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Exploring Muon Decay with TWIST

Exploring Muon Decay with TWIST. Carl A. Gagliardi Texas A&M University for the TWIST Collaboration Outline. Physics of muon decay TWIST experiment TWIST results to date How will we do better?. Muon decay matrix element.

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Exploring Muon Decay with TWIST

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  1. Exploring Muon DecaywithTWIST Carl A. Gagliardi Texas A&M University for the TWIST Collaboration Outline • Physics of muon decay • TWIST experiment • TWIST results to date • How will we do better?

  2. Muon decay matrix element • Most general local, derivative-free, lepton-number conserving muon decay matrix element: • In the Standard Model, gVLL = 1, all others are zero • Pre-TWIST global fit results (all 90% c.l.): • Theoretical constraints have recently been derived on the LR and RL terms from neutrino mass limits (hep-ph/0608163)

  3. Muon decay spectrum • The energy and angle distributions of positrons following polarized muon decay obey the Michel spectrum: • Pre-TWIST accepted values for the Michel parameters: where SM  = 0.7518 ± 0.0026 3/4  = -0.007 ± 0.013 0 P = 1.0027 ± 0.0079 ± 0.0030 1  = 0.7486 ± 0.0026 ± 0.0028 3/4 P(/) > 0.99682 (90% c.l.) 1

  4. Goal of TWIST • Search for new physics that can be revealed by order-of-magnitude improvements in our knowledge of ρ, δ, and Pμξ • Model-independent limit on muon handedness • Left-right symmetric models • ….. Two examples

  5. What is required? • Must: • Understand sources of muon depolarization • -- Pμ and ξ come as a product • Determine spectrum shape • -- All three parameters • Measure forward-backward asymmetry • -- For Pμξ and δ • to within a few parts in 104

  6. TRIUMF M13 beam line Surface muon beam

  7. TWIST spectrometer

  8. Typical events • Use pattern recognition (in position and time) to sort hits into tracks, then fit to helix • Must also recognize beam positrons, delta tracks, backscattering tracks

  9. 2-d momentum-angle spectrum In angular fiducial In momentum fiducial Acceptance of the TWIST spectrometer

  10. Fitting the data distributions αMC hidden  blind analysis • Fit data to sum of a MC base spectrum plus MC-generated derivative distributions. • Decay distribution is linear in the Michel parameters, so this is exact, no matter what values (αMC) are used in the MC base spectrum.

  11. Physics data sets • Fall 2002 • Test data-taking procedures and develop analysis techniques • First physics results – ρ and δ • Graphite-coated Mylar target not suitable for Pμξ • Fall 2004 • Al target and Time Expansion Chamber enabled first Pμξ measurement • Improved determinations of ρ and δ are underway • 2006-07 • Achieve ultimate TWIST precision for ρ, δ, and Pμξ

  12. Fitting the 2002 data to determine ρ and δ Normalized residuals [(Data-Fit)/sigma] of the 2-d momentum-angle fit Fit describes the data well, even when extrapolated far outside the fiducial region Angle-integrated results

  13. Fitting the 2004 data to determine Pμξ Separating the asymmetry into components

  14. Results to date • From Fall, 2002 run: • ρ= 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η) PRL 94, 101805 • δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst) PRD 71, 071101 • New global analysis (PRD 72, 073002) using the ρ andδ results, together with previous measurements and recent e+ transverse polarization measurements (PRL 94, 021802): • Significant improvements in the limits for gS,V,TLR • η= -0.0036 ± 0.0069 • From Fall, 2004 run (so far): • Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst) PRD 74, 072007 • Factors of 2-3 improvements on pre-TWIST precisions

  15. New limits in left-right symmetric models Restricted (“manifest”) LRS model General LRS model Initial TWIST measurements already provide significant new limits

  16. Results to date • From Fall, 2002 run: • ρ= 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η) PRL 94, 101805 • δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst) PRD 71, 071101 • New global analysis (PRD 72, 073002) using the ρ andδ results, together with previous measurements and recent e+ transverse polarization measurements (PRL 94, 021802): • Significant improvements in the limits for gS,V,TLR • η= -0.0036 ± 0.0069 • From Fall, 2004 run (so far): • Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst) PRD 74, 072007 • Factors of 2-3 improvements on pre-TWIST precisions

  17. Systematics in the previous measurements The same effects tend to dominate the systematic uncertainties for all three parameters.

  18. Reducing the leading systematics • Issues that were unique to 2002 data • Stopping target thickness uncertainty • Chamber orientation uncertainty with respect to magnetic field • For all three Michel parameters • Chamber response • Improved gas system regulation and monitoring • Improved determination of foil geometry • Improved treatment of drift chamber behavior • Positron interactions • Specific to Pμξ • Muon depolarization when crossing fringe field • Muon depolarization in the stopping target

  19. Controlling positron interaction uncertainties – “upstream stops” • Our Monte Carlo must simulate positron interactions properly • Need a well-understood test beam to validate the Monte Carlo • We use the Michel spectrum!

  20. Validating the Monte Carlo with “upstream stops” (Downstream fit result) – (upstream fit result) • The TWIST Monte Carlo provides an excellent description of the hard interaction physics • Took ~50 times more upstream stop events in 2004 than in 2002 Bremsstrahlung Scattering

  21. Muon depolarization across the fringe field Use Time Expansion Chamber (TEC) to measure andoptimize the muon beam • First installed for 2004 run • Found vertical beam offset – now corrected • Now take frequent beam characterizations • Have techniques to identify when the beam changes between TEC measurements 2004 muon beam spot

  22. Muon depolarization after stopping • Observed significant depolarization in 2004 data • New techniques allow us to veto muons that stop outside the metal stopping target • More sensitive analysis procedures to determine the residual depolarization rate • Now taking data with a Ag target to explore material dependence

  23. Conclusions • The initial TWIST measurements have improved our knowledge of the Michel parameters ρ, δ, and Pμξ by factors of 2-3 • Improvements by additional factors of 3-5 are anticipated • Stay tuned!

  24. TRIUMF Ryan Bayes¤y Yuri Davydov Jaap Doornbos Wayne Faszer Makoto Fujiwara David Gill Alex Grossheim Peter Gumplinger Anthony Hillairet¤y Robert Henderson Jingliang Hu John A. Macdonald x Glen Marshall Dick Mischke Mina Nozar Konstantin Olchanski Art Olin y Robert Openshaw Tracy Porcelli z Jean-Michel Poutissou Renée Poutissou Grant Sheffer Bill Shin zz Alberta Andrei Gaponenko¤¤ Peter Kitching Robert MacDonald¤ Maher Quraan Nate Rodning x John Schaapman Glen Stinson British Columbia James Bueno¤ Mike Hasinoff Blair Jamieson¤¤ Montréal Pierre Depommier Regina Ted Mathie Roman Tacik TWIST Participants Kurchatov Institute Vladimir Selivanov Vladimir Torokhov Texas A&M Carl Gagliardi Jim Musser¤¤ Bob Tribble Maxim Vasiliev Valparaiso Don Koetke Paul Nord Shirvel Stanislaus ¤ Graduate student ¤¤ Graduated y also U Vic z also Manitoba zz also Saskatchewan x deceased Supported under grants from NSERC (Canada) and DOE (USA). Additional support from TRIUMF, NRC (Canada), and the Russian Ministry of Science. Computing facilities of WestGrid are gratefully acknowledged.

  25. Coupling constants and Michel parameters • The Michel parameters are bilinear combinations of the coupling constants:

  26. Detector array • 56 low-mass high-precision planar chambers symmetrically placed around thin target foil • Measurement initiated by single thin scintillation counter at entrance to detector • Beam stop position controlled by variable He/CO2 gas degrader

  27. Analysis method • Extract energy and angle distributions for data: • Apply (unbiased) cuts on muon variables. • Reject fast decays and backgrounds. • Calibrate e+ energy to kinematic end point at 52.83 MeV. • Fit to identically derived distributions from simulation: • GEANT3 geometry contains virtually all detector components. • Simulate chamber response in detail. • Realistic, measured beam profile and divergence. • Extra muon and beam positron contamination included. • Output in digitized format, identical to real data.

  28. Data set by data set 2-d fit results 2002 ρ 2004 Pμξ δ

  29. Recent muon decay global analysis PRD 72, 073002 • Fit also finds η = -0.0036 ± 0.0069, a factor of ~2 more precise than the previously accepted value, -0.007 ± 0.013. • Significant improvement in η comes from new measurements of the transverse polarization of the e+ in muon decay (PRL 94, 021802), but the TWISTρ and δ measurements also play an important part. These improvements arise from the TWISTρ and δ measurements.

  30. Validating the soft physics simulationwith “upstream stops” • Multiple scattering is well reproduced • Small differences in dE/dx are seen between data and MC • Exploring differences between GEANT3 and GEANT4 Multiple Scattering dE/dx

  31. Tracking (in)efficiency • Measured in upstream stop events • Reconstruct track upstream, then ask if downstream also reconstructs • Overestimates true inefficiency – not all tracks reach the downstream half Difference between inefficiencies in data and Monte Carlo events

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