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
東京大学素粒子物理国際研究センター久松康子 他MEG Collaboration
~ muon radiative events がトリガーに及ぼす影響について ~
Contents
• MEG Physics• MEG and muon radiative decay events• Trigger Consideration• Trigger Rates estimates concerning accide
ntal overlap of R.D. and M.D.
˜
MEG Searches down to Br ~10-14
Br. SU(5) GUT 10-15 -10-13 @ meR 100-300 GeV
Clear Signal to Physics beyond SM !
• SM – LFC by ignoring neutrino masses
• SUSY-GUT– Large slepton mixing as a result of large top-qu
ark Yukawa coupling• Enhancement of lepton flavor mixing in left
-handed/ right-handed slepton sector
Exp. Limit by MEGA (1999) Br ~1.2*10-11
Research proposal accepted at PSI (1999)Intensive muon beam 107 ~ 108 muons/secPhys. Run Start at 2006
MEG ~ Probing SUSY-GUT through LFV
MEG and μ radiative decay
• Background for searching μ eγdecay– Prompt background– Accidental overlap with e+ from Michel decay
• Timing Calibration for whole detector
Back to back Same momentum 52.8MeV/c (mμ /2)at the same time
γ
e+
Arrival time of γ : tγ
Arrival time of e+ : te
Michel decay
Radiative decay
Radiative decays as background to μ eγ
• Prompt background – Phase space extremely limited
• Accidental background– Rejection is crucial for μ eγsearch
Suppression in trigger levelHigh resolution detectors for γand e+
Super Conducting Magnet and DCH for e+ spectrometer
Scintillator bars for measuring exact arrival time of e+
Liq. Xe photon detector
Detector perfomance(FWHM)δEe =0.7%δEγ =4%δθeγ =17mradδteγ = 150psec
Kuno and Okada,Phys.Rev.Lett Vol77
e+
re
solu
tion
γ resolution
Trigger algorithm for μ eγ
• 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
Same momentum 52.8MeV/c Back to back at the same time
γ
Timing counters
PMTs
PMT
PMT
Trigger system
Timing counters
PMTs
PMT
PMT
Liq. Xe γ ray detector
Drift chamber
~105 Hz γ impinging Liq.Xe detector~106Hz e+ hitting Timing Counters
Environment
Need to digitize continuouslyRead by FADC Stored in ring buffer memory
All read by 100MHz FADCSummed up to evaluate charge
Scintillator bars viewed by 2PMTs PMT signals read by FADC
Positron arrival time is estimated by the average of rise time of 2 PMTs
Trigger thrshold
trigger
Trigger system
Liq. Xe detector : 369ch (216PMTs+612PMTs/4)Timing Counters : 60ch (30 bars, read by 2 PMTs)Drift chamber : 32 ch (16 chambers ×2)
Detectors joining trigger system:
Trigger Types μ e γ trigger 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
PMT
PMT
PMTs
Liq. Xe γ ray detector
Drift chamber
Many channelsVarious physical quantities to evaluate
Signals processed by FPGA
PMT signal 100MHz FADC FPGA VME
Need of flexibilityPMT inputs
FPGA
control signals.
FADC 100MHz
• Trigger algorthm for γ
γ
γ Energy deposit Sum of PMT charge
Energy deposit @
Liq.Xenon Number of p.e. observed
PMTs
PMT signal 100MHz FADC FPGA
PMT signal bufferingPMT gain equalizationBaseline subtractionSum up collected charge
γ
θ Muon beame+
θ
Trg. Efficiency ~80%
Trigger
1FADC / 1 PMT at front face1FADC / 4 PMT at other faces
FADC readout simulated using exponential function
FADC readout Efficiency 97%
• Trigger algorithm for e+
γ
PMTs
Timing counters
PMT position θ
γ m
omentum
θ
γdirection PMT which outputs maximum signal amplitude
~10°Estimates well
e+ direction specific 5 counters
z
Timing counters
PMT
PMTφ
PMT signal 100MHz FADC FPGA VME
Trigger
PMT at front face of γdetectorPMT at Timing Counters
Search PMT which has maximum pulse heightDirection match between TC and PMTDefinition of signal timing δT < 10nsec
e+ hit position z e+ hit position φ
Trigger rate estimates for accidental b.g. eventsTrigger Efficiency for μ eγ
• γenergy selection• Direction matching between e+ and γ• Timing selection between e+ and γ
Muon Beam Rate: 2*107 muon/secEγ> 45MeV 375 Hze+ from MD hitting Timing Counter 0.67MHzΔt e γ < 10nsec 5.0HzE+ γ direction match 0.68Hz
M.D.R.D.
+
Acc.B.G. events
R.D. γspectrumnumber of p.e. observed
[GeV]
γenergy selection
Acceptable for DAQ running speed~100Hz
θ
~75%
Summary
The detail study on the efficiency of Trigger system for μ eγ search was conducted.
The trigger rates for accidental background events are estimated.
Study on radiative decay events continues….
•Study on data preselection both in online and offlineBackground? Signal events?
•Study on trigger system for timing calibration using radiative decay events