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 a higher sensitivity, we plan to upgrade the experiment, including an upgrade of the liquid xenon gamma-ray detector with readout of smaller photosensors such as MPPC. It was turn out by a simulation that the energy and position resolution will be significantly improved especially for events γ-ray convert at a shallow part of the detector. It is needed to develop MPPCs which is operational in liquid xenon detectors, because commercial MPPCs have little sensitivity to vacuum-ultra-violet range photons, from xenon scintillation. μ → e +γ decay MEG experiment liquid xenon detector Performed in PSI (Switzerland) using the world’s 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 coincidence of random γ & Michel e+. Therefore, good resolutions are needed to achieve a high sensitivity. ○ 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 ) Stopping target muon beam timing counter drift chamber 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 → ~5×10-14 Phys. Rev. Lett., 107:171801, 2011 Current UL (90% C.L.) Goal of 1st stage of MEG Aimed in next stage positron bent by COBRA magnet ← feature of COBRA magnet e+ quickly swept out. (left) Radius independent of angle. (right) ○Until 2013 ○DAQ for 3 - 4 years MEG ○Construction of new detectors for 2 years Annu. Ref. Nucl. Part. Sci. 2008. 58:315-41 W. J. Marciano, T.Mori, and J. M. Roney Latest physics result → H.Natori’s talk Positron spectrometer upgrade→ M.Nishimura’s poster Expected performance LXe detector upgrade concept Energy resolution is expected to significantly be improved. → Position resolutions improve especially for event where γ converts near the inner face. ↓ Detection efficiency improves by about 10%. Present Upgraded MC Replace current 2” PMT with smaller photosensors 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 γ MPPC Package drawing We plan to mount MPPCs on PCB. And arrange those PCBs onto detector inner face. Development of UV-sensitive MPPC It is necessary to develop MPPCs which is operational in liquid xenon, because commercial products don’t have sensitivity to UV light, and 3x3mm2 active area is largest. UV-sensitive MPPC is under development in collaboration with Hamamatsu. CG image PMT Depth [cm] Position resolution σ [mm] Horizontal Red : PMT (present) Blue : MPPC (upgraded) MC MPPC (12mm) MPPC Improve sensitivity to xenon scintillation (λ~175nm) ・Remove protection layer ・Reduce thickness of insensitive layer ・Anti reflection coating ・ Refractive index of sensor surface better matched to LXe Larger MPPCs are needed to suppress the number of channels With 12x12 mm2 active region, (15mm in package dimension) ~4000 channels will be needed to fill inner face. (currently 216 channels of PMT) γ-ray Result of UV-sensitive MPPC Test photon detection efficiency (PDE) measurement Gain is calculated from 1p.e peak. PDE is evaluated with a correction for effect of cross-talk (CT) and after-pulsing (AP). Testing Facility charge charge 1 p.e. 2 p.e. 3 p.e. pedestal If CT or AP occur, charge of 1 photon event become larger. Test bench in LXe Xenon handling system xenon chamber refrigerator gas xenon recovery tank control panel getter Sample MPPCs are mounted on PCB and immersed in liquid xenon. PMT, α source & LEDs for calibration are also attached. Estimated number of the scintillation photons of 5.5MeV α event from 241Am. ↑ Flashing LED at very low intensity. Probabilities of CT and AP are estimated from deviation from Poisson statistics. “F” type “G” type “H” type Temperature dependence 3mm x 3mm MPPC sample Dark count rate 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 waveform digitizer We take waveform with　waveform digitizer DRS4, the same system as in MEG experiment. Currently, the most sensitive MPPC sample (“G” type) has ~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. Plan of LXe upgrade sample type 2012 2013 2014 ・ Development of UV-sensitive MPPC ・PCB & Feed-thru design, test ・Prototype test ・Detector construction Conclusion ・Liquid xenon detector will be upgraded with smaller photosensor such as MPPC ・We are developing MPPCs sensitive to LXe scintillation ・PDE of most sensitive sample is currently about 10% ・Prototype test in 2013, detector construction start in 2014 CG image MPPC ←MPPCs are arranged on gamma incident face γ-ray PMT