Development of a 5-D active tracker target for the PIONEER experiment

PIONEER is a next-generation rare-pion decay experimental program recently approved at the Paul Scherrer Institute in Switzerland. The first phase of the experiment will focus on a 0.01% precision measurement of the charged-pion branching ratio to electrons versus muons, Re/µ = Γ (π → eν(γ)) /Γ (π → µν(γ)). Comparison with existing theory predictions at this precision level will provide an unprecedented test of lepton flavor universality and a powerful probe of physics beyond the Standard Model, with sensitivity to quantum effects from new particles at mass scales up to the PeV range.

Following an overview of the experimental strategy, the presentation will focus on PIONEER’s segmented active target (ATAR)—a key advancement over previous experiments and essential to achieving the required precision. ATAR employs an emerging detector technology based on low-gain avalanche diodes (LGADs). Our R&D program aims to develop this technology into a unique five-dimensional tracking system, combining precision timing, 3D spatial tracking, and accurate energy measurement. The detector will consist of forty-eight stacked LGAD strip sensors with minimal inactive material, providing approximately 4800 readout channels with large dynamic range (several hundred) and minimal cross talk. The talk will describe the construction of a large-scale prototype of the PIONEER experiment and present new results from studies at the UW CENPA Tandem accelerator on gain saturation in LGADs, a critical effect influencing their energy resolution.