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