Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University1 MEG 実験による   e  探索 Run2009 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 ２０１０年３月２３日 日本物理学会 第６５回年次大会 岡山大学津島キャンパス Contents   e  search & MEG Experiment MEG Run2009 and detector performance in 2009 Analysis procedure & the current status Prospects in 2010

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University2 Background Accidental background is our main background   e  search Signal Positive  decay at rest Clear two body kinematics Back to back (180°) E e ≈E  =52.8MeV Coincident (T e =T  )  BG e + BG Michel positron Random .etc   ee,   e    e Good detector performance is essential High rate e + measurement with intense  beam

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University3 MEG Experiment Target sensitivity : Br(   e  )~ Current best limit : 1.2x by MEGA experiment World’s most intense continuous muon beam MEG detector1.3MW PSI Proton cyclotron Physics DAQ has started since 2008 The result of Run2008 : Br(  +  e +  )≤2.8x (90%C.L.) (Sensitivity was 1.3x ) (to be published, cf. J.Adam et al, MEG Collaboration, arXiv: )

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University4 MEG Run2009 Full DCH modules worked well LXe light yield as good as expected by prototype Physics run condition 6.3Hz trigger rate 38TB in total including calibration data Physics run2009 was shorter than run2008 Apr Jul Oct Jan 2010 PSI accelerator shutdown Other experiment DCH modification Maintenance work for MEG Detector setup Calibration Waveform digitizer update Physics run ( 2 months ) PSI accelerator shutdown 2.9x10 7  /s beam intensity In total, 6.5x10 13  stopped on target Three days maintenance Run2009

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University5 Positron spectrometer Discharge problem of positron tracker (drift chamber, DCH) happened in The problem was finally figured out in HV distribution cards after intensive works All DCHs were repaired before physics DAQ in 2009, and the detection efficiency (14%  40%) and performance of DCH was much improved. Hit map  e 18  11mrad  e 10  8mrad

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University6 Photon Detector (XEC) Light yield of liquid xenon in 2008 was improving during physics run by purification Gaseous purification was done for all xenon before physics run in 2009, and the light yield in 2009 was as good as expected by our prototype Light yield in 2009 had been stable during physics run without any purification. Run2008 gas&liquid purification Run2009 Light Yield Energy resolution Calibration is not finalized yet in 2009, but comparable resolution is already achieved signal

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University Detector performance (preliminary) Gamma Energy (%)2.0 (w>2cm)  Gamma Timing (psec)80>67 Gamma Position (mm)5(u,v)/6(w)  Gamma Efficiency (%)63  e + Timing (psec)<125  e + Momentum (%) e + angle (mrad) 10(  )/18(  )8(  )/11(  ) e + efficiency (%)1440 e + - gamma timing (psec)148<180 Muon Decay Point (mm)3.2(R)/4.5(Z)2.2(R)/3.1(Z) Trigger efficiency (%)6688 Stopping Muon Rate (sec -1 )3x x10 7 (300  m) DAQ time / Real time (days)48/7835/43 box5x x Expected N BG Sensitivity1.3x x BR upper limit (obtained)2.8x Measured in  Analysis is still being improved Resolution  : comparable with 2008 e + : all  are improved Efficiency  : comparable with 2008 e + : x 2.9 Trigger efficiency: x 1.3 Running time : ~ 0.73 Data statistics in 2009 : x 2.2 N BG : 0.7 Sensitivity: 6.6x For the first time, we will search for   e  decay with a sensitivity beyond the current limit! (2009 numbers are preliminary)

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University8 Analysis procedure Preselection: ~20% of all events Blind analysis to avoid any bias T e  (-1ns<T e  <1ns), E  (48<E  <57.6MeV) Optimization of analysis algorithms and background study Open the box Likelihood fit for data, and calculate confidence intervals with toy MC simulation N sig normalized to the number of observed Michel decays BR(  +  e +  ) = N sig / (Normalized factor) Analysis box Blinding box Signal box Preselection

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University9 Maximum likelihood fitting PDF preparation (S, R, B) : PDF for (   e , RMD, Background) Product of PDFs for the five observables ( E , E e, t e ,  e  and  e  ) Fitting parameters (N BG, N RMD, N Signal ) All figures shown here are run2008 results All PDFs are being prepared now for run2009 Blue: All Pink: BG Red: RD Green: Signal EE EeEe TeTe ee ee

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University10 Normalization Count the number of detected Michel positrons (Michel trigger) –N e obtained simultaneously with the signal (MEG trigger) The upper limit on BR (  +  e +  ) is calculated by normalizing N sig to the N e Independent of instantaneous beam rate and insensitive to positron acceptance and efficiency Theory Resolution Selection efficiency

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University11 Run2009 PDF preparation We are still calibrating the detector and optimizing algorithms Several times, re-processing for preselected events will be done after certain improvement Hopefully in June 2010, we’ll open the blind box, and do the final analysis Target: publish the result at this summer conference E  sideband data  PDF for E  BG E e data  PDF for E e BG T e  data  PDF for T e  signal

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University12 Run 2010 Detector performance should be improved by reconstruction algorithm and calibration methods – Positron beam ( efficiency, resolution ) – DCH noise removal ( resolution ) – Gamma energy (1.5%  at the best point) – Trigger efficiency ( TC fiber counter ) – Optimization of beam stopping position Data statistics in 2010 will be 11 times higher than 2008 ( 6 months data taking ), and the sensitivity is estimated to be 1.3x Three years data taking ( ) will enable us to reach the sensitivity of ~ level

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University13 Summary MEG experiment has started physics run since 2008, and 2008 results were already presented at LP09, Br(  + ->e +  )<2.8x Running time in 2009 was shorter than 2008, but data statistics is about 2.2 times higher than 2008 because of improved DC efficiency. Because the sensitivity is expected to be 6.6x (this is still preliminary), we will really search for   e  decay with a sensitivity beyond the present upper limit (1.2x ). Run2009 data are being analyzed, and the goal is to publish the result at this summer conference. Sensitivity of MEG experiment will reach level in three years ( ).

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University14 Prospects in (preliminary)2010 (preliminary) Gamma Energy (%)2.0 (w>2cm)  1.5 (w>2cm) Gamma Timing (psec)80>6768 Gamma Position (mm)5(u,v)/6(w)  Gamma Efficiency (%)63  e + Timing (psec)<125  90 e + Momentum (%) e + angle (mrad) 10(  )/18(  )8(  )/11(  )8(  )/8(  ) e + efficiency (%)1440 e + - gamma timing (psec)148< Muon Decay Point (mm)3.2(R)/4.5(Z)2.2(R)/3.1(Z)1.4(R)/2.5(Z) Trigger efficiency (%) Stopping Muon Rate (sec -1 )3x x10 7 (300  m) 3x10 7 DAQ time / Real time (days)48/7835/43133/162 box5x x x Expected N BG Sensitivity1.3x x x BR upper limit (obtained)2.8x

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University15 Optimization of beam stopping position Degrader thickness (300  m  200  m) in 2009 to match 6% air contamination in COBRA by means of MC study. Strong asymmetry was observed by TC upstream/downstream hit map Beam test for optimization of the degrader is planned

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University16 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) TC bars (  measurement ) 207 Bi on TC bar center Better z position calibration z 

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University17 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

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University18 PMT gain shift Problem in photo cathode production stage –Might be different mixture at certain period, too much alkali?

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University19 “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

Mar Toshiyuki Iwamoto (ICEPP) JPS 2010 Spring meeting, Okayama University20  e ,  e  Opening angle resolution combination of positron angle resolution target position resolution photon detector position 2008 results