モンテカルロシミュレーションを用いた MEG 実験における muon radiative decays に関する考察

1. モンテカルロシミュレーションを用いたMEG実験におけるmuon radiative decays に関する考察 ～muon radiative events がトリガーに及ぼす影響について ～ 東京大学素粒子物理国際研究センター 久松康子　他MEG Collaboration

2. Contents • MEG Physics • MEG and muon radiative decay events • Trigger Consideration • Trigger Rates estimates concerning accidental overlap of R.D. and M.D.

3. SM LFC by ignoring neutrino masses SUSY-GUT Large slepton mixing as a result of large top-quark Yukawa coupling Enhancement of lepton flavor mixing in left-handed/ right-handed slepton sector MEG ~ Probing SUSY-GUT through LFV Clear Signal to Physics beyond SM ! Exp. Limit by MEGA (1999) Br ~1.2*10-11 Br. SU(5) GUT 10-15 -10-13@ meR 100-300 GeV ˜ MEG Searches down to Br ~10-14 Research proposal accepted at PSI (1999) Intensive muon beam 107～ 108 muons/sec Phys. Run Start at 2006

4. MEG and μ radiative decay Michel decay Back to back Same momentum 52.8MeV/c (mμ /2) at the same time • Background for searching μeγdecay • Prompt background • Accidental overlap with e+ from Michel decay • Timing Calibration for whole detector Arrival time of γ: tγ Radiative decay γ e+ Arrival time of e+: te

5. Radiative decays as background to μeγ e+ resolution • Prompt background • Phase space extremely limited • Accidental background • Rejection is crucial for μ eγsearch Suppression in trigger level High resolution detectors for γand e+ γresolution Kuno and Okada, Phys.Rev.Lett Vol77 Super Conducting Magnet and DCH for e+ spectrometer Liq. Xe photon detector Detector perfomance (FWHM) δEe =0.7% δEγ =4% δθeγ =17mrad δteγ =150psec Scintillator bars for measuring exact arrival time of e+

6. PMT PMT Trigger algorithm for μeγ Same momentum 52.8MeV/c Back to back at the same time • γenergy selection@ Liq.Xe detector • Eγ>45MeV • Direction matching between e+ and γ • @PMT at Liq.Xe detector and TC bars • Timing selection between e+ and γ @ Liq.Xe and TC bar • Δteγ<10nsec γ Timing counters PMTs

7. PMT PMT Trigger system Environment Liq. Xe γ ray detector ~105 Hz γimpinging Liq.Xe detector ~106Hz e+ hitting Timing Counters Drift chamber Need to digitize continuously Read by FADC Stored in ring buffer memory Timing counters PMTs All read by 100MHz FADC Summed up to evaluate charge Trigger thrshold trigger • Scintillator bars viewed by 2PMTs • PMT signals read by FADC • Positron arrival time is estimated by the • average of rise time of 2 PMTs

8. PMT PMT Trigger system Detectors joining trigger system: Liq. Xe γ ray detector Liq. Xe detector :369ch (216PMTs+612PMTs/4) Timing Counters : 60ch (30 bars, read by 2 PMTs) Drift chamber : 32 ch (16 chambers ×2) Drift chamber Trigger Types μe γtrigger Debugging trigger Calibration trigger calibration of Liq.Xe detector using π0 γγ timing calibration using radiative decay calibration of drift chamber using Michel decay Trigger for other physics run PMTs control signals. Many channels Various physical quantities to evaluate FPGA PMT inputs Need of flexibility FADC 100MHz Signals processed by FPGA PMT signal 100MHz FADC FPGA VME

9. 1FADC / 1 PMT at front face 1FADC / 4 PMT at other faces PMT signal buffering PMT gain equalization Baseline subtraction Sum up collected charge γ FADC readout Efficiency 97% FADC readout simulated using exponential function Trg. Efficiency ~80% θ γEnergy deposit Sum of PMT charge PMT signal 100MHz FADC FPGA Trigger • Trigger algorthm for γ PMTs Energy deposit @ Liq.Xenon Number of p.e. observed γ θ Muon beam e+

10. e+ hit position z e+ hit position φ PMT signal 100MHz FADC FPGA VME PMT PMT at front face of γdetector PMT at Timing Counters φ Trigger Search PMT which has maximum pulse height Direction match between TC and PMT Definition of signal timing δT < 10nsec Timing counters PMT γdirection PMT which outputs maximum signal amplitude γ momentumθ ~10° Estimates well • Trigger algorithm for e+ γ PMT position θ e+ direction specific 5 counters Timing counters PMTs z

11. R.D. M.D. Acc. B.G. events + number of p.e. observed R.D. γspectrum γenergy selection [GeV] • Muon Beam Rate: 2*107 muon/sec • Eγ> 45MeV 375 Hz • e+ from MD hitting Timing Counter 0.67MHz • Δt e γ < 10nsec 5.0Hz • E+ γ direction match 0.68Hz Acceptable for DAQ running speed ~100Hz Trigger rate estimates for accidental b.g. events Trigger Efficiency for μeγ ~75% • γenergy selection • Direction matching between e+ and γ • Timing selection between e+ and γ θ

12. Summary • The detail study on the efficiency of Trigger system for μ eγ searchwas conducted. • The trigger rates for accidental background events are estimated. Study on radiative decay events continues…. • Study on data preselection both in online and offline • Background? Signal events? • Study on trigger system for timing calibration • using radiative decay events