1. Progress on the final TWIST measurement of James Bueno, University of British Columbia and TRIUMF on behalf of the Triumf Weak Interaction Symmetry Test

2. Outline Muon production and decay Previous measurements of Sources of depolarisation, and their simulation. –Depolarisation in fringe field of solenoid. –Depolarisation in the muon stopping target. James Bueno, WNPPC 2008, 15 February 1/20

3. Muon production and James Bueno, WNPPC 2008, 15 February Muons from stationary pion decay are perfectly polarised (for massless neutrinos) TWIST only accepts a narrow momentum range  muon beam is highly polarised 2/20

4. Muon decay and James Bueno, WNPPC 2008, 15 February TWIST measures e + energy (x) and angle (θ) relative to muon spin. 3/20

5. Muon decay and James Bueno, WNPPC 2008, 15 February TWIST measures e + energy (x) and angle (θ) relative to muon spin. determines forward- backward asymmetry 3/20

6. Muon decay and James Bueno, WNPPC 2008, 15 February TWIST measures e + energy (x) and angle (θ) relative to muon spin. determines forward- backward asymmetry appears as an inseparable product 3/20

7. Physics motivation muon handedness SM predicts LH muon decays to LH positron. Probability of RH muon decay to a LH or RH positron is James Bueno, WNPPC 2008, 15 February 4/20

8. Physics motivation muon handedness left-right symmetric models SM predicts LH muon decays to LH positron. Probability of RH muon decay to a LH or RH positron is James Bueno, WNPPC 2008, 15 February 4/20

9. Previous measurements of 1987, Beltrami et al. (PSI) 1.0027 ± 0.0079 (stat.) ± 0.0030 (syst.) 2006, Jamieson et al. (TWIST, TRIUMF) 1.0003 ± 0.0006 (stat.) ± 0.0038 (syst.) James Bueno, WNPPC 2008, 15 February Standard model, 5/20

10. Previous measurements of 1987, Beltrami et al. (PSI) 1.0027 ± 0.0079 (stat.) ± 0.0030 (syst.) 2006, Jamieson et al. (TWIST, TRIUMF) 1.0003 ± 0.0006 (stat.) ± 0.0038 (syst.) 2004, Jodidio et al. with TWIST ρ, δ (Indirect) 0.9960 < < 1.0040 (90% confidence) James Bueno, WNPPC 2008, 15 February Standard model, 5/20

11. Previous measurements of 1987, Beltrami et al. (PSI) 1.0027 ± 0.0079 (stat.) ± 0.0030 (syst.) 2006, Jamieson et al. (TWIST, TRIUMF) 1.0003 ± 0.0006 (stat.) ± 0.0038 (syst.) 2004, Jodidio et al. with TWIST ρ, δ (Indirect) 0.9960 < < 1.0040 (90% confidence) 2009, final result from TWIST (goal) ± 0.0003 (stat.) ± 0.0010 (syst.) James Bueno, WNPPC 2008, 15 February Standard model, 5/20

12. Previous measurements of 1987, Beltrami et al. (PSI) 1.0027 ± 0.0079 (stat.) ± 0.0030 (syst.) 2006, Jamieson et al. (TWIST, TRIUMF) 1.0003 ± 0.0006 (stat.) ± 0.0038 (syst.) 2004, Jodidio et al. with TWIST ρ, δ (Indirect) 0.9960 < < 1.0040 (90% confidence) 2009, final result from TWIST (goal) ± 0.0003 (stat.) ± 0.0010 (syst.) James Bueno, WNPPC 2008, 15 February Standard model, Previous TWIST direct measurement limited by uncertainties in simulating 5/20

13. TWIST measurement of 1 x = E/Emax +1 0 cos θ Michel spectrum from real data 0 6/20

14. TWIST measurement of 1 x = E/Emax +1 0 cos θ Michel spectrum from real data Spectrum from GEANT simulation hidden difference in is measured Spectra from decay (depolarised) μ are compared  depolarisation in simulation must match data 0 6/20

15. Sources of depolarisation James Bueno, WNPPC 2008, 15 February symmetric tracking volume 7/20

16. Sources of depolarisation James Bueno, WNPPC 2008, 15 February symmetric tracking volume solenoid fringe field depends on trajectory 7/20

17. Sources of depolarisation James Bueno, WNPPC 2008, 15 February high purity metal target to stop muons symmetric tracking volume depends on time solenoid fringe field depends on trajectory 7/20

18. Simulating the depolarisation James Bueno, WNPPC 2008, 15 February Beam is measured by time expansion chambers BEFORE the fringe field symmetric tracking volume 7/20 X Y

19. Simulating the depolarisation James Bueno, WNPPC 2008, 15 February Beam is measured by time expansion chambers BEFORE the fringe field Time dependence of e + forward-backward asymmetry gives in target symmetric tracking volume 7/20 X Y

20. Simulating muon trajectories James Bueno, WNPPC 2008, 15 February 1. Measure each muon’s position and angle with time expansion chambers. projection in x 8/20

21. Simulating muon trajectories James Bueno, WNPPC 2008, 15 February 1. Measure each muon’s position and angle with time expansion chambers. projection in x 2. Correct angles for multiple scattering. 3. Simulate muons with GEANT and track spin to predict at target. Uncertainties due to beam instability, initial position & angle of beam, position & shape of B-field. 8/20

22. Beam stability (1 of 2) James Bueno, WNPPC 2008, 15 February During data acquisition, we needed a stable muon beam for weeks on end. 9/20

23. Beam stability (1 of 2) James Bueno, WNPPC 2008, 15 February During data acquisition, we needed a stable muon beam for weeks on end. And a method of detecting beam instabilities without the time expansion chambers in place. (too much multiple scattering) 9/20

24. Beam stability (2 of 2) James Bueno, WNPPC 2008, 15 February For 2006/7, internal beamspots are fit to a helix. Helix fit parameters indicate position and momentum. most of data set beam instability 10/20 5mm

25. Misalignment uncertainties James Bueno, WNPPC 2008, 15 February For 2006/2007 data, the beam was steered onto the solenoid axis. 2004 beam 2006/7 beam y (cm) 11/20

26. Misalignment uncertainties James Bueno, WNPPC 2008, 15 February For 2006/2007 data, the beam was steered onto the solenoid axis. 2004 beam 2006/7 beam y (cm) 2006 2004 Sensitivity to misalignments is now reduced 11/20

27. Spin tracking in the B-field James Bueno, WNPPC 2008, 15 February To validate GEANT’s spin tracking: two weeks of low data taken in 2006/7. nominal steered at angle 12/20

28. Spin tracking in the B-field James Bueno, WNPPC 2008, 15 February To validate GEANT’s spin tracking: two weeks of low data taken in 2006/7. nominal steered at angle inside detector nominal steered 12/20

29. Spin tracking in the B-field James Bueno, WNPPC 2008, 15 February To validate GEANT’s spin tracking: two weeks of low data taken in 2006/7. nominal steered at angle inside detector nominal steered Validation (in progress now) will require: 12/20

30. Stopping target depolarisation James Bueno, WNPPC 2008, 15 February Ideally, the muons would not depolarise at the stopping target (it’s 99.999% pure aluminium, or silver). However, an individual muon spin can interact with local magnetic fields and reverse sign. 13/20

31. Stopping target depolarisation James Bueno, WNPPC 2008, 15 February The simulation must include the correct model for in the target. 14/20

32. Stopping target depolarisation James Bueno, WNPPC 2008, 15 February The simulation must include the correct model for in the target. TWIST result for silver theoretically preferred also possible 14/20

33. Stopping target depolarisation James Bueno, WNPPC 2008, 15 February The simulation must include the correct model for in the target. TWIST result for silver theoretically preferred also possible no data below ~ 1 μs (or above 10 μs) A subsidiary experiment would be helpful… 14/20

34. Subsidiary muSR experiment collimator target trigger scintillators Schematic, not to scale 15/20

35. Subsidiary muSR experiment collimator backward e + counter target forward e + counter trigger scintillators Schematic, not to scale 15/20

36. Subsidiary muSR experiment collimator backward e + counter target forward e + counter trigger scintillators Schematic, not to scale 15/20

37. Subsidiary muSR experiment Advantages: Higher rate (~30kHz compared to ~3kHz) Larger time fiducial (~5 ns  14 μs, compared to ~1 μs  ~9 μs). 16/20 James Bueno, WNPPC 2008, 15 February

38. Subsidiary muSR experiment Advantages: Higher rate (~30kHz compared to ~3kHz) Larger time fiducial (~5 ns  14 μs, compared to ~1 μs  ~9 μs). Disadvantages: Background from beam positrons. A fraction of muons stop in the scintillator. 16/20 James Bueno, WNPPC 2008, 15 February

39. Subsidiary muSR experiment Unfortunately, still no discrimination between and 17/20

40. Subsidiary muSR experiment exp(-at) Aluminium a (x10 -6 ns -1 ) Silver a (x10 -6 ns -1 ) TWIST1.4 ± 0.11.1 ± 0.3 muSR1.7 ± 0.21.2 ± 0.2 Unfortunately, still no discrimination between and 17/20

41. Subsidiary muSR experiment exp(-at) Aluminium a (x10 -6 ns -1 ) Silver a (x10 -6 ns -1 ) TWIST1.4 ± 0.11.1 ± 0.3 muSR1.7 ± 0.21.2 ± 0.2 Unfortunately, still no discrimination between and Experiment confirmed no fast depolarisation between ~5 ns and 1 μs, and results are consistent with TWIST. 17/20

42. Using target PCs in TWIST proportional chambers closest to stopping target stopping target μ can stop in chamber gas 18/20 PC6 PC5

43. Using target PCs in TWIST proportional chambers closest to stopping target stopping target μ can stop in chamber gas pulse width in final PC greater for muons that stop here (more energy deposited) 18/20 PC6 PC5

44. Using target PCs in TWIST proportional chambers closest to stopping target stopping target μ can stop in chamber gas pulse width in final PC greater for muons that stop here (more energy deposited) PC 5 (ns) PC 6 (ns) 18/20 PC6 PC5

45. Using target PCs in TWIST proportional chambers closest to stopping target stopping target μ can stop in chamber gas pulse width in final PC greater for muons that stop here (more energy deposited) deep cut selects target stop μ PC 5 (ns) PC 6 (ns) 18/20 PC6 PC5 and rejects these

46. Summary James Bueno, WNPPC 2008, 15 February The final measurement is dominated by uncertainties in simulating. Validation of the spin tracking is well underway. Improvements in data acquisition in 2006/7 will reduce uncertainties. 19/20

47. Summary James Bueno, WNPPC 2008, 15 February The final measurement is dominated by uncertainties in simulating. Validation of the spin tracking is well underway. Improvements in data acquisition in 2006/7 will reduce uncertainties. Subsidiary muSR experiment has confirmed no fast depolarisation exists (this could not be measured by TWIST). Time dependent depolarisation uncertainties can be minimised by selecting genuine target stops. 19/20

48. James Bueno, WNPPC 2008, 15 February The TWIST collaboration 20/20