Upgrade of LXe gamma-ray detector in MEG experiment Daisuke Kaneko, ICEPP, University of Tokyo, on behalf of the MEG collaboration kaneko@icepp.s.u-tokyo.ac.jp Introduction We are searching for the μ → e +γ decay in the MEG experiment at Paul Scherrer Institute in Switzerland, and improved experimental upper limit on the branching ratio. In order to achieve the higher sensitivity, we plan to upgrade the experiment, including the upgrade of the liquid xenon gamma-ray detector. It was turn out by simulation that energy and position resolution will be significantly improved especially for events γ-ray convert at shallow part of detector. It is needed to develop MPPCs which operational in liquid xenon detectors, because conventional MPPCs have little sensitivity to vacuum-ultra-violet range photons, from xenon scintillation. μ → e +γ decay MEG experiment muon beam positron bent by COBRA magnet Stopping target μ → e +γ is a good probe for new physics, because charged lepton flavor violations are suppressed in the standard model. B ~ O (10-50) Many theories such as SUSY-GUT, SUSY-seesaw, etc. predict μ → e +γ in the reachable branching ratio. B ~ O (10-12 ~ 15 ) 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 background is accidental event coincident γ& e+. Therefore, good resolutions are needed to achieve high sensitivity. cLFV search has a long history, but no signal has been observed yet. S. Antush et al. JHEP, 11:090, 2006. Expectation of μ → e +γ branching ratio , from SUSY-seesaw model. recent θ13 measurement MEG excluded 90% C.L. Present limit and future sensitivities 2.4×10-12 → 6×10-13 → 6×10-14 Phys. Rev. Lett., 107:171801, 2011 Current UL (90% C.L.) Goal of 1st stage of MEG Aimed in next stage ← feature of COBRA magnet e+ quickly swept out. (left) Radius independent from angle,(right) MEG Latest physics result → H.Natori’s talk Positron spectrometer upgrade→ M.Nishimura’s poster Expected performance LXe detector upgrade concept Present Upgraded MC Energy resolution is expected to significantly be improved. → Position resolutions improve especially event that γ convert near the inner face. ↓ Detection efficiency improve by about 10%. γ Replace current 2” PMT with smaller detectors for example, ・MPPC ・Smaller, square PMT ・Flat panel PMT Modification in lateral PMT arrangement ・Slant angle 　　　for better uniformity ・Wider inner face 　　　reduce energy leakage MPPC Package drawing We plan to mount MPPCs on PCB. And arrange those PCBs onto detector inner face. Requirements for MPPC CG image Depth [cm] position resolution [mm] Horizontal Red : PMT Blue : MPPC It is necessary to develop MPPCs which operational in liquid xenon, because commercial products don’t have sensitivity, and 3x3mm2 active area is largest. MC MPPC (12mm) Testing specifications to detect xenon scintillation (λ~175nm) 　・Remove protection layer 　・Reduce insensitive layer 　・Anti reflection coating 　・Match refractive index to LXe Larger MPPCs are needed to reduce channels. With 12x12 mm2 active region, (15mm in package dimension) ~4000 channels will be needed to fill inner face. (currently 216 channels of PMT) Result of UV sensitive MPPC Test photon detection efficiency (PDE) measurement Gain is calculated from 1p.e peak charge. And PDE is corrected by removing effect of optical cross-talk & after-pulse. Testing Facility charge 1 p.e. 2 p.e. 3 p.e. pedestal charge If C.T or A.P occur, charge of 1 photon event become larger. Xenon handling system Test bench used in LXe xenon chamber refrigerator gas xenon recovery tank control panel getter Sample MPPCs are mounted on board and immersed in liquid xenon. PMT, α source & LEDs for calibration and Pt-100 RTDs also attached. “F” type “G” type “H” type ↑ Flashing LED at very low intensity. Difference from poisson statistics is effect from C.T. & A.P . Temperature dependence 3mm x 3mm MPPC sample Thermal noise can be reduced at low temperature. Typically, dark count rate (3x3mm2 MPPC) is a few Hz in LXe temp:165K. Shift of Break-down voltage and quench resistance by temperature are observed. 241Am source 5.5 MeV α Anti-reflection cylinder DAQ with DRS Currently, the most sensitive MPPC sample (“G” type) have ~10% PDE at nominal operation voltage. If MPPCs with this PDE are used to detector, almost the same number of photon can be detected compared to current detector. We take waveform with　waveform-digitizer DRS4, the same system as MEG experiment. Plan of the LXe γ-ray detector development sample type 2012 2013 2014 ・Element test ・PCB & Feed-thru design, test ・Proto-type test ・Detector construction Conclusion ・Liquid xenon detector will be upgraded with smaller detector such as MPPC ・We are developing MPPCs to use in liquid xenon ・PDE of most sensitive sample is currently about 10% ・Prototype test in 2013, detector construction in 2014 CG image ←MPPCs are arranged on gamma incident face