Speaker
Description
The PIONEER experiment at the Paul Scherrer Institute (PSI) will observe rare pion decay processes; in its first phase, it will provide a sensitive test of the Standard Model by measuring the branching ratio for pion decays directly into positrons ($\pi^+ \to e^+ \nu_e$) with a precision of 0.01%. We are developing a demonstrator for the active target (ATAR) system, a key novel component of the experiment. This 5-D tracker (3-D position, time, and charge) will directly observe the stopping points and decays of pions and muons, allowing the topology of each event to be determined. The device consists of an array of eight trench-isolated LGAD devices manufactured by Fondazione Bruno Kessler (FBK), each 10 $\times$ 10 mm$^2$ and segmented into 0.2 mm strips. After transmission through a 15 cm flex cable, the signals are amplified by FAST3 ASICs and recorded by high-density waveform digitizers (SAMPIC or HDSoC). The amount of non-active material is minimized, both within the ATAR and between it and a surrounding calorimeter. This talk will present the status of the mechanical assembly and electronic characterization of the demonstrator, which is being prepared for a test beam run at PSI in December 2026.