An ultra-light helium cooled pixel detector for the Mu3e experiment

Abstract

The Mu3e experiment searches for the lepton flavour violating decay ${\mu }^{+}\to e^{+}{e}^{-}{e}^{+}$ with an ultimate aimed sensitivity of $1$ event in $10^{16}$ decays. To achieve this goal, the experiment must minimize the material budget per tracking layer to $X/X_0\approx 0.1\%$ and use gaseous helium as coolant. The pixel detector uses High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) which are thinned down to $50\mu m$. Both helium cooling and HV-MAPS are a novelty for particle detectors. Here, the work on successfully cooling a pixel tracker using gaseous helium is presented. The thermal studies focus on the two inner tracking layers, the Mu3e vertex detector, and the first operation of a functional thin pixel detector cooled with gaseous helium. The approach, which circulates gaseous helium under ambient pressure conditions with a gas temperature around $0°C$ using a miniature turbo compressor with a mass flow of $2g/s$ allows the vertex detector to operate below $70°C$ at heat densities of up to $350mW/c{m}^2$. Finally, performance data of the final HV-MAPS used by Mu3e, the MuPix11, is presented. These results demonstrate the feasibility of using HV-MAPS combined with gaseous helium as a coolant for an ultra-thin pixel detector exploring new frontiers in lepton flavor.

conference proceedings submitted to JINST, New Frontiers on Lepton Flavor 2023, Pisa