The liquid xenon detector for the MEG II experiment to detect 52.8 MeV [formula omitted] with large area VUV-sensitive MPPCs

Abstract

The MEG II experiment is searching for new physics beyond the SM (e.g. SUSY-GUT, SUSY-seesaw) through the lepton flavor violating μ+→e+γ decay with ten times better sensitivity than the MEG experiment. The MEG collaboration published the result of B(μ+→e+γ)<4.2×10−13 at 90% C.L. in 2016, which was thirty times better result than the previous limit. As the sensitivity of the MEG experiment was already limited by the accidental background, the MEG detector had to be upgraded to reach one order of magnitude better sensitivity. The MEG experiment utilized 846 2 inch PMTs to detect scintillation light in a 900 l liquid xenon γ calorimeter. In the MEG II experiment, 216 2 inch PMTs on the γ incident face were replaced with 4092 MPPCs (SiPMs produced by Hamamatsu) to improve the energy and position resolutions. We started the detector commissioning with the full electronics readout channels for the first time in 2021, and soon after, we started the physics data taking. Here the LXe detector status including initial photon sensor calibration and performance will be summarized together with the expected detector performance. The PDE decrease of the SiPM observed in the high rate muon beam environment and our possible remedy will also be discussed.