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The Muon: A Laboratory for Particle Physics

The Muon: A Laboratory for Particle Physics. B.L. Roberts Department of Physics Boston University. roberts @bu.edu http://physics.bu.edu/roberts.html. Outline. Introduction to the muon Selected weak interaction parameters Magnetic and electric dipole moments Lepton Flavor Violation

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The Muon: A Laboratory for Particle Physics

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  1. The Muon:A Laboratory for Particle Physics B.L. Roberts Department of Physics Boston University roberts @bu.edu http://physics.bu.edu/roberts.html B. Lee Roberts, PSI - 15 August 2006

  2. Outline • Introduction to the muon • Selected weak interaction parameters • Magnetic and electric dipole moments • Lepton Flavor Violation • Summary and conclusions. B. Lee Roberts, PSI - 15 August 2006

  3. The Muon: Discovered in 1936 Discoveredin cosmic rays by Seth Neddermeyer and Carl Anderson B. Lee Roberts, PSI - 15 August 2006

  4. Confirmed by Street and Stevenson It interacted too weakly with matter to be the “Yukawa” particle which was postulated to carry the nuclear force B. Lee Roberts, PSI - 15 August 2006

  5. The discovery of the muon was abig surprise… • Lifetime ~2.2 ms, practically forever • 2nd generation lepton • mm/me = 206.768 277(24) • produced polarized • in-flight decay: both “forward” and “backward” muons are highly polarized • Paul Scherrer Institut has 108m/s in a beam B. Lee Roberts, PSI - 15 August 2006

  6. Death of the Muon • Decay is self analyzing B. Lee Roberts, PSI - 15 August 2006

  7. What have we learn from the m’s death? • The strength of the weak interaction • i.e. the Fermi constant GF (more properly Gm) • The fundamental nature of the weak interaction • i.e. is it scalar, vector, tensor, pseudo-scalar, pseudo-vector or pseudo-tensor? • Lepton flavor conservation in m-decay • VEV of the Higgs field: • Induced form-factors in nuclear m-capture B. Lee Roberts, PSI - 15 August 2006

  8. A precise measurement of tm+ leads to a precise determination of the Fermi constantGF from radiative corrections B. Lee Roberts, PSI - 15 August 2006

  9. d B. Lee Roberts, PSI - 15 August 2006

  10. tm helped predict the mass of the top quark B. Lee Roberts, PSI - 15 August 2006

  11. Predictive power in weak sector of SM. Difference between the charged current and neutral current propagators The radiative correction shown above depends on mt2. Comparisons of charged, vrs. neutral currents gives information on mt. tm helped predict the mass of the top quark B. Lee Roberts, PSI - 15 August 2006

  12. The Electro-Weak Working Group Fits: • Predicted • Input: GF(17 ppm), • (4 ppb at q2=0), • MZ (23 ppm), • Measured: from GF The mLan experiment at PSI will accumulate 1012m-decays and measure Gm to ~1 ppm. If LHC provides a Higgs Mass, then the precision of the confrontation with the SM will greatly improve B. Lee Roberts, PSI - 15 August 2006

  13. The Weak Lagrangian (Leptonic Currents) • Lepton current is (vector – axial vector) “(V – A)” • It might have been: V±A or S±V±A or most general form: Scalar ± Vector ± Weak-Magnitism ± PseudoScalar ± Axial-Vector ± Tensor There have been extensive studies at PSI by Fetscher, Gerber, et al. to look for other couplings in muon decay. Search continues with TWIST at TRIUMF. At present, none have been found. B. Lee Roberts, PSI - 15 August 2006

  14. If the Strong Interaction is Present • Then we have a more general current, which in principle can have 6 induced form factors in the current. B. Lee Roberts, PSI - 15 August 2006

  15. Leptonic and hadronic currents • For nuclear m- capture (and also in b-decay) there are induced form-factors and the hadronic V-A current contains 6 terms. • inm capture the induced pseudoscaler term becomes important b - decay vector weak magnitism scalar 2nd class axial vector pseudoscalar tensor B. Lee Roberts, PSI - 15 August 2006

  16. Muon Magnetic Dipole Momoment amchiral changing Muon EDM The Muon Trio: B. Lee Roberts, PSI - 15 August 2006

  17. Muon Magnetic Dipole Momoment amchiral changing Muon EDM Lepton Flavor Violation The Muon Trio: B. Lee Roberts, PSI - 15 August 2006

  18. Magnetic Moments (Field started by Stern) (in modern language) (and in English) B. Lee Roberts, PSI - 15 August 2006

  19. Dirac Equation Predicts g=2 Non-relativistic reduction of the Dirac Equation for an electron in a weak magnetic field. B. Lee Roberts, PSI - 15 August 2006

  20. Dirac + Pauli moment Schwinger B. Lee Roberts, PSI - 15 August 2006

  21. Radiative corrections change g Schwinger Kusch-Foley Dirac Stern-Gerlach B. Lee Roberts, PSI - 15 August 2006

  22. The SM Value for electron and muon anomalies e*, e, e, e, e, e, e, e, e, e, e, e vrs. m : relative contributionof heavier things B. Lee Roberts, PSI - 15 August 2006

  23. aμ is sensitive to a wide range of new physics • substructure • SUSY (with large tanβ ) • many other things (extra dimensions, etc.) B. Lee Roberts, PSI - 15 August 2006

  24. We measure the difference frequency between the spin and momentum precession With an electric quadrupole field for vertical focusing 0 B. Lee Roberts, PSI - 15 August 2006

  25. Experimental Technique Central orbit Kicker Modules R=711.2cm d=9cm Electric Quadrupoles Protons Pions Inflector (from AGS) p=3.1GeV/c Target (1.45T) Injection orbit • Muon polarization • Muon storage ring • injection & kicking • focus by Electric Quadrupoles • 24 electron calorimeters Storage ring (from Q. Peng) B. Lee Roberts, PSI - 15 August 2006

  26. Muon lifetime tm = 64.4 ms (g-2) period ta = 4.37 ms Cyclotron period tC = 149 ns muon (g-2) storage ring B. Lee Roberts, PSI - 15 August 2006

  27. Detectors and vacuum chamber B. Lee Roberts, PSI - 15 August 2006

  28. We count high-energy electrons as a function of time. B. Lee Roberts, PSI - 15 August 2006

  29. The ± 1 ppm uniformity in the average field is obtained with special shimming tools. We can shim the dipole, quadrupole sextupole independently 0.5 ppm contours B. Lee Roberts, PSI - 15 August 2006

  30. Calibration to a spherical water sample that ties the field to the Larmor frequency of the free protonwp. So we measure wa and wp The magnetic field is measured and controlled using pulsed NMR and the free-induction decay. B. Lee Roberts, PSI - 15 August 2006

  31. When we started in 1983, theory and experiment were known to about 10 ppm. Theory uncertainty was ~ 9 ppm Experimental uncertainty was 7.3 ppm B. Lee Roberts, PSI - 15 August 2006

  32. E821 achieved 0.5 ppm and the e+e- based theory is also at the 0.6 ppm level. Both can be improved. All E821 results were obtained with a “blind” analysis. B. Lee Roberts, PSI - 15 August 2006

  33. To compare with theory, there are two hadronic issues: • Lowest order hadronic contribution • Hadronic light-by-light ≈ B. Lee Roberts, PSI - 15 August 2006

  34. Lowest Order Hadronic from e+e- annihilation B. Lee Roberts, PSI - 15 August 2006

  35. Two experiments at the Budker Insitute at Novosibirsk have measured R(s) to better than a percent. 1994-1995 114k π+π- 1996 4k π+π- 1997 33k π+π- 1998 ~1M π+π- 2000 ~2M π+π- CMD-2 SND 98,2000 95,98 96 96,98 97 r-w meson interference B. Lee Roberts, PSI - 15 August 2006

  36. R(s) measurements at low s Babar/Belle (ISR) KLOE (ISR) VEPP-2000 VEPP-2M At low s the cross-section is measured independently for each final state from Davier/Höcker B. Lee Roberts, PSI - 15 August 2006

  37. Taking the hadronic contribution from Simon Eidelman’s talk at ICHEP06, including new CMD2 & SND e+e- data B. Lee Roberts, PSI - 15 August 2006

  38. Electric and Magnetic Dipole Moments Phys. Rev. 78 (1950) B. Lee Roberts, PSI - 15 August 2006

  39. Transformation Properties of Electric and Magnetic Dipole Moments An EDM implies both PandT are violated. Assuming CPT symmetry, an EDM at a measureable level would imply non-standard model CP. B. Lee Roberts, PSI - 15 August 2006

  40. However, it should be emphasized that while such arguments are appealing from the point of view of symmetry, they are not necessarily valid. Ultimately the validity of all such symmetry arguments must rest on experiment. N.F. Ramsey, Phys. Rev. 109, 225 (1958) B. Lee Roberts, PSI - 15 August 2006

  41. B. Lee Roberts, PSI - 15 August 2006

  42. Present EDM Limits *not yet final B. Lee Roberts, PSI - 15 August 2006

  43. B. Lee Roberts, PSI - 15 August 2006

  44. Muon EDM: Naïve scaling would imply that but in some models the dependence is greater. B. Lee Roberts, PSI - 15 August 2006

  45. Spin Frequencies: m in B field with MDM & EDM 0 spin difference frequency = ws - wc The motional E - field, β X B, is much stronger than laboratory electric fields (~GV/m). The EDM causes the spin to precess out of plane. B. Lee Roberts, PSI - 15 August 2006

  46. Dedicated EDM Experiment 0 Use a radial E-field to turn off the wa precession With wa = 0, the EDM causes the spin to steadily precess out of the plane. wh B. Lee Roberts, PSI - 15 August 2006

  47. “Frozen spin” technique • Turn off the (g-2) precession with radial E • Look for an up-down asymmetry building up with time B. Lee Roberts, PSI - 15 August 2006

  48. A very interesting proposal by Adelmann and Kirch B = 1 T pm = 125 MeV/c bm = 0.77, gm = 1.57 P ≈ 0.9 E = 0.64 MV/m R = 0.35 m In 1 year of running @ PSI B. Lee Roberts, PSI - 15 August 2006

  49. The standard-model gauge bosons do not permit leptons to mix, but new physics at the TeV scale such as SUSY does. Relevant quark level interactions for m-e conversion Dipole Scalar Vector Lepton Flavor Violation (the transition moment) R.Kitano, M.Koike and Y.Okada. 2002 B. Lee Roberts, PSI - 15 August 2006

  50. m→ e MDM, EDM ~ ~ SUSY connection between am , Dμ , μ→ e B. Lee Roberts, PSI - 15 August 2006

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