TWIST: Precision Measurement of the Muon Decay Parameters Alexander Grossheim TRIUMF for the TWIST Collaboration Muon Decay and Weak Interaction TWIST experiment Latest Results Prospects NUFACT08

2 Muon Decay EM radiative corrections calculable Strong interactions are at < 1e-6 level μ+μ+ μ+μ+ νμνμ νμνμ νeνe νeνe e+e+ e+e+ W+W+ W+W+ ~2 μs lifetime

3 Weak Matrix Element In Standard Model (“V- A”): otherwise g κ εm constrained by muon decay, inverse decay, etc.

Decay (“Michel”) Spectrum Pre-TWISTSM ρ ± η ± PμξPμξ ± δ ± pepe SμSμ θ Latest results: new ρ and δ ρ, η, δ, ξ are bilinear combinations of g κ εm Michel, Kinoshita & Sirlin

“State of affairs” TWIST goal: reduce uncertainties by factor of 10

6 The TWIST Detector Target & PCs NIM A548 (2005) 206 Drift Chambers (4 mm, DME) Drift Chambers (4mm, DME) Proportional Chambers (2 mm, CF4/Iso) Proportional Chambers (2 mm, CF4/Iso) He in gaps Trigger Scintillator Low mass, symmetric, high-precision construction Assembled by hand at TRIUMF

7 The TWIST Experiment TRIUMF Weak Interaction Symmetry Test

8 “Blind” Analysis 8 Experimental Data Geant3 Simulation Analysis Δρ, Δδ, Δξ ρ, δ, ξ Analysis running on WestGrid Spectrum Fitter hidden ρ MC, δ MC, ξ MC revealed ρ MC, δ MC, ξ MC

9 Verifying the Simulation Muon stops here decay positron fit track upstream fit track downstream Specialized data, reproduced in simulation ➞ independent of Michel parameters For example: “upstream stops”: Eloss/ Scattering in target reproduced by MC?

Energy LossScattering Logarithmic Linear

1 Determining Systematics MC or Data Analysis Δρ, Δδ, Δξ Bremsstrahlung Chamber geometry … Systematic Uncertainty (Δρ, Δδ, Δξ) Exaggeration = = Exaggerated Analysis Magnetic field Chamber alignment … Spectrum Fitter exaggerated MC or Data

1212 δ ρ Latest Results for ρ,δ 12 TWIST published: ± (stat) ± (sys) TWIST published: ± (stat) ± (sys) Pre-TWIST: ± In prep.: ± (stat) ± (sys) ± (η) Pre-TWIST: ± (stat)± (sys) In prep. : ± (stat) ± (sys) In prep, to be submitted to PRD

1313 PRELIMINARY Phys. Rev. Lett. 94, (2005) ρ: Phys. Rev. D 71, (R) (2005) δ: Details on Systematics

1414 Improvements to Systematics for the latest analysis Chamber response online monitoring, increased instrumentation Target thicknessprecision target geometry Positron interactions better&more calibration data Alignment improved techniques, better understanding of uncertainties Momentum calibration new calibration techniques, uncertainty is statistical Radiative corrections higher-order corrections, uncertainty tested directly And more improvements for the final analysis.

Global Analysis of Muon Data Weak Coupling pre-TWIST Gagliardi et. al.* MacDonald '08 (in prep.) |g S LR | < 0.125< 0.088< |g V LR | < 0.066< 0.036< |g T LR | < 0.036< 0.025< *Phys. Rev. D 72, (2005) 90% Confidence Limits

16 μ- decay Some data was taken with negative muons ● Muon gets captured almost immediately in Al target ● Decay in orbit changes lifetime, decay spectrum ● Unfortunately only Al target and modest statistics ● Want to look at ● decay time ● eventual residual polarization/asymmetry ● smearing of momentum edge (52.8 MeV) ● extrapolation to endpoint (105 MeV)

17 μ- decay decay time: ~ 866 ns ns Challenges: Detector acceptance drops for >53 MeV Standard reconstruction and MC not optimized for >53 MeV Standard analysis not set up to fit an arbitrary spectrum

18 μ + vs μ- decay acceptance correction missing! +-+-

19 Something like TWIST in a high intensity muon beam? Another x10 reduction of uncertainties in Michel Parameters? statistical error becomes important: need more rate, but there are limits: decay time, drift time, CPU time There are almost “un-reducible” systematics (like positron interactions, momentum scale) need (at best) more dedicated calibrations -> more data -> more rate Measuring the full decay spectrum: Another order of magnitude improvement in all Michel parameters would require a modified approach.

20 Summary TWIST has measured full mu+ decay spectrum to extract Michel parameters Current results with no evidence for new physics, but new limits En route to final results more statistics (for physics and calibration) all leading systematics addressed original goal of reducing uncertainties by a factor of 10 in reach Some non-Michel analyses like mu-, rare decays, etc planned

The TWIST Collaboration TRIUMF Ryan Bayes ✪✻ Yuri Davydov Wayne Faszer Makoto Fujiwara David Gill Alexander Grossheim Peter Gumplinger Anthony Hillairet ✪✻ Robert Henderson Jingliang Hu Glen Marshall Dick Mischke Mina Nozar Konstantin Olchanski Art Olin ✻ Robert Openshaw Jean-Michel Poutissou Renée Poutissou Grant Sheffer Bill Shin ✧ Alberta Andrei Gaponenko ✭ Peter Kitching Robert MacDonald ✪ Nate Rodning ✾ Maher Quraan U British Columbia James Bueno ✪ Mike Hasinoff Blair Jamieson ✭ U Montréal Pierre Depommier U Regina Ted Mathie Roman TacikKurchatov Institute Vladimir Selivanov Texas A&M Carl Gagliardi Jim Musser ✭ Bob Tribble Valparaiso Don Koetke Shirvel Stanislaus ✪ graduate student ✭ graduated ✻ also U Vic ✧ also Saskatchewan ✾ deceased Supported under grants from NSERC and the US DOE. Additional support from TRIUMF, NRC, and the Russian Ministry of Science.

backup slides

2323 Muon Production and Transport p+p+ Polarized Muon Production TEC Stopping Target Fringe Field TWIST TEC: NIM A566 (2006), 563 M13 X X Y Y

2424 Decay Parameters and Coupling Constants

2525 Radiative Corrections O(α): Full tree-level RCs, exact electron mass O(α 2 ): Leading-Order (LO) and NLO in L=ln(m μ /m e )

Spectrum Fitter =+ ++ α = {ρ, δ, ξ}

2727 Spectrum Fit Quality

2828 Parameter Sensitivity Full spectrum (no RCs) ρ, ξδ η*300 ξ cosθ=1 E / E max Arbitrary Units

2929 Left-Right Symmetry TWIST Published: Pre-TWIST: Current:

Limits on Right-Handed Muon Decay Gagliardi: Pre-TWIST: Current: Gagliardi et al., PRD (2005)