Grant: Subatomic Physics Envelope - Project

Tensions between some flavor physics measurements and Standard Model predictions are accumulating. In particular some discrepancies hint at non-universality of lepton couplings and non-unitarity of the CKM quark mixing matrix which are triggering great interest in high precision measurements. A new experiment, PIONEER, is being developed to provide highly precise measurements which could shed new light on both lepton universality and CKM unitarity. PIONEER is aimed at studying the pienu branching ratio (pion decay to positron over pion decay to muon), pion beta decay and other rare reactions. It may be located at TRIUMF, Canada or at the Paul Scherer Institut (PSI), Switzerland. These facilities can provide intense low energy pion beams which can be stopped in a target for precise measurements of rare pion decays. Proposals have been (or are being) submitted to both laboratories. The newly formed PIONEER collaboration consists of experts from the PIENU experiment at TRIUMF, PEN at PSI, as well as international collaborators who have been involved in rare kaon decay and low energy stopped muon experiments, in the Fermilab Muon g-2 experimental campaign and ATLAS. This application seeks initial support for the Canadian PIONEER team consisting of four principal investigators from TRIUMF and UBC. The application focuses on developments related to a central element of the experiment: the calorimeter. Targeting an order of magnitude improvement in precision with respect to existing measurements of the pienu branching ratio and pion beta decay, the calorimeter has to fulfill stringent requirements in terms of energy resolution and pile-up suppression to cope with higher pion rates and enable unequaled suppression of systematic effects. A large liquid xenon detector instrumented with state-of-the-art vacuum ultraviolet sensors is the leading contender but advanced simulation studies and associated R&D have to be performed to determine the optimal configuration and to predict the overall performance of the detector. These issues are addressed in this proposal.