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Muon (g-2)

Muon (g-2). Past, Present and Future. B. Lee Roberts Department of Physics Boston University. roberts@bu.edu http://physics.bu.edu/roberts.html. 673 (1924). (in modern language). Dirac + Pauli moment. Schwinger term. Lepton Flavor Violation

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Muon (g-2)

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  1. Muon (g-2) Past, Present and Future B. Lee Roberts Department of Physics Boston University roberts@bu.edu http://physics.bu.edu/roberts.html B. Lee Roberts, NuFact WG4: 24 June 2005

  2. 673 (1924) (in modern language) B. Lee Roberts, NuFact WG4: 24 June 2005

  3. Dirac + Pauli moment Schwinger term B. Lee Roberts, NuFact WG4: 24 June 2005

  4. Lepton Flavor Violation Muon MDM (g-2) chiral changing Muon EDM The Muon Trio: B. Lee Roberts, NuFact WG4: 24 June 2005

  5. Muon (g-2) : Four Past Experiments • CERN 1 - 1950s • SC m precessed in a gradient field • CERN 2 - 1960s • Dedicated Storage Ring, pm = 1.28 GeV/c • protons from PS injected into the storage ring • CERN 3 - 1970s • Dedicated Storage Ring • used p injection + p→m decay to give the kick, The “magic” g = 29.7; pm = 3.09 GeV/c, • BNL E821 • Superconducting “superferric” storage ring • magic g, direct muon injection, fast non-ferric kicker B. Lee Roberts, NuFact WG4: 24 June 2005

  6. Spin Precession Frequencies: m in B field spin difference frequency = ws - wc B. Lee Roberts, NuFact WG4: 24 June 2005

  7. Use an E field for vertical focusing 0 spin difference frequency = ws - wc B. Lee Roberts, NuFact WG4: 24 June 2005

  8. Spin Precession Frequencies: m in B field with both an MDM and EDM The motional E - field, β X B, is much stronger than laboratory electric fields. The EDM causes the spin to precess out of plane. B. Lee Roberts, NuFact WG4: 24 June 2005

  9. Muon (g-2): Store m ± in a storage ring magnetic field averaged over azumuth in the storage ring B. Lee Roberts, NuFact WG4: 24 June 2005

  10. Muon (g-2) Present precision: ± 0.5 ppm B. Lee Roberts, NuFact WG4: 24 June 2005

  11. Theory and Experiment • Using these hadronic contributions M. Davier et al., Eur. Phys. J. C 31, 503 (2003), A Höcker, hep-ph/0410081 K. Hagiwara, et al., Phys. Rev. D69, 093003 (2004) B. Lee Roberts, NuFact WG4: 24 June 2005

  12. D am with standard model ~2.7 s With this discrepancy, a compelling case can be made to do better, and resolve whether this “discrepancy” is significant or not. B. Lee Roberts, NuFact WG4: 24 June 2005

  13. Can we do a more precise measurement? • Yes • E969 at BNL has scientific approval to reach 0.2ppm • At a more intense muon facility we could do better. Will Theory Improve? • Yes • First, let’s look at the pieces which might contribute to a potential discrepancy. B. Lee Roberts, NuFact WG4: 24 June 2005

  14. Why might this be interesting? • what sources of new physics are there? B. Lee Roberts, NuFact WG4: 24 June 2005

  15. 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, NuFact WG4: 24 June 2005

  16. SUSY connection between am , Dμ , μ→ e B. Lee Roberts, NuFact WG4: 24 June 2005

  17. SUSY, dark matter, (g-2) DE821 CMSSM B. Lee Roberts, NuFact WG4: 24 June 2005

  18. DE969 = Dnow B. Lee Roberts, NuFact WG4: 24 June 2005

  19. DE969= 0 B. Lee Roberts, NuFact WG4: 24 June 2005

  20. SM value dominated by hadronic issues: • Lowest order hadronic contribution ( ~ 60 ppm) • Hadronic light-by-light contribution ( ~ 1 ppm) The error on these two contributions will ultimately limit the interpretation of a more precise muon (g-2) measurement. B. Lee Roberts, NuFact WG4: 24 June 2005

  21. Lowest Order Hadronic contribution from e+e- annihilation B. Lee Roberts, NuFact WG4: 24 June 2005

  22. Magnitude of the errors • present hadronic uncertainty ~0.6 ppm • present experimental uncertainty 0.5 ppm • theory: better R measurements • KLOE • BaBar • SND and CMD2 at Novosibirsk • More work on the strong interaction • experiment: E969 @ BNL or elsewhere How could we do better? B. Lee Roberts, NuFact WG4: 24 June 2005

  23. Recent News from Novosibirsk • SND has just released their results for the cross section e+e- → p +p - over the r. • Error on dispersion integral 50% higher than CMD2 • Good agreement with CMD2 • Completely independent from CMD2 • Preprint should be on the web soon B. Lee Roberts, NuFact WG4: 24 June 2005

  24. How much could the theory improve? • In their “Annual Reviews” articleDavier and Marciano guess a factor of 2 or so for argument let’s assume theory uncertainty will get to • 0.3 to 0.1 ppm • Experiment • E969 at BNL (if it runs) could achieve a factor of 2.5 for a total error of 0.2 ppm • future experiment could reach 0.06 ppm How much could experiment improve? B. Lee Roberts, NuFact WG4: 24 June 2005

  25. E969 at BNL • Scientific approval in September 2004 • at present: no funds for construction or running • Goal: total error = 0.2 ppm • lower systematic errors • more beam B. Lee Roberts, NuFact WG4: 24 June 2005

  26. Strategy of the improved experiment • More muons – E821 was statistics limited sstat = 0.46 ppm, ssyst = 0.3 ppm • Backward-decay, higher-transmission beamline • Double the quadrupoles in the p decay line • New, open-end inflector • Upgrade detectors, electronics, DAQ • Improve knowledge of magnetic field B • Improve calibration, field monitoring and measurement • Reduce systematic errors on ωa • Improve the electronics and detectors • New parallel “integration” method of analysis B. Lee Roberts, NuFact WG4: 24 June 2005

  27. Improved transmission into the ring Inflectoraperture Inflector Storage ring aperture E821 Closed End P969 Proposed Open End B. Lee Roberts, NuFact WG4: 24 June 2005

  28. Near side Far side Pedestal vs. Time E821: forward decay beam Pions @ 3.115 GeV/c Decay muons @ 3.094 GeV/c This baseline limits how early we can fit data B. Lee Roberts, NuFact WG4: 24 June 2005

  29. Expect for both sides E969: backward decay beam Pions @ 5.32 GeV/c Decay muons @ 3.094 GeV/c No hadron-induced prompt flash Approximately the same muon flux is realized x 1 more muons B. Lee Roberts, NuFact WG4: 24 June 2005

  30. E969: Systematic Error Goal • Field improvements will involve better trolley calibrations, better tracking of the field with time, temperature stability of room, improvements in the hardware • Precession improvements will involve new scraping scheme, lower thresholds, more complete digitization periods, better energy calibration B. Lee Roberts, NuFact WG4: 24 June 2005

  31. Beyond E969? • It’s not clear how far we can push the present technique. • To get to 0.06 ppm presents many challenges. • Perhaps a new storage ring design, and a smaller aperture. • detectors for another factor of 4 will be very challenging. • At a neutrino factory we certainly we can get more muons B. Lee Roberts, NuFact WG4: 24 June 2005

  32. A new idea (F.J.M. Farley) • Sector focused storage ring, which uses polarized protons to measure ∫B.dℓ No need to know mm / mp Need to know ∫B.dℓ to 20 ppb!!!!! (while E821 already achieved: Can run well above the magic g , so that there are more (g-2) cycles per lifetime. Many details to be worked out. B. Lee Roberts, NuFact WG4: 24 June 2005

  33. As always, there are questions … • Will E969 be funded and reach 0.2 ppm? • How far can theory be improved? • a observation from history . . . . B. Lee Roberts, NuFact WG4: 24 June 2005

  34. Where we came from: B. Lee Roberts, NuFact WG4: 24 June 2005

  35. Today with e+e- based theory: All E821 results were obtained with a “blind” analysis. world average B. Lee Roberts, NuFact WG4: 24 June 2005

  36. Summary • (g-2)m provides a precise check of the standard model, and accesses new physics in a way complementary to other probes. • (g-2)m is dependent on a standard model value, part of which must be taken from data (e+ e-→ hadrons ) • The hadronic contribution will eventually set the limit on useful precision, but substantial improvement can be made beyond the present situation. B. Lee Roberts, NuFact WG4: 24 June 2005

  37. B. Lee Roberts, NuFact WG4: 24 June 2005

  38. Fourier Transform: residuals to 5-parameter fit beam motion across a scintillating fiber – ~15 turn period B. Lee Roberts, NuFact WG4: 24 June 2005

  39. Effects of the CBO on e- spectrum • CBO causes modulation of N, amplitude ~0.01 • CBO causes modulation of observed energy distribution • which in turn causes oscillation in A(E), f(E), with amplitudes ~0.001, ~1 mrad. B. Lee Roberts, NuFact WG4: 24 June 2005

  40. Functional form of the time spectrum • A1 and A2 → artificial shifts in wa up to 4 ppm in individual detectors when not accounted for. B. Lee Roberts, NuFact WG4: 24 June 2005

  41. Other Systematic Effects: wa • muon losses • gain changes and pedistal shifts • pulse pileup B. Lee Roberts, NuFact WG4: 24 June 2005

  42. E821: Systematic Errors Muon spin precession Magnetic field B. Lee Roberts, NuFact WG4: 24 June 2005

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