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Lepton Flavor Violation : Goals and Status of the MEG Experiment at PSI

Lepton Flavor Violation : Goals and Status of the MEG Experiment at PSI. Stefan Ritt Paul Scherrer Institute, Switzerland. Agenda. Search for m  e g down to 10 -13 Motivation Experimental Method Status and Outlook. Motivation. Why should we search for m  e g ?.

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Lepton Flavor Violation : Goals and Status of the MEG Experiment at PSI

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  1. Lepton Flavor Violation:Goals and Status of the MEG Experiment at PSI Stefan Ritt Paul Scherrer Institute, Switzerland

  2. Agenda • Search for m e g down to 10-13 • Motivation • Experimental Method • Status and Outlook Particle Colloquium Heidelberg

  3. Motivation Why should we search for m  e g ?

  4. The Standard Model Generation I II III *) Yet to be confirmed Particle Colloquium Heidelberg

  5. The success of the SM • The SM has been proven to be extremely successful since 1970’s • Simplicity (6 quarks explain >40 mesons and baryons) • Explains all interactions in current accelerator particle physics • Predicted many particles (most prominent W, Z ) • Limitations of the SM • Currently contains 19 (+10) free parameters such as particle (neutrino) masses • Does not explain cosmological observation such as Dark Matter and Matter/Antimatter Asymmetry Today’s goal is to look for physics beyond the standard model CDF Particle Colloquium Heidelberg

  6. High Energy Frontier • Produce heavy new particles directly • Heavy particles need large colliders • Complex detectors • High Precision Frontier • Look for small deviations from SM (g-2)m , CKM unitarity • Look for forbidden decays • Requires high precision at low energy Beyond the SM Find New Physics Beyond the SM Particle Colloquium Heidelberg

  7. Neutron beta decay • Neutron b decay via intermediate heavy W- boson Neutron mean life time: 886 s ~80 MeV ne W - e- ~5 MeV u d • decay discovery: ~1934 W- discovery: 1983 p n d d u u n  p+ + e - + ne Particle Colloquium Heidelberg

  8. New physics in m decay • Can’t we do the same in m decay? g ? m- e-  Probe physics at TeV scale with high precision m decay measurement Particle Colloquium Heidelberg

  9. The Muon • Discovery: 1936 in cosmic radiation • Mass: 105 MeV/c2 • Mean lifetime: 2.2 ms Seth Neddermeyer ne W- e- Carl Anderson ≈ 100% m- nm 0.014 < 10-11 led to Lepton Flavor Conservationas “accidental” symmetry Particle Colloquium Heidelberg

  10. Lepton Flavor Conservation • Absence of processes such as m  e g led to concept of lepton flavor conservation • Similar to baryon number (proton decay) and lepton number conservation • These symmetries are “accidental” because there is no general principle that imposes them – they just “happen” to be in the SM (unlike charge and energy conservation) • The discovery of the failure of such a symmetry could shed new light on particle physics Particle Colloquium Heidelberg

  11. g W- m- e- nm ne LFV and Neutrino Oscillations • Neutrino Oscillations  Neutrino mass  m  e g possible even in the SM  LFV in the charged sector is forbidden in the Standard Model n mixing Particle Colloquium Heidelberg

  12. g g W- m- e- m- e- nm ne LFV in SUSY • While LFV is forbidden in SM, it is possible in SUSY ≈ 10-12 Current experimental limit: BR(m e g) < 10-11 Particle Colloquium Heidelberg

  13. LFV Summary • LFV is forbidden in the SM, but possible in SUSY (and many other extensions to the SM) though loop diagrams ( heavy virtual SUSY particles) • If m e g is found, new physics beyond the SM is found • Current exp. limit is 10-11, predictions are around 10-12 … 10-14 • First goal of MEG: 10-13 • Later maybe push to 10-14 ($$$) • Big experimental challenge • Solid angle * efficiency (e,g) ~ 3-4 % • 107 – 108m/s DC beam needed • ~ 2 years measurement time • excellent background suppression Particle Colloquium Heidelberg

  14. m→ e g mA→ eA m→ eee History of LFV searches • Long history dating back to 1947! • Best present limits: • 1.2 x 10-11 (MEGA) • mTi → eTi < 7 x 10-13 (SINDRUM II) • m → eee < 1 x 10-12 (SINDRUM II) • MEG Experiment aims at 10-13 • Improvements linked to advancein technology cosmic m 10-1 10-2 10-3 10-4 10-5 stopped p 10-6 10-7 m beams 10-6 stopped m 10-9 10-10 10-11 SUSY SU(5) BR(m e g) = 10-13 mTi  eTi = 4x10-16BR(m eee) = 6x10-16 10-12 10-13 MEG 10-14 10-15 1940 1950 1960 1970 1980 1990 2000 2010 Particle Colloquium Heidelberg

  15. ft(M)=2.4 m>0 Ml=50GeV 1) Current SUSY predictions current limit MEG goal tan b “Supersymmetric parameterspace accessible by LHC” • J. Hisano et al., Phys. Lett. B391 (1997) 341 • MEGA collaboration, hep-ex/9905013 W. Buchmueller, DESY, priv. comm. Particle Colloquium Heidelberg

  16. g m- e- LFV link to other SUSY proc. me-LFV slepton mixing matrix: (g-2)m g In SO(10), eEDM is related to meg: m- m- mEDM g R. Barbieri et al., hep-ph/9501334 m- m- Particle Colloquium Heidelberg

  17. Experimental Method How to detect m  e g ?

  18. Decay topology m  e g 52.8 MeV m e g N g 52.8 MeV m 180º Eg[MeV] 10 20 30 40 50 60 e N 52.8 MeV • m→ e g signal very clean • Eg = Ee = 52.8 MeV • qge = 180º • e and g in time 52.8 MeV Ee[MeV] 10 20 30 40 50 60 Particle Colloquium Heidelberg

  19. n m n e Michel Decay (~100%) • Three body decay: wide energy spectrum N 52.8 MeV Theoretical m  e nn Ee[MeV] N 52.8 MeV Convoluted withdetector resolution Ee[MeV] Particle Colloquium Heidelberg

  20. g n m n e Radiative Muon Decay (1.4%) N 52.8 MeV m  e nn g Eg[MeV] “Prompt” Background Particle Colloquium Heidelberg

  21. g g n m n n n m e e “Accidental” Background Background m e g g m  e nn m Annihilation in flight 180º e m  e nn • m→ e g signal very clean • Eg = Ee = 52.8 MeV • qge = 180º • e and g in time Good energy resolution Good spatial resolution Excellent timing resolution Good pile-up rejection Particle Colloquium Heidelberg

  22. How can we achieve a quantum step in detector technology? Previous Experiments Particle Colloquium Heidelberg

  23. How to build a good experiment? Photon Calorimeter Muon Beam Positron Detector Electronics Particle Colloquium Heidelberg

  24. Collaboration • ~70 People (40 FTEs) from five countries Particle Colloquium Heidelberg

  25. Proton Accelerator Swiss Light Source Paul Scherrer Institute Particle Colloquium Heidelberg

  26. PSI Proton Accelerator Particle Colloquium Heidelberg

  27. Generating muons Carbon Target p+ m+ 590 MeV/c2 Protons 1.8 mA = 1016 p+/s 108m+/s m+ p+ Particle Colloquium Heidelberg

  28. Muon Beam Structure • Muon beam structure differs for different accelerators Pulsed muon beam, LANL DC muon beam, PSI Instantaneous rate much higher in pulsed beam Duty cycle: Ratio of pulse width over period Duty cycle: Ratio of pulse width over period Duty cycle: 100 % Duty cycle: 6 % Particle Colloquium Heidelberg

  29. - x x x m+ x x x e+ + Muon Beam Line • Transport 108m+/s to stopping target inside detector with minimal background Lorentz Force vanishes for given v: Wien Filter m+ from production target Particle Colloquium Heidelberg

  30. Results of beam line optimization Rm ~ 1.1x108m+/s at experiment e+ m+ s ~ 10.9 mm m+ Particle Colloquium Heidelberg

  31. Previous Experiments Particle Colloquium Heidelberg

  32. The MEGA Experiment • Detection of g in pair spectrometer • Pair production in thin lead foil • Good resolution, but low efficiency (few %) • Goal was 10-13, achieved was 1.2 x 10-11 • Reason for problems: • Instantaneous rate 2.5 x 108 m/s • Design compromises • 10-20 MHz rate/wire • Electronics noise & crosstalk • Lessons learned: • Minimize inst. rate • Avoid pair spectrometer • Carefully design electronics • Invite MEGA people! Particle Colloquium Heidelberg

  33. Photon Detectors (@ 50 MeV) • Alternatives to Pair Spectrometer: • Anorganic crystals: • Good efficiency, good energy resolution,poor position resolution, poor homogeneity • NaI (much light), CsI (Ti,pure) (faster) • Liquid Noble Gases: • No crystal boundaries • Good efficiency, resolutions g induced shower 25 cm CsI CsI CsI Liquid Xenon: PMT PMT PMT Particle Colloquium Heidelberg

  34. H.V. Refrigerator Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb Liq. Xe window PMT filler Plastic 1.5m Liquid Xenon Calorimeter • Calorimeter: Measure g Energy, Positionand Time through scintillation light only • Liquid Xenon has high Z and homogeneity • ~900 l (3t) Xenon with 848 PMTs(quartz window, immersed) • Cryogenics required: -120°C … -108° • Extremely high purity necessary:1 ppm H20 absorbs 90% of light • Currently largest LXe detector in theworld: Lots of pioneering work necessary g m Particle Colloquium Heidelberg

  35. LXe g response • Light is distributed over many PMTs • Weighted mean of PMTs on front face  x • Broadness of distribution Dz • Position corrected timing Dt • Energy resolution depends on light attenuation in LXe x z Particle Colloquium Heidelberg

  36. LXe g response • Light is distributed over many PMTs • Weighted mean of PMTs on front face Dx • Broadness of distributionDz • Position corrected timingDt • Energy resolution depends on light attenuation in LXe x z Particle Colloquium Heidelberg

  37. Use GEANT to carefully study detector • Optimize placement of PMTs according to MC results Particle Colloquium Heidelberg

  38. LXe Calorimeter Prototype • ¼ of the final calorimeter was build to study performance, purity, etc. 240 PMTs Particle Colloquium Heidelberg

  39. How to get 50 MeV g’s? • p- p  p0 n (Panofsky)p0 g g • LH2 target • Tag one g with NaI & LYSO Particle Colloquium Heidelberg

  40. Resolutions • NaI tag: 65 MeV < E(NaI) < 95 MeV • Energy resolution at 55 MeV:(4.8 ± 0.4) % FWHM • LYSO tag for timing calib.:260 150 (LYSO) 140 (beam)= 150 ps (FWHM) • Position resolution:9 mm (FWHM) FWHM = 4.8% FWHM = 260 ps To be improved with refinedanalysis methods Particle Colloquium Heidelberg

  41. Lessons learned with Prototype • Two beam tests, many a-source and cosmic runs in Tsukuba, Japan • Light attenuation much too high (~10x) • Cause: ~3 ppm of Water in LXe • Cleaning with “hot” Xe-gas before filling did not help • Water from surfaces is only absorbed in LXe • Constant purification necessary • Gas filter system (“getter filter”) works, attenuation length can be improved, but very slowly (t ~350 hours) • Liquid purification is much faster First studies in 1998, final detector ready in 2007 Particle Colloquium Heidelberg

  42. GXe pump (10-50L/min) Heat exchanger GXe storage tank Getter+Oxysorb Cryocooler LN2 (100W) LN2 Cryocooler (>150W) Liquid pump (100L/h) Purifier 1000L storage dewar LXe Calorimeter Liquid circulating purifier Xenon storage • ~900L in liquid, largest amount of LXe ever liquefied in the world Particle Colloquium Heidelberg

  43. Final Calorimeter • Currently being assembled, will go into operation summer ‘07 Particle Colloquium Heidelberg

  44. Homogeneous Field Gradient Field (COnstant-Bending-RAdius) Positron Spectrometer • Ultra-thin (~3g/cm2) superconducting solenoid with 1.2 T magnetic field high pt track constant |p| tracks e+ from m+e+g Particle Colloquium Heidelberg

  45. Drift Chamber • Measures position, time and curvature of positron tracks • Cathode foil has three segments in a vernier pattern  Signal ratio on vernier strips to determine coordinate along wire Particle Colloquium Heidelberg

  46. Positron Detection System • 16 radial DCs with extremely low mass • He:C2H6 gas mixture • Test beam measurementsand MC simulation: • Dp/p = 0.8% • Dq = 10 mrad • Dxvertex = 2.3 mm FWHM Particle Colloquium Heidelberg

  47. Timing Counter • Staves along beam axis for timing measurement • Resolution 91 ps FWHM measured at Frascati e- - beam • Curved fibers with APD readout for z-position Particle Colloquium Heidelberg

  48. The complete MEG detector Particle Colloquium Heidelberg

  49. MC Simulation of full detector g e+ “Soft” gs TC hit Particle Colloquium Heidelberg

  50. Beam induced background • 108m/s produce 108 e+/s produce 108g/s Cable ductsfor Drift Chamber Particle Colloquium Heidelberg

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