MEG 2009 現状と展望 東京大学素粒子物理国際研究センター 岩本敏幸 他 MEG コラボレーション 日本物理学会 2009 年秋季大会 甲南大学岡本キャンパス

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University2 Contents Detector performance in 2008 Problems in 2008 and solutions for 2009 run DCH XEC TC Upgrade plan  Waveform digitizer (DRS2 to DRS4)  TC fiber readout into trigger Prospects in 2009

MEG detector performance in 2008 Efficiency%  66 x 91 e+e+ 30 x 40 Trigger66 Selection63x70x90 3 DAQ80x93 Calibration Run~70 Running Time11.5 (week) Resolution E  (%)2.0 T  (ps)<80 Position  (mm) p e + (%)0.7, 2, 3.8 T e + (ps)<60-90  e + (mrad, ,  ) 18~10  Decay Point (mm) 3-4 T e +  (ps)150 Br(  +  e +  ) < 3.0x (90% CL) (expected 90%CL sensitivity was 1.3x ) 90% signal box

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University4 Problems in 2008 Drift chamber (DCH) Operation Discharge problem Lower efficiency Worse  p,  ,   Timing counter fiber operation Noisy, couldn’t be used for trigger worse efficiency for e + -  direction match Liquid xenon calorimeter (XEC) light yield Light yield: less (~70% of prototype) Because the light yield can be monitored by 17.6MeV CW calibration, gaseous purification was continued during physics run

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University5 Solutions for discharge problem Investigated weak points for all HV connections HV soldering spot on PCB, and HV via on PCB are suspicious DCH discharge reproduced at Lab test finally after many trials ! Solutions New design of PCB Individual layers with only HV and only GND ( 3 -> 4 layers ) Potting of HV soldering spot with epoxy

After modification Two chambers with new anode HV pcb into “Aquarium” to see long term operation with He/Ethane inside and pure He outside and nominal HV 16 DC modules are mounted in the support structure inside the helium cabin. Signal check by nominal HV Stable operation is confirmed for ~ 5 months Aquarium Helium Cabin

Expected performance (DCH) These MC study will be verified by this year’s Michel data taken with the physics data Efficiency ~ 3 times better by MC including DCH, DCH-TC matching efficiency Resolution E e : tail part reduced 374keV(60%), 1.06MeV(33%), 2.00MeV(7%) ~ 1.3% (2008)  e  e : 18-10mrad

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University8 XEC detector maintenance Detector maintenance during winter shutdown Fix some (possible) leak positions Fix bad PMTs Gaseous purification for 900L xenon In July 2009, XEC detector filled with liquid again XEC detector’s been stable since then Light yield monitoring by CW, alpha, CR Liquid purification (~9 times volume circulation)

Gaseous & Liquid purification Gaseous purification Liquid purification 1000L liquid storage tank XEC detector 8 x 250L gas storage tank

New purifier system O 2 getter cartridge Developed for LAr use at CERN H 2 O also removed by Oxidization-reduction of Cu New pump speed ~ 180l/h 43 hours operation  7740l circulation  9 times circulation

Light yield in MeV  of Li(p,  )Be, Light yield monitoring Higher light yield ~ 40% improved Consistent with prototype  E  might be improved by this light yield increase X / /7

 waveform Resolution, energy scale will be measured by  0 calibration before physics run  decay time ~ 45ns (as expected, ~30ns in last year) Longer  waveform after purification 2008/9 2008/ /7

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University13 TC modification Plastic scintillating fiber + APD ( z measurement ) To provide precise z position for online trigger New front-end boards design S/N ~ 8 (noise~40mV) Shorter shaping time for trigger (250ns -> 20ns) Direction match 66%  >90% z 

Direction match To get back-to-back e +  events –  direction Inner face PMT index with maximum charge in XEC detector ( ,  ) – e + direction TC bar counter (  ) TC bar counter charge asymmetry or TC fiber ( , or z) – Direction matching at trigger level LUT based on MC z reconstruction resolution – ~7cm at online by using TC bar charge asymmetry – ~5mm by using TC fiber

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University15 Electronics modification Waveform digitizer (DRS2  DRS4) 700MHz ~ 5GHz, 8(+1) channels with 1024 storage cells Eliminate temperature drifts Linearity improved ( 0~1V ) Timing accuracy below 30ps Currently being tested Trigger Dead time reduction Dynamic range Some front face PMTs saturated with input range(0-1V) -> (0-2V) Dedicated  trigger Improved separation between  ’s and  ’s Direction matching efficiency improvement by using TC APD information

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University16 Prospects in 2009 Expected sensitivity(90%CL) in 2009 ~ 2.4x ( ~5 times better) Resolution improvement  E   calibration  light yield  Ee   e,   e  Te  stable DC operation Background level is still low Limited by statistics Efficiency improvement e + (~0.4) Trigger efficiency (>0.9) DAQ livetime Less calibration time Running Time ~ 8.6 week In total, ~ 5 times better

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University17 Schedule Other experiment had used  E5 beam line until the beginning of Aug. All detectors are ready at Mid. Sep. Calibration run until beg. Oct. Physics run from Oct. ~ 3 months Aug Sep Oct Nov Dec Detector installation  0 calibration Physics DAQ Lamb shift experiment

Sep Toshiyuki Iwamoto (ICEPP) JPS 2009 Autumn meeting, Konan University18 Summary In 2008 physics run, Br(   e  )<3x (90%CL) Main problem was DCH discharge, and this problem should be solved in this years run Efficiencies, resolutions of all detectors will be improved, and 5 times better statistics than 2008 will be available in All detectors will be ready in mid. Sep., and physics run will be started at Oct after a calibration run using  0 in late Sep. MEG sensitivity in 2009 will still be limited by statistics, and the expected sensitivity is ~5 times better (2.4x %CL). Two more years will be needed to reach a sensitivity goal. 素粒子実験領域シンポジウム 主題： Japan-Korea Joint Symposium: Lepton Flavor Violation Experimental Search for LFV Muon Decay U-Tokyo ICEPP Mori Toshinori

“Ghost pulse” problem R “Ghost pulse” 2 GHz “Ghost pulse” 2 GHz After sampling a pulse, some residual charge remains in the capacitors on the next turn and can mimic wrong pulses Solution: Clear before write write clear

 0 calibration  - p   0 n   n Back-to-back photons – 55, 83 MeV Opposite detector – NaI + APD array LH 2 target  - beam condition wasn’t the same as  + beam ( many e - background ) This year, 1/4  - beam rate will be used to estimate energy, timing, position resolution as close condition as the  + -- LH 2 NaI+APD   XEC

Xe scintillation photon Excitation process (fast) – Xe + Xe*  Xe* 2  2Xe + h Recombination process (slow) – Xe + + Xe  Xe + 2 – Xe e  Xe** + Xe – Xe**  Xe* + heat – Xe + Xe*  Xe* 2  2Xe + h