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The Muon: A Laboratory for Particle Physics

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  1. The Muon: A Laboratory for Particle Physics Everything you always wanted to know about the muon but were afraid to ask. B. Lee Roberts Department of Physics Boston University roberts@bu.edu http://physics.bu.edu/roberts.html B. Lee Roberts, College of William and Mary, 24 March 2006

  2. Outline • Introduction to the muon • Selected weak interaction parameters • Muonium • Lepton Flavor Violation • Magnetic and electric dipole moments • Summary and conclusions. B. Lee Roberts, College of William and Mary, 24 March 2006

  3. The Muon: Discovered in 1936 Discoveredin cosmic rays by Seth Neddermeyer and Carl Anderson B. Lee Roberts, College of William and Mary, 24 March 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, College of William and Mary, 24 March 2006

  5. The Muon’s Discovery was a big surprise • Lifetime ~2.2 ms, practically forever • 2nd generation lepton • mm/me = 206.768 277(24) I. I. Rabi B. Lee Roberts, College of William and Mary, 24 March 2006

  6. The Standard Model (Our Periodic Table) Interact weakly through the Leptons Electroweak gage bosons Quarks Interact strongly through the gluons g B. Lee Roberts, College of William and Mary, 24 March 2006

  7. Production of The Muon • produced polarized from the death of a pion For p decay in flight, “forward” and “backward” muons are highly polarized. • It can be produced copiously in pion decay • e.g. Paul Scherrer Institut has 108m/s in a beam B. Lee Roberts, College of William and Mary, 24 March 2006

  8. Death of the Muon • Decay is self analyzing B. Lee Roberts, College of William and Mary, 24 March 2006

  9. What can we learn from the m’s death? • The strength of the weak interaction • i.e. the Fermi constant GF • The fundamental nature of the weak interaction • i.e. is it scalar, vector, tensor, pseudo-scalar, pseudo-vector or pseudo-tensor? B. Lee Roberts, College of William and Mary, 24 March 2006

  10. A precise measurement of tm+ leads to a precise determination of the Fermi constantGF from radiative corrections B. Lee Roberts, College of William and Mary, 24 March 2006

  11. d B. Lee Roberts, College of William and Mary, 24 March 2006

  12. tm helped predict the mass of the top quark • Predictive power in weak sector of SM. Difference between the charged and neutral current propagator: • Top quark mass prediction: mt = 177  20 GeV • Input: GF(17 ppm),a(4 ppb at q2=0),MZ (23 ppm), • 2004 Update from D0 mt = 178  4.3 GeV B. Lee Roberts, College of William and Mary, 24 March 2006

  13. Experiment at a glance • Collect handful of muons in a few ms • Turn off beam • Watch them decay • Repeat e+ m in target Time Measurement Period Accum. Period B. Lee Roberts, College of William and Mary, 24 March 2006

  14. mLan @ Paul Scherrer Institut aims for a factor of 20 improvement on tm B. Lee Roberts, College of William and Mary, 24 March 2006

  15. 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 Gerber, Fetscher, et al. to look for other couplings in muon decay. None were found B. Lee Roberts, College of William and Mary, 24 March 2006

  16. If the Strong Interaction is Present • Then we have a more general current, which in principle can have all 6 of these components to the current. B. Lee Roberts, College of William and Mary, 24 March 2006

  17. 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. • the induced pseudoscaler term is important vector weak magnitism scalar 2nd class axial vector pseudoscalar tensor B. Lee Roberts, College of William and Mary, 24 March 2006

  18. An Aside: The induced pseudoscalar coupling in m-capture further enhanced in radiative muon capture A new experiment at PSI MuCap hopes to resolve the present 3 s discrepancy with PCAC butstop the press! new measurement of the atomic “ortho to para transition rate” seems to remove much of this problem, Clark, Armstrong, et al., PRL 96, 073401 (2006) B. Lee Roberts, College of William and Mary, 24 March 2006

  19. Muonium m+ e- (not m+m-) Hydrogen (without the proton) Named by Val Telegdi discovered by Vernon Hughes B. Lee Roberts, College of William and Mary, 24 March 2006

  20. Muonium Zeeman splitting mm/mp = 3.183 345 24(37) (120 ppb) where mp comes from proton NMR in the same B field B. Lee Roberts, College of William and Mary, 24 March 2006

  21. muonium and hydrogen hfs → proton structure B. Lee Roberts, College of William and Mary, 24 March 2006

  22. Lepton Flavor • Remember the puzzle with b-decay? • it appeared that energy conservation did not hold in the decay n → p + e- which should have a mono-energetic e+ in the final state. e- B. Lee Roberts, College of William and Mary, 24 March 2006

  23. Lepton Flavor • It took Pauli to propose that energywas conserved, but there was a new neutral particle emitted in the decay (named neutrino by Fermi), so the decay was a 3-body decay with a continuous spectrum. B. Lee Roberts, College of William and Mary, 24 March 2006

  24. Lepton Flavor • We have found empirically that lepton family number is conserved in muon decay. • e.g. • What about or B. Lee Roberts, College of William and Mary, 24 March 2006

  25. Lepton Flavor in Muon Decay me = 0.511 MeV mm = 104.7 MeV Why don’t we see m+→ e+g ? Neutrinos oscillate – however, the predicted Standard Model Charged Lepton Flavor Violation unmeasureably small (from loops). The standard model gauge bosons (interactions) do not permit lepton flavor-changing interactions, i.e. there is conservation ofeachlepton flavor separately. B. Lee Roberts, College of William and Mary, 24 March 2006

  26. SM charged leptons do not mix • Expect charged lepton flavor to be enhanced if there is new dynamics at the TeV scale, in particular if there is Supersymmetry B. Lee Roberts, College of William and Mary, 24 March 2006

  27. In Standard Model we have: antiparticles particles SUSY: supersymmetric partners (spartners) (with thanks to Bruce Winstein) B. Lee Roberts, College of William and Mary, 24 March 2006

  28. Supersymmetry Permits Charged Lepton Mixing • In supersymmetry there is mixing between the charged sleptons • Many people believe that SUSY is the new physics which will be found at LHC B. Lee Roberts, College of William and Mary, 24 March 2006

  29. Lepton Flavor Violation Muon MDM (g-2) chiral changing Muon EDM Beyond the SM: The Muon Trio: B. Lee Roberts, College of William and Mary, 24 March 2006

  30. The First -N  e-N Experiment Steinberger and Wolf • After the discovery of the muon, it was realized it could decay into an electron and a photon or convert to an electron in the field of a nucleus. • Without any flavor conservation, the expected branching fraction for +e+ is about 10-5. • Steinberger and Wolf looked for -N  e-Nfor the first time, publishing a null result in 1955, with a limit Re < 2  10-4 Absorbs e- from - decay 9” Conversion e- reach this counter B. Lee Roberts, College of William and Mary, 24 March 2006

  31. The MECO Experiment Straw Tracker Muon Stopping Target Muon Beam Stop Superconducting Transport Solenoid (2.5 T – 2.1 T) Crystal Calorimeter Superconducting Detector Solenoid (2.0 T – 1.0 T) Superconducting Production Solenoid (5.0 T – 2.5 T) Collimators 10-17BR single event sensitivity p beam B. Lee Roberts, College of William and Mary, 24 March 2006

  32. Past and Future of LFV Limits MEGm → e g • 10-13 BR sensitivity • under construction at PSI, first data in 2006 MECOm++A→e++A • 10-17 BR sensitivity • Was approved at Brookhaven, not funded m+e-→m-e+ Branching Ratio Limit B. Lee Roberts, College of William and Mary, 24 March 2006

  33. Electric and Magnetic Dipole Moments In 1950, Purcell and Ramsey propose to search for a neutron EDM to check parity violation In 1957, Landau and Ramsey independently point out that an EDM violates both P and T B. Lee Roberts, College of William and Mary, 24 March 2006

  34. Electric and Magnetic Dipole Moments Transformation properties: An EDM implies both PandT are violated. An EDM at a measureable level would imply non-standard model CP. The baryon/antibaryon asymmetry in the universe, needs new sources of CP. B. Lee Roberts, College of William and Mary, 24 March 2006

  35. Present EDM Limits *projected B. Lee Roberts, College of William and Mary, 24 March 2006

  36. Magnetic Dipole Moments The field was started by Otto Stern B. Lee Roberts, College of William and Mary, 24 March 2006

  37. Z. Phys. 7, 249 (1921) B. Lee Roberts, College of William and Mary, 24 March 2006

  38. (in modern language) B. Lee Roberts, College of William and Mary, 24 March 2006

  39. Dirac + Pauli moment B. Lee Roberts, College of William and Mary, 24 March 2006

  40. Dirac Equation Predicts g=2 • radiative corrections change g Schwinger B. Lee Roberts, College of William and Mary, 24 March 2006

  41. The CERN Muon (g-2) Experiments The muon was shown to be a point particle obeying QED (Quantum Electrodynamics) The final CERN precision was 7.3 ppm B. Lee Roberts, College of William and Mary, 24 March 2006

  42. Standard Model Value for (g-2) relative contribution of heavier things B. Lee Roberts, College of William and Mary, 24 March 2006

  43. Lowest Order Hadronic from e+e- annihilation Cauchy’s theorem and the optical theorem B. Lee Roberts, College of William and Mary, 24 March 2006

  44. aμ is sensitive to a wide range of new physics • muon substructure • anomalous couplings • SUSY (with large tanβ ) • many other things (extra dimensions, etc.) B. Lee Roberts, College of William and Mary, 24 March 2006

  45. SUSY connection between am , Dμ , μ→ e B. Lee Roberts, College of William and Mary, 24 March 2006

  46. Spin Precession Frequencies: m in B field B. Lee Roberts, College of William and Mary, 24 March 2006

  47. If we use an electric quadrupole field for vertical focusing we get 0 B. Lee Roberts, College of William and Mary, 24 March 2006

  48. Experimental Technique Spin Momentum Central orbit Kicker Modules R=711.2cm d=9cm Electric Quadrupoles polarized m 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 B. Lee Roberts, College of William and Mary, 24 March 2006

  49. muon (g-2) storage ring B. Lee Roberts, College of William and Mary, 24 March 2006

  50. B. Lee Roberts, College of William and Mary, 24 March 2006