1. Upgrade of the MEG liquid xenon calorimeter with VUV-light sensitive large area SiPMs Kei Ieki for the MEG-II collaboration 1 II

2. Contents 2  MEG and MEG II  Upgrade of liquid Xe detector  VUV-sensitive MPPC  Signal transmission  MPPC performance tests  Simulated dector performance

3. MEG: μ  e γ search 3 e+e+ γ μ+μ+ μ+μ+ e+e+ γ New physics prediction Upper limit (MEG): 5.7×10 -13 MEG II goal: 4×10 -14 History of μ  e γ upper limits BSM (e.g. SUSY) LFV decay μ  e γ is a good probe for new physics! 52.8 MeV

4. Signal and background 4 e+e+ γ μ+μ+ Signal Main BG e+e+ γ from μ  e νν from μ  e ννγ or e + e -  γγ Same energy (52.8MeV) Same timing Back to back Energy, position and timing resolution are important! Accidental coincidence +

5. The MEG experiment 5 μ + beam e+e+ γ Liquid Xe γ -ray detector Liquid Xe γ -ray detector e + drift chamber & timing counter e + drift chamber & timing counter Gradient magnetic field

6. The MEG experiment 6 μ + beam e+e+ γ 900l liquid Xe large & homogeneous High light yield (~75% of NaI) Fast signal ( τ decay ~45ns) Short radiation length (2.8cm) Liquid Xe γ -ray detector Liquid Xe γ -ray detector Successfully operated in 2009~2013 e + drift chamber & timing counter e + drift chamber & timing counter

7. Upgrade of the Xe detector 7 Resolution was limited by the non-uniformity of the photon collection efficiency. 12x12mm 2 MPPC × 4096 2’’ PMT × 216  Replace the PMTs at the γ incident face to 12x12 mm 2 MPPCs. (CG) Upgrade!

8. Upgrade of the Xe detector 8 MPPC+PMT PMT Imaging power will be significantly improved.  Better energy & position resolution Imaging power will be significantly improved.  Better energy & position resolution The layout of the PMTs will also change.  less energy leakage, better uniformity

9. VUV-sensitive MPPC 9 ● Sensitive to VUV-light  Protection coating is removed, VUV-transparent quartz window is used for protection. ● Large area (12x12 mm 2 )  signal tail become long due to large capacitance.  Reduce capacitance by connecting 4 chips in series. We have successfully developed VUV-MPPC in collaboration with Hamamatsu Photonics. K.K. 2.5mm - 50 μ m pitch pixel Hamamatsu S10943-3186(X) - metal quench resister - 4 independent chips 4-segments 2-segments or

10. Signal transmission 10 PCB inside chamber outside chamber DAQ (WaveDream) “co-axial like structure” MPPC chips are connected in series on PCB. Co-axial like structure PCB and feed-through.  good shielding, high bandwidth, small crosstalk (<0.3%) feed- through

11. MPPC performance tests 11  Small sample test in liquid Xe  Mass test in room temperature  Mass test in liquid Xe

12. MPPC small sample test 12 In liquid Xe LEDMPPC α source ( 241 Am) on wire Several performance tests have been done in 2 litter Xe chamber. Measurement of MPPC properties with LED  Gain, crosstalk, afterpulse etc. α source  PDE, energy resolution example of LED signal 100ns Charge example of charge distribution with LED 0p.e. 1p.e.

13. MPPC small sample test 13 PDE vs. over voltage Energy resolution vs. Np.e. Basic performance of final model MPPC @ V over ~7V (4 segment series connection): Gain: 8x10 5, Crosstalk+Afterpulse probability: 25%, Signal decay time: 30ns PDE: 16~25% (large uncertainty due to geometry of the setup) VUV sensitivity and short decay time are confirmed.

14. Mass test in room temperature 14 LED 600 prototype MPPCs have been tested in room temperature. Basic properties at T=20 deg are measured for each chip No dead channel found. Same setup is being used for the test of final model MPPCs (4000 pcs). Readout PCB relays to change readout ch MPPC insertion/removal tools Temperature controlled chamber

15. Mass test in liquid Xe 15 feed-through α sources 241 Am LED PCBs (12+12) in LXe 600 prototype MPPCs are tested in liquid Xe Goal: Test all the readout system (PCB etc.) MPPC & PCB

16. Mass test in liquid Xe 16 feed-through in LXe 600 prototype MPPCs are tested in liquid Xe Goal: Test all the readout system (PCB etc.) MPPC & PCB

17. Mass test in liquid Xe 17 feed-through in LXe 600 prototype MPPCs are tested in liquid Xe Goal: Test all the readout system (PCB etc.) MPPC & PCB

18. Mass test in liquid Xe 18 Response to the LED light and α scintillation signal are measured for all PCBs. We confirmed that MPPC, PCB and feed-through work OK in liquid Xe. Example of α event for one PCB Example of LED charge distribution α There were ~5% of bad channels found. Most of those were caused by bad connections of MPPCs and cables at PCB and feed-through.  Assembly procedure and the design of the connectors and boards will improve. reduce density of connectors direct soldering of cables

19. Expected detector performance 19 Simulation based on measured properties of MPPCs. Waveform of 1p.e. signal PDE, gain Crosstalk & afterpulse Reconstruction algorithm has been optimized to exploit the advantages of MPPC. u v w

20. 5254565850 E [MeV] 48 depth > 2cm 1.7% 1.0% Timing resolution 0100200300-100-200-300 t rec – t MC [ps] σ =60ps Expected detector performance 20 ResolutionMEGIMEGII u (mm)52.4 v (mm)52.2 w (mm)63.1 E γ (w<2cm)2.4%1.1% E γ (w>2cm)1.7%1.0% t γ (ps)6760 Position resolution 5254565850 E [MeV] 48 depth < 2cm 2.4% 1.1% Position & energy resolution improve by a factor of 2! Energy resolution Also, γ det. efficiency 63%  69% thanks to less material

21. Summary & prospect 21  MEG liquid Xe calorimeter will be upgraded by replacing some PMTs to VUV-sensitive MPPCs.  MPPC, PCB and feed-through have been successfully developed and tested in liquid Xe.  Thanks to the high granularity, the position and the energy resolution will improve by a factor of 2.  Mass test of final model MPPCs is ongoing. Construction of the detector will start this autumn.

22. Backup slides 22

23. Mass test results 23 1p.e. charge @ V over =3.0V 1p.e. pulse height @ V over =3.0V Noise rate @ V over =3.0V CTAP @ V over =3.0V Breakdown voltage

24. Bad channels 24 ~1% of the channels had problems in th PCB and cable connecors. Bad soldering of chips Connector too hard Imperfect isolation of pin

25. Series connections parallel 2 segmented 4 segmented 100ns 25