Upgrade of LXe gamma-ray detector in MEG experiment Daisuke Kaneko, ICEPP, Univ. of Tokyo, on behalf of the MEG collaborations LXe detector upgrade concept μ → e +γ decay Replacement current 2” PMT with　 ・MPPC 　　　　・Smaller, square PMT 　　　　・Panel type PMT Conventional MPPCs don’t have sensitivity to xenon scintillation light (λ~178nm) Developing with Hamamatsu Modification in lateral PMT arrangement ・Slant angle 　　　for better uniformity ・Wider inner face 　　　reduce energy leakage μ → e +γ is a good probe for new physics, because cLFV is suppressed in SM. B ~ O (10-50) Many theories such as SUSY-GUT predict μ → e +γ in the reachable branching ratio. B ~ O (10-12 ~ 15 ) ↑ MPPC Package drawing 4000ch of MPPCs will be needed to cover all inner face. cLFV search has a long history, but no signal has been observed yet. Expectation of μ → e +γ branching ratio , from SUSY-seesaw model. CG image recent θ13 measurement MPPCs on PCB Cross-section of PCB S. Antush et al. JHEP, 11:090, 2006. Arrangement for detector inner surface MEG experiment Latest physics result → H.Natori’s talk Positron Spectrometer upgrade→ N.Miki’s poster Performed in PSI (Switzerland) in pursuit of most intense DC μ-beam Signal is 52.8MeV γ-ray and 52.8MeV e+ emitted back-to-back. γ-ray → Liquid Xenon Detector positron → Drift Chamber (tracker) Timing Counter Main back-ground is accidental event, occasionally coincident γ& e+. Therefore, good resolution is needed to achieve high sensitivity. muon beam positron bent by COBRA magnet Stopping target Expected performance PMT MPPC Energy resolution is expected to significantly improved. → Position resolutions improve especially at shallow part. ↓ Detection efficiency improve by about 10%. feature of COBRA magnet, e+ quickly swept out. (left) Radius independent from angle,(right) Depth [cm] position resolution [mm] Horizontal Red : PMT Blue : MPPC MPPC (12mm) Achieved & Aiming Branching Ratio Current UL (90% C.L.) Goal of 1st stage of MEG Aimed in next stage 2.4×10-12 → 6×10-13 → 6×10-14 Phys. Rev. Lett., 107:171801, 2011 Testing Facility Result of Recent Element Test Xenon handling system xenon chamber refrigerator gas xenon recovery tank control panel getter PDE(photon detection efficiency) measurement Gain is calculated from subtraction of 1p.e & 2p.e. peaks, and corrected by removing effect of optical cross-talk & after-pulse. We can maintain condition inside of chamber stably. ⊿T ~ 0.5K, ⊿p ~ 0.001MPa charge 1 p.e. 2 p.e. 3 p.e. pedestal charge If C.T or A.P occur, charge of 1 p.e. event become larger. ↑ Flashing LED at very low intensity. Difference from poisson statistics is effect from C.T. & A.P . Test bench used in LXe Sample MPPCs are mounted on board and immersed in liquid xenon. PMT, LEDs for calibration and Pt-100 RTDs also attached. Currently, the most sensitive MPPC sample have ~10% PDE at nominal operation voltage. Dark noise rate Quench resistance Thermal noise could be reduced at low temperature. Typically, dark count rate is few Hz in LXe temp:165K. Shift of quench resistance by temperature is observed. 3x3 mm “G” type 102kΩ 272kΩ “H” type “F” type DAQ with DRS We take waveform with waveform-digitizer DRS4, the same system as MEG experiment. Resistance obtained by fitting I-V curve with MPPC model function.