The Mu3e Experiment: Status and Short-Term Plans

Abstract

Mu3e is an experiment currently under construction at the Paul Scherrer Institute in Switzerland, designed to search for the Lepton Flavor Violating (LFV) decay $\mu^+ \rightarrow e^+e^-e^+$. In extensions of the Standard Model (SM) that account for neutrino masses, this decay is theoretically allowed but occurs only through extremely rare loop processes, with a predicted branching ratio of approximately $\mathcal{O}(10^{-54})$. Such a small probability implies that any observation of this decay would provide clear evidence for physics beyond the SM. The Mu3e experiment aims to probe the $\mu^+ \rightarrow e^+e^-e^+$ decay with a sensitivity of approximately $\mathcal{O}(10^{-15})$ in its Phase-1 and plans to achieve a sensitivity of $\mathcal{O}(10^{-16})$ after future upgrades. To reach its Phase-1 ambitious goals, Mu3e is going to use the most intense continuous muon beam in the world, generating 10$^{8}$ muon stops per second in the target placed at the center of the Mu3e. Mu3e will use three main technologies for particle detection. The tracking will done through ultra-thin (50 - 70 $\mu m$) pixel detectors based on MuPix11 sensors. These are high-voltage monolithic active pixel sensors (HV-MAPS) with a $\sim$ 23~$\mu$m spatial resolution. The timing will be done through scintillating fibres ($\sim$ 250 ps) and tiles ($\sim$ 40 ps), coupled to silicon photomultipliers and read out by MuTRiG3 ASICs. A triggerless DAQ system based on FPGAs will collect data from the detectors, which will then undergo reconstruction in a GPU filter farm. The assembly of the detectors has started, with a detector commissioning beam time planned for 2025. This document reports on the status of the construction, installation, and data-taking plans for the near future.