Tracking detector for the precision measurements of rare pion decays with PIONEER experiment

A state of the art next-generation rare pion decay experiment, PIONEER, has recently been approved at the Paul Scherrer Institute (PSI,CHRISP). The main motivation of the PIONEER experiment is to improve the test of the Standard Model (SM) hypothesis of lepton flavor universality. It will probe non-SM explanations of possible anomalies through sensitivity to quantum effects of new particles at very high mass scales. Further, the experiment will enable determination of the the CKM matrix element Vud with improved sensitivity. The main observables for PIONEER are the branching ratio for pion decays to positrons relative to muons and the rare process π+ → π0 e+ ν(γ) aiming for an order of magnitude improvement in precision in each case. Pioneer will also search for sterile neutrinos and other exotic processes. The PIONEER experiment design contains an active target (ATAR) based on low-gain avalanche detectors (LGADs), where an intense pion beam is brought to rest and a 3π sr, 25 radiation length (X0) liquid Xe electromagnetic calorimeter. This presentation will introduce a new cylindrical tracker design and discuss the simulation studies in the PIONEER environment. It will surround the ATAR, to link the locations of pions stopping in the target to showers in the calorimeter. The compact tracker design proposal is based on μRWELL micro patter gaseous detector (MPGD) to provide high spatial and timing resolutions with very low radiation length and is suitable for high rate pion beams.