${{\mathit \mu}}$ DECAY PARAMETERS

$\delta $ PARAMETER

INSPIRE  
($\mathit V−\mathit A$) theory predicts $\delta $ = 0.75.
VALUE EVTS DOCUMENT ID TECN CHG  COMMENT
$\bf{ 0.75047 \pm0.00034}$ OUR AVERAGE
$0.75049$ $\pm0.00021$ $\pm0.00027$ 1
BAYES
2011
TWST + Surface ${{\mathit \mu}^{+}}$
$0.7486$ $\pm0.0026$ $\pm0.0028$ 2
BALKE
1988
SPEC + Surface ${{\mathit \mu}^{+}}$
• • We do not use the following data for averages, fits, limits, etc. • •
$0.75067$ $\pm0.00030$ $\pm0.00067$
MACDONALD
2008
TWST + Surface ${{\mathit \mu}^{+}}$
$0.74964$ $\pm0.00066$ $\pm0.00112$ 6G
GAPONENKO
2005
TWST + Surface ${{\mathit \mu}^{+}}$
3
VOSSLER
1969
$0.752$ $\pm0.009$ 490k
FRYBERGER
1968
ASPK + 25$-$53 MeV ${{\mathit e}^{+}}$
$0.782$ $\pm0.031$
KRUGER
1961
$0.78$ $\pm0.05$ 8354
PLANO
1960
HBC + Whole spectrum
1  The quoted systematic error includes a contribution of 0.00006 (added in quadrature) from uncertainties on radiative corrections and on the Michel parameter $\eta $.
2  BALKE 1988 uses $\rho $ = $0.752$ $\pm0.003$.
3  VOSSLER 1969 has measured the asymmetry below 10 MeV. See comments about radiative corrections in VOSSLER 1969 .
References:
BAYES 2011
PRL 106 041804 Experimental Constraints on Left-Right Symmetric Models from Muon Decay
Also
PR D85 092013 Precision Muon Decay Measurements and Improved Constraints on the Weak Interaction
MACDONALD 2008
PR D78 032010 Precision Measurement of the Muon Decay Parameters ${{\mathit \rho}}$ and ${{\mathit \delta}}$
GAPONENKO 2005
PR D71 071101 Measurement of the Muon Decay Parameter $\delta $
BALKE 1988
PR D37 587 Precise Measurement of the Asymmetry Parameter $\delta $ in Muon Decay
VOSSLER 1969
NC 63A 423 A Measurement of the Muon Decay Asymmetry at Low Energy
FRYBERGER 1968
PR 166 1379 Measurement of the Muon Decay Spectrum with a Wire Spark Chamber Spectrometer
KRUGER 1961
UCRL 9322 unpub. Momentum Dependence of the Asymmetry in Muon Decay
PLANO 1960
PR 119 1400 Momentum and Asymmetry Spectrum of ${{\mathit \mu}}$ Meson Decay