 N-eN Rare Process M. Aoki, Osaka University MuSAC-2009, Tokai, 2010/3/10-11

Contents Introduction – Muon in Particle Physics – Charged Lepton Flavor Violation –  N→eN Rare Process Experiment Searching for  N→eN Rare Process – Principle of experiment – COMET – A new idea Test MLF/D2 DeeMe – Single Event Sensitivity – Background Summary

Standard Model of Particle Physics There are three generations (flavors) of Quarks and Leptons. Muon was found at – I.I. Rabi said “Who ordered that?” Is the muon excited state of electron? – The world-first search for  -> e – Null Result → hint of generation/flavor BR theory (  ->e  )~ – But exp. already gave BR exp. < 2 x → Two neutrinos model e ≠ BNL – Toward the establishment of the concept of “generation/flavor”. 3 Muon played very important role in the development of particle physics.

Flavor Mixing Quark Mixing – Cabbibo-Kobayashi-Maskawa (CKM) Matrix – Established --- Novel Neutrino Mixing – Pontecorvo-Maki-Nakagawa-Sakata (PMNS) Matrix – Homestake, Kamiokande, SNO etc. – Observed and Established. Charged Lepton Flavor Violation (CLFV) – Not observation yet at all. – Forbidden in the Standard Model of Particle Physics. 4 sbd uct  e e    ??

5 History of CLFV Searches Since 1948 E.P. Hincks and B. Pontecorvo, PR 73 (1948) 257   →  e  : Concept of “Flavor”  History of CLFV = History of particle physics. The best limits from muons. Current Limits   →  e  : < 1.2 x (MEGA)    N →  e - N: < 7x (SINDRUM II)  On-going program MEG/PSI  →  e  : < After Yoshitaka Kuno

Standard Model ≠ “the theory of everything”. – Hierarchy Problem – Unification of Force Supersymmetry (SUSY) If SUSY exists → SUSY flavor mixing → CLFV 6 SUSY and CLFV

7 Theoretical Predictions Process Current Limit SUSY- GUT SUSY- Seesaw Future  N → e N , ,  →  e   →  PRISM MEG Courtesy Hisano PRISM MEG SUSY+Seesaw, MSW Large Angle SUSY-GUT COMET COMET tanβ=3 tanβ=10 tanβ=30 → Physics at TeV scale, or even much higher energy domain.

   N →  e - + N Muonic Atom (1S state) – 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 8 nuclei −− Muon Decay in Orbit (MDO) charged Lepton Flavor Violation (CLFV) Muon Capture(MC)

9 Principal of Experiment Signal : μ - +(A,Z) → e - +(A,Z) – A single mono-energetic electron 100 MeV Delayed ： ~1μS No accidental backgrounds Physics backgrounds – Muon Decay in Orbit (MDO) ΔE e =350 keV (BR: ) – Beam Pion Capture π - +(A,Z) → (A,Z-1)* → γ+(A,Z-1) γ → e + e - Prompt timing SINDRUM II BR[Al] < 7 ×

General Idea of Setup Sensitivity – High  - yield Background – Pulsed Proton

J-PARC/MR COMET: BR[Al] <

12 μ-e electrons may directly coming from a production target. an electron analogue of the surface muon. Experiment could be very simple, quick and low-cost.

Issues to be Checked Muonic Atom Formation Rate Measurement – Yield of Michel e - from target. – Decay constant of e - e - from carbon: 2.0  sec Otherwise, << 2.0  sec Extinction Ratio Measurement – Time structure of high momentum e - (E e- > 52 MeV) 2009A0023: 3 days 2009A0032: 1 day How many  - actually stop in the muon target? What is the potential source of backgrounds.

Detector for the Test Meas. D2 Exit Pb (4mm t ) Plastic Scintillator μ-μ- e-e- B1B2B3 B1: gating-PMT readout B2: gating-PMT readout B3: ND filter (1/1000), normal PMT readout Have to detect delayed e - after prompt burst (~10 4 /pulse). Beam time approved is very short → Use gating-PMT to increase delayed-time efficiency. Background e - coming from the decay of prompt  - stopped in counters. → Pb plate to absorbe  - by muon capture process. Electron-detection efficiency ~ 50%

Result Intrinsic Counter Efficiency:~100% τ=2.10±0.02 μsec – Potential contamination of e - from Bhabha scattering of e + from  + decay. – But only order 2 at most. Existence of Michel Edge: confirmed – The shape of spectrum is consistent with that obtained with G4Beamline Simulation. → e - from Muonic Carbon Atom: Confirmed Yield: 4.4 detector → 8 × 10 9 /sec/MW in the current Muon Target. N e+ /N c = 450 B1 pulse height (B2 tagged) B2 pulse height (B1 tagged)

Sensitivity π-π- μ-μ- D2 and the current muon target – R μC = 8 × 10 9 sec/MW – D2 Acceptance: 105 MeV/c – Muon Capture Rate(Carbon) = 0.08 – Time Window Acceptance = 66% – μ-e relative strength (normalized to Al) = 0.7 – Running Time 10 7 sec – S.E.S. = 8 × SINDRUM II limit = 7 × Place Al μ-stopper: Capture rate 0.08 → 0.60 New Beam line with larger acceptance: x4 Exclusive use of the New Beam line: x2 or more S.E.S < for Al

New Beamline Concept by Jaap Dornbos (TRIUMF) Detailed Design is on-going. Double Thin Solenoid to preserve momentum-dispersive plane. The first Solenoid could be a single type. Kicker to reduce prompt burst 1/1000 – Spec: J-PARC RCS type is OK Acceptance = 140 msr Δp/p = 10 MeV/c(FWHM) H line

Backgrounds Should Delay by ~  sec ** Only coming from μ decay.** – Background: Muon Decay in Orbit : mostly E e < 55 MeV – N DIO MeV – Signal: μ-e Electron: P e = 105 MeV/c If there is any off-timing protons, that could become potential background. – Extinction < – R extinction [COMET] < 10 -9, R π-survive [COMET] = How much difficult to achieve the extinction ? Fast Extraction No scattering by septum during the slow extraction. Fall time of kicker is very fast. < 300 nsec. There is a room to install additional kickers in the primary beam line.

J-PARC MLF mu-e conversion SensitivitySchedule DeeMe< ~2015 COMET< ~ DeeMe does not replace COMET. DeeMe and COMET are complementary. Sound scenario to secure the world-first discovery.

Summary A new idea of  -e conversion experiment (DeeMe) that could improve the current limit by 2 orders of magnitudes (BR < ) was shown. DeeMe will never replace COMET, but they are complementary. – DeeMe → COMET Test Measurement at MLF/D2 was performed. – The time constant of e - from D2 is consistent with  - decay in carbon. – The shape of spectrum is consistent with Michel Spectrum. – The rate of muonic carbon atom formation is 8 × 10 9 Muon Target – Very good agreement with Geant4 MC: 7 × 10 9 /sec/MW The extinction should be < , which is much severe than COMET. But, – Fast-extracted beam should have much better extinction than slow-extracted beam. – Extra Extinction Kicker (The same type as the current RCS kicker) can be located in the primary beamline. Expression of Interest (EoI) is out. Working for the detailed design of experiment.