Upgrade of the liquid xenon detector in the MEG experiment

The MEG experiment is searching for the lepton flavor violating decay, μ + → e + + γ , with the highest sensitivity. A major upgrade of the experiment is planned where the performance of the liquid xenon (LXe) γ - ray detector is expected to be significantly improved. The current 2-in. photomultiplier tubes (PMT) on the γ - ray incident face will be replaced with smaller photo-sensors such as multi-pixel photon counters (MPPC) in order to improve the resolutions and the efficiency. A UV-sensitive MPPC operational in LXe is under development in collaboration with Hamamatsu Photonics. A high photon detection efficiency (PDE) of 17% to LXe scintillation light is achieved with a prototype with an active area of 12×12 mm2 and a single-photoelectron peak is clearly observed. The engineering design of the detector and the development of the related technologies such as the signal transmission and assembly are also steadily in progress.