Commissioning of liquid Xe detector with VUV-MPPC readout for MEG II experiment

MEG II experiment is an upgrade of MEG experiment to search for charged lepton flavor violating decay of muon, $\mu \rightarrow e\gamma$. Target sensitivity of MEG II is $6 \times 10^{-14}$ at 90% C.L., which is one order of magnitude better than MEG. Liquid xenon detector was used to detect $53 \,\mathrm{MeV}$ signal $\gamma$-ray in MEG, and this detector has been upgraded for MEG II. Replacement of 216 PMTs on the entrance face with 4092 newly developed VUV-sensitive MPPCs has been carried out to have better granularity and uniformity of the scintillation readout. In 2017, commissioning of this detector has been started. Sufficient performance has been confirmed for most of the MPPCs. Pilot run of this detector with muon beam was performed, and $\gamma$-ray near signal energy has been successfully detected. By the even-odd analysis of the obtained data, good timing resolution ($44 \,\mathrm{ps}$ for $50 \,\mathrm{MeV}$ $\gamma$-ray) has been achieved.