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Muon (g-2): Past, Present, and Future

This article discusses the history of muon (g-2) experiments, the present precision of measurements, and the potential for more precise future measurements. It explores the theoretical and experimental factors that contribute to the discrepancy and the possibility of new physics.

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Muon (g-2): Past, Present, and Future

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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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