Precision measurements of pion decays with the PIONEER Experiment

PIONEER is a next-generation precision experiment proposed at PSI to perform high precision measurements of rare pion decays. By improving the precision on the experimental result on the charged pion branching ratio to electrons vs. muons and the pion beta decay by an order of magnitude, PIONEER will provide a pristine test of Lepton Flavour Universality and the Cabibbo angle anomaly. In addition, various exotic rare decays involving sterile neutrinos and axions will be searched for with unprecedented sensitivity.

In the PIONEER experiment, a charged-pion beam is stopped in a segmented active target (ATAR). The proposed technology for the ATAR is based on fast silicon sensors, which can provide precise spatial and temporal resolution and thus separate even very closely spaced decays and decay products. An additional tracking layer will surround the ATAR. For the electromagnetic calorimeter, two alternative designs - liquid Xe or LYSO:Ce crystal scintillators - are actively explored.

This presentation will cover the theoretical motivations for PIONEER, as well as the ongoing simulations efforts to precisely determine the systematic performance, informing decisions on the experiment design. It will display results from recent beam test campaigns aimed at different aspects of the experiment setup: the beamline itself, sensors for the ATAR, and calibration measurements of the LYSO calorimeter crystal prototypes. In addition, new developments on the path to a prototype detector system will be presented.