μ-e conversion 探索実験--- COMET --Masaharu Aoki, Osaka U.
「J-PARCハドロン物理の将来研究計画を考える」研究会
理化学研究所2008/9/1-2

内容• イントロダクション• μ-e conversion 探索実験 @J-PARC --- COMET• パルス陽子• パイオン捕獲ソレノイド• カーブソレノイドスペクトロメータ• 実験感度• mu2e@FNAL• まとめμ-e Conversion• Muonic Atom (1S state)
Muon Capture(MC)µ−Muon Decay in Orbit (MDO)

nuclei• MC:MDO = 1:1000(H), 3:2(Al), 13:1(Cu)• τ(free-μ-) = 2.2 μs• τ(μ-;Al) = 0.88 μs• μ-e Conversion
Coherent Process−−Γ[µ+(A,Z)→e+ (A, Z)]−−
BR[µ + (A, Z) → e + (A, Z)] ≡Γ[µ− + (A, Z) → νµ + (A, Z − 1)]3• μ- + (A,Z) → e- + (A,Z)• レプトン・フレーバー非保存過程：標準理論では禁止されている。
•

ニュートリノ混合による高次効果は非常に小さい• 標準理論を越える物理に敏感
A. de Gouvea

超対称性理論slepton 質量行列の物理
4

理論的予想Process
µN→eN
µ→eγ
τ→µγ

Current SUSY-GUT
Limit
level
10-13
10-16
10-11
10-14
10-6

FutureSUSY+Seesaw, MSW Large Angle

10-16,10-18
10-13

10-910-8

SUSY-GUT

MEG

MEG
COMET

COMET

PRISM/PRIME
PRISM/PRIMECourtesy Hisano5Golden Triog-2

c-LFV

EDMlarge top Yukawa coupling˜"˜˜"˜˜mixingẽe~B!0!0RealImaginary

slepton mass matrix 2mẽẽ
2∆mµ̃ẽ
2∆mτ̃ ẽ2∆mẽµ̃
2mµ̃µ̃
2∆mτ̃ µ̃τ-LFV2∆mẽτ̃
2∆mµ̃τ̃
2mτ̃ τ̃Physics Capabilities
J-PARC PAC report

US P5 Report

COMET Overview
Pion Capture Section

ProductionTarget

A section to capture pions with a largesolid angle under a high solenoidal
magnetic field by superconducting
magetDetector Section

A detector to search formuon-to-electron conversion processes.

Stopping
TargetPion-Decay andMuon-Transport Section

A section to collect muons fromdecay of pions under a solenoidal magnetic field.

• Pulsed Proton Beam
• π-b.g. suppression• J-PARC/MR• Large μ yields• J-PARC/MR

only 60 kW out of 450kW• π-capture SC-solenoid• 1011 μ/s (PSI:108 μ/s)

• Curved-solenoid detector• Lower detector rate
• Upgradability to PRISM• add Phase-Rotator-Ringパルス陽子ビーム• バックグランド• π-+(A,Z) → (A,Z-1)* → γ+(A,Z-1)，γ → e+ e- ：一次陽子ビームに同期• μ- decay-in-flight, e- scattering, neutron streaming• 信号•μ- +(A,Z) → e- +(A,Z) : 遅延 (~1μs)
0.7 μsNbg = NP×Rext×Yπ/P×Aπ×Pγ×A
NP : total # of protons (~1021)
Rext : Extinction Ratio (10-9)
Yπ/P : π yield per proton (0.015)
Aπ : π acceptance (1.5×10-6)
Pγ : Probability of γ from π (3.5×10-5)
A : detector acceptance (0.18)
BR=10-16, Nbg < 0.12
⇔ Extinction < 10-9パルス陽子 from J-PARC/MR
Tomizawa Scheme• 8 GeV, 7 μA
• Beam Pulsing• Empty bucket Scheme• Bunched Slow Extraction• Beam Emittance• V: Reduce RCS painting area
• Reduce NP in a bunch:small space charge eﬀect• H: SX < 5π mm.mrad• Proton Yield• High Repetition Rate
Extinction: < 10-9Power: 60 kW (4×1013 pps@8 GeV)

空バケツ法

Pion Capture
SC Coilpt (GeV/c)targetπ production ratebeam
pl (GeV/c)

3600

Magnetic field (Tesla)

ProtonW shield
300
1000

140064200100200300

Z position along solenoid axis (cm)

• Tungsten Target• 16cm × 0.8cmφ• water or radiation cooling• ΔE@Target: 3-4 kW• yields

p (GeV/c)

0.002 stopping-μ-/proton(M. Yoshida)pt (GeV/c)

μ-yield vs. Bmax

pt (GeV/c)

DIO/Stopping-µ

Curved Solenoid Spectrometer
-2

10-4

10-610-8

10-10

10-12Collimator

Detector solenoid

30MeV/c
60MeV/c
105MeV/cCurved solenoid
spectrometer

Acceptance

10020

4060

80

10080

100Eth (MeV)

0.6
0.5
0.4
0.30.2

Traget solenoid

0.1

Top view

Side view

00204060Momentum (MeV/c)

Transmission eﬃciency

Detector Acceptance &Signal Sensitivity
mu-e Conversion and Decay-in-Orbit

Geometrical Acc.
Electron Transport
pt > 52 MeV/c
χ2 Cut (χ2 < 9)Energy Selection
Timing cut
Total

Acceptance
0.73
0.44
0.67
0.86
0.56
0.38
0.04

Events per 0.192 MeV/c

103

Decay-in-Orbit

102mu-e Conversion Signal

10110-1
10-210-3100

101

102

103

104
105
106
107
Electron Momentum (MeV/c)

B(µ + Al → e + Al) =−−1
Nµ · fcap · Ae

Proton Intensity
Running Timeμ’s yields per proton
μ-stopping eﬃciency

4 × 1013 Hz
2 × 107 sec
0.007
0.26

•Nμ = 1.5 × 1018
•fcap = 0.6 for Aluminum•Ae = 0.04
•B(μ- + Al → e- + Al) = 2.8 × 10-17Total

1.5 × 1018 stopped μ’s< 5 × 10-17 (90% C.L.)

(Still) Straw-man’s Layouts

DRJ-PARC
NP-HallAFT

Recent Activities• muon task force (chaired by S. Mihara)• to address charges given by PAC• J-PARC/MR extinction study• pulsed FX measurement: MR abort line
• pulsed SX measurement: K1.8BR• COMET location study

mu2e @Fermilab• mu2e(FNAL + xMECO)• Revive of MECO
• After the shutdown of Tevatron• Parasite on SNuMI-2• 2012 ~• Renovate a Debuncher ring forbeam bunching• Single Event Sensitivity: 2×10-17

A-D Line

AP4 Line

AP5 Lineまとめ• μ-e電子転換過程は標準理論を越えたc-LFV過程の一つであり、TeVスケールの物理を研究する為の重要な手段を提供する。

• 10-16まで探索すればイベントが見つかるかもしれない。
• MEG(μ→eγ 探索@ PSI)とは相補的である。
• COMET• BR=10-16 で μN→eN を探索する実験• J-PARC/MRとハドロンホールを活用• Fermilab/mu2eとCOMETの競争• muon-TF (chaired by S. Mihara)• extinction study• location study

End of Slides