Speaker
Description
PIONEER is a recently approved, next-generation, rare-pion decay experimental program at the Paul Scherrer Institute (PSI) in Switzerland. The first phase of the experiment will focus on a measurement of the charged-pion branching ratio to electrons vs. muons Re/μ = Γ (π → eν(γ)) /Γ (π → μν(γ)). This is a test of lepton flavor universality to be performed at an order of magnitude greater sensitivity than any other experiment. At present, the SM prediction for Re/μ is known to 1 part in 10000, which is 15 times more precise than the current experimental result. An experiment reaching this theoretical accuracy will probe non-SM explanations of these anomalies through sensitivity to quantum effects of new particles up to the PeV mass scale.
After introducing the experimental strategy, we will focus on PIONEER’s segmented stopping target (ATAR) which is a key enhancement over earlier experiments and crucial for achieving its precision goal in Re/μ . ATAR will define the pion stop fiducial region, and will suppress decay in flight and accidental positrons from earlier stopped pionsemphasized text to allow the experiment to run at its high beam rate of 300 kHz. It is indispensable for suppressing the dominant π-μ-e chain by more than six orders of magnitude to directly measure the tail of the π→ e+ν positrons in the calorimeter response.
ATAR is using an emerging detector technology called low gain avalanche diodes (LGADs). Our R&D program aims to develop this technology into a unique 5-D tracking device, featuring precision time information (0.2 ns time and 2ns pulse-pair resolution), precision 3-D tracking at the 100 μm scale and good energy resolution. Forty-eight individual LGAD strip sensors of 20x20 mm2 area and 120 μm thickness will be stacked with minimal dead material. A total of 4800 channels will be read-out with large dynamic range (several 100) and minimal cross talk.
Slow pions and muons in the PIONEER experiment generate large local energy deposition (dE/dx), where the LGAD gain is quenched by gain saturation, compromising the linearity of the sensor. In order to study, characterize and optimize the sensor behavior, we developed a test beam set-up at the UW CENPA Tandem accelerator. The response of sensors from several suppliers with differing properties is mapped as a function of energy and angle of incidence using protons of 1-8 MeV. We demonstrated the method in 2023 and significantly increased its scope and efficiency during 2024. These results and their impact will be discussed.
