Speaker
Description
We present the development of an Ultra-Fast Silicon Pixel Detector (UFSPD) for Phase II of the Mu3e experiment, which aims to detect the rare decay of a muon into three electrons. To achieve the required sensitivity of $10^{−16}$, enhanced time and vertex resolution are essential. The UFSPD should replace the Phase I SciFi detector and targets a time resolution of ~100 ps. The first test sensor, Picopix, built in 180 nm HV-CMOS, includes low-power pixels and integrated TDCs. Initial measurements show a TDC resolution of 56 ps RMS and sensor-level time resolution down to 200 ps RMS.
Summary (500 words)
The Mu3e experiment at PSI is currently under construction. Its primary goal is to search for the rare charged lepton flavour violating decay of a muon into three electrons. The cylindrical pixel detector comprises four layers equipped with High Voltage Monolithic Active Pixel Sensors (HVMAPS). The final sensor version, MuPix11, has been successfully produced and tested. Following the commissioning of the High Intensity Muon Beamline (HIMB), the new experimental area is expected to be available for Mu3e in 2029. The ultimate goal for the Mu3e collaboration - single event sensitivity of $10^{−16}$ - can only be achieved with the high muon stopping rates provided by the HIMB. However, the increased muon rate will raise the accidental background rate by a factor of 400, necessitating significantly improved background suppression. This improvement requires enhanced vertex resolution through optimization of the pixel resolution and reduced thickness of the innermost tracking layer, along with improved detector time resolution. The latter will be achieved by replacing the Phase I scintillating fiber (SciFi) detector with an Ultra-Fast Silicon Pixel Detector (UFSPD), targeting a time resolution of 100 ps or better. Development of the UFSPD sensor chip is already underway. The first UFSPD test sensor, named Picopix, has been implemented using TSI 180 nm HV-CMOS technology. The pixel design features a cascade of voltage amplifiers without feedback, resulting in minimal detector capacitance and low-voltage operation to reduce power consumption. Each pixel is equipped with a Time-to-Digital Converter (TDC) employing a novel analog time-stretching technique. The TDC has demonstrated a time resolution of approximately 56 ps RMS. The sensor time resolution has been measured at around 200 ps RMS using an injection circuit, and approximately 900 ps RMS with a preliminary test using a Sr-90 source. Design details and measurement results will be presented.