1. Development of Liquid Xenon Photon Detector for μ → eγ Search Experiment at PSI μ → eγ 崩壊探索実験用液体キセノン検出器の開発 Wataru OOTANI International Center for Elementary Particle Physics University of Tokyo For the MEG collaboration 1.Introduction 2.μ → eγSearch Experiment at PSI 3.Status of R&D of Liquid Xenon Detector 4.Summary

2. MEG collaboration （仮名） in Japan 東大素セ 浅井祥仁、大谷航、小曽根健嗣、佐伯学行、西口創、 真下哲郎、三原智、森俊則、八島純、山下了、吉村浩司 東大理 折戸周治 早大理工総研 岡田宏之、菊池順、澤田龍、鈴木聡、 寺沢和洋、道家忠義、山下雅樹、吉村剛史 高エ研 久野良孝、杉本康博、春山富義、真木昌弘、山本明 名大 増田公明

3. Introduction Charged lepton flavor violating (LFV) process Forbidden in the Standard Model Sensitive to physics beyond the Standard Model SUSY-GUT, SUSY+ν R, … Present experimental bound Br( μ ＋ → e ＋ γ) < 1.2 x 10 － 11 (MEGA experiment) μ＋→e＋γμ＋→e＋γ μ+μ+ e+e+ γ Ee=Ee= m μ 2 +m e 2 2m μ Eγ=Eγ= mμ2－me2mμ2－me2 ~ 52.8MeV e + and γ Back-to-back Coincident

4. SUSY-GUT SU(5) SUSY-GUT predicts Br （ μ ＋ → e ＋ γ ） ＝ 10 -15 - 10 -13 (SO(10) SUSY-GUT: even larger value 10 -13 - 10 -11 ) L.J.Hall et al. Nucl. Phys. B267(1986)415

5. Neutrino Oscillation and SUSY Signature of μ → eγ could be discovered at the sensitivity of Br ~ 10 -14 “MSW small angle mixing’’ and “Just-so’’ are disfavored by recent Super Kamiokande results

6. μ → eγsearch experiment at PSI Liquid Xe photon detector Positron spectrometer with gradient magnetic field Thin superconducting solenoid DC muon beam at PSI 10 8 μ/sec Expected sensitivity Br(μ → eγ) ~ 10 -14

7. Sensitivity and Backgrounds N μ =1x10 8 /sec, T =2.2x10 7 sec, Ω/4π=0.09, ε γ =0.7,ε e =0.95 Br( μ ＋ → e ＋ γ ) ~ 0.94 x 10 -14 Major backgrounds Single event sensitivity Accidental Coincidence Michel decay(μ ＋ → e ＋ ν e ν μ ) + random γ Radiative muon decays (inner bremsstrahlung) μ ＋ → e ＋ ν e ν μ γ

8. Expected Detector Performance ΔEeΔEe 0.7% (FWHM) ΔEγΔEγ 1.4 – 2.0 % (FWHM) ΔeγΔeγ 12 – 14 mrad (FWHM) ΔteγΔteγ 0.15 nsec (FWHM) Backgrounds can be suppressed well below 10 -14 B accidental ∝ ΔE e Δt eγ ( ΔE γ ) 2 ( Δ  eγ ) 2 Accidental background rate

9. Positron Spectrometer Superconducting solenoid spectrometer with gradient magnetic field Constant bending radius independent of emission angle Positrons are quickly swept out Uniform field Gradient field Uniform field See also : 西口創他 μ ＋ → e ＋ γ 崩壊探索実験のための e ＋ スペクトロメータの研究開発 (25pYE-10)

10. Liquid Xenon photon detector Scintillation light is viewed by ~800 PMTs (Mini-Kamiokande type) effective coverage ~ 35% Good energy resolution Fast response Spatially uniform response See also : 八島純他 μ ＋ → e ＋ γ 崩壊探索実験の 高性能液体 Xe photon detector の R&D (25pYE-9)

11. Liquid Xenon as Scintillator High light yield (75% of NaI(Tl)) Fast signals Spatially uniform response Excitation Xe+Xe * → Xe 2 * → 2Xe+hν(175nm) Recombination Xe + +Xe → Xe 2 + Xe 2 + +e → Xe ** +Xe Xe ** → Xe * Xe+Xe * → Xe 2 * → 2Xe+hν(175nm)

12. Properties of Liquid Xenon Scintillator Mass number131.29 Density3.0 g/cm 3 Boiling and melting points165 K, 161 K Energy per scintillation photon24 eV Radiation length2.77 cm Decay time4.2 nsec (fast component) 22 nsec (slow component) 45 nsec (recombination) Scintillation light wave length175 nm Refractive index1.57

13. R&D of PMT Hamamatsu R6041Q Dynode structureMetal channel Photo cathodeRb-Cs-Sb WindowQuartz Quantum efficiency10-15 % PMT size57 mm dia. Effective area46 mm dia. PMT Length32 mm Typical H.V.1000 V Current amplification9x10 6 TTS0.3 ns typ. Dynode structure

14. R&D of PMT R6041Q can be stably operated at liquid Xe temperature (165K)

15. First Prototype of Liquid Xenon Detector 32 x PMTs (R6041Q) Active Xe volume 116 x 116 x 174 mm 3

16. PMT Frame of First Prototype

17. Energy Resolution Measurements Possible to achieve   < 1% for 52.8MeVγ

18. Position Resolution Measurements Positions are determined by means weighed by PMT output Possible to achieve  < 1mm for 52.8MeV γ

19. Timing Resolution Measurements Possible to achieve σ~ 50psec for 52.8MeV γ

20. PMT Calibration with Gas Xe PMT calibration with scintillation light (175nm) from gas Xe Quantum efficiency Gain Position dependence on photo-cathode plane Movable α- source with collimator spread of light spot ~ 2mm Operating temperature 190K – 300K

21. Position Dependence on Photo-cathode Plane 300K190K

22. Liquid Xenon Level Meter Capacitance level meter Level resolution < 3mm

23. Large Prototype of Liquid Xenon Detector Prototype of larger size 1/4 - 1/3 size of final detector 264 PMTs Measurement of Resolutions for high energy γ Energy, timing, position, … Light attenuation length, light absorption length Establishment of calibration technique and cryogenics,, purification system, … Tests with large prototype will start at the beginning of 2001 …

24. Tests with High Energy Photon Beam TERAS electron storage ring of Electrotechnical Laboratory (ETL) Inverse Compton gamma rays with an energy up to 40MeV ETL electron LINAC facility TERAS See also : 豊川弘之他 蓄積リング TERAS における汎用 LCS ラインの建設 (23aYC-2)

25. Support structure of PMTs on the front wall of the large prototype PMT G10 Lucite

26. Schedule of μ → eγsearch experiment 2000 Construction of large prototype of liquid Xe detector R&D of positron tracker and timing counter Beam test of positron tracker at PSI (18/10/200~) Design work of superconducting solenoid 2001 Test with large prototype at ETL Construction of superconducting solenoid (winding, cryostat, … ) Beam line studies at PSI Design work of final version of liquid Xe detector 2002 Fabrication and assembly of detector component Tests of each detector component 2003 Engineering/physics run

27. Summary New experiment to search for μ ＋ → e ＋ γwith a sensitivity of 10 -14 at PSI is in preparation R&D works of Liquid Xe photon detector with good energy and timing resolutions are under way Performance of the liquid Xe detector for high energy γ will be checked with large prototype early 2001 Preparations of other detector components are on- going Engineering/physics run will be started in 2003

28. Mechanical Analysis of Superconducting Solenoid Stress distribution in the coil (cross-sectional view) Max 180MPa Center of the solenoid →

29. Stray Magnetic Field in Liquid Xe detector Region Iron yoke Active shields (compensation coil) PMT with fine mesh dynode structure