1. 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?
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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/ )
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
 = ± /4
 = ±
P = ± ±
 = ± ± /4
P(/) > (90% c.l.)
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

4. Goal of TWIST Two examples
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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

6. Surface muon beam TRIUMF M13 beam line
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

7. TWIST spectrometer
The TWIST Spectrometer
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

9. 2-d momentum-angle spectrum
In angular fiducial
In momentum fiducial
Acceptance of the TWIST spectrometer
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

11. Physics data sets Fall 2002 Fall 2004 2006-07
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
Achieve ultimate TWIST precision for ρ, δ, and Pμξ
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

13. Fitting the 2004 data to determine Pμξ
Separating the asymmetry
into components
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

14. Results to date From Fall, 2002 run:
ρ = ± (stat) ± (syst) ± (η)
PRL 94,
δ = ± (stat) ± (syst)
PRD 71,
New global analysis (PRD 72, ) using the ρ and δ results, together with previous measurements and recent e+ transverse polarization measurements (PRL 94, ):
Significant improvements in the limits for gS,V,TLR
η = ±
From Fall, 2004 run (so far):
Pμξ = ± (stat) ± (syst)
PRD 74,
Factors of 2-3 improvements on pre-TWIST precisions
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

15. New limits in left-right symmetric models
Restricted (“manifest”) LRS model
General LRS model
Initial TWIST measurements already provide significant new limits
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

16. Results to date From Fall, 2002 run:
ρ = ± (stat) ± (syst) ± (η)
PRL 94,
δ = ± (stat) ± (syst)
PRD 71,
New global analysis (PRD 72, ) using the ρ and δ results, together with previous measurements and recent e+ transverse polarization measurements (PRL 94, ):
Significant improvements in the limits for gS,V,TLR
η = ±
From Fall, 2004 run (so far):
Pμξ = ± (stat) ± (syst)
PRD 74,
Factors of 2-3 improvements on pre-TWIST precisions
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

17. Systematics in the previous measurements
The same effects tend to dominate the systematic uncertainties for all three parameters.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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!
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

20. Validating the Monte Carlo with “upstream stops”
(Downstream fit result) – (upstream fit result)
Bremsstrahlung
Scattering
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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

21. Muon depolarization across the fringe field
Use Time Expansion Chamber (TEC) to measure and optimize 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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

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!
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

24. TWIST Participants 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
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.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

25. Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

26. Coupling constants and Michel parameters
The Michel parameters are bilinear combinations of the coupling constants:
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

27. 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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

28. 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 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.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

29. Data set by data set 2-d fit results
2002 ρ
2004
Pμξ δ
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

30. Recent muon decay global analysis
PRD 72,
These improvements arise from the TWIST ρ and δ measurements.
Fit also finds η = ± , a factor of ~2 more precise than the previously accepted value, ±
Significant improvement in η comes from new measurements of the transverse polarization of the e+ in muon decay (PRL 94, ), but the TWIST ρ and δ measurements also play an important part.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

31. Validating the soft physics simulation with “upstream stops”
Multiple Scattering
dE/dx
Multiple scattering is well reproduced
Small differences in dE/dx are seen between data and MC
Exploring differences between GEANT3 and GEANT4
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06

32. 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
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06