% at 70 MeV. These specifications may be achieved using a large array of LYSO crystals. LYSO is a fast, dense, high light-yield scintillator whose intrinsic properties suggest it would be a natural candidate for the experiment. Despite its advantages, a large, LYSO-based calorimeter has never been developed. Ongoing bench tests have demonstrated impressive single crystal resolution and uniformity at low energies when crystals are wrapped in a well-fitted specular reflector. We present the results of these tests using radioactive sources at the 0.51-4.14 MeV energy scale. A beam test of a 2x2 array of LYSO crystals using 17.6 MeV gamma rays is planned in the late summer and a beam test at PSI using a 100 MeV positron beam incident on a 3x3 array in November.