# $\delta$ PARAMETER INSPIRE search

($\mathit V−\mathit A$) 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
 2011
TWST + Surface ${{\mathit \mu}^{+}}$
$0.7486$ $\pm0.0026$ $\pm0.0028$ 2
 1988
SPEC + Surface ${{\mathit \mu}^{+}}$
• • • We do not use the following data for averages, fits, limits, etc. • • •
$0.75067$ $\pm0.00030$ $\pm0.00067$
 2008
TWST + Surface ${{\mathit \mu}^{+}}$
$0.74964$ $\pm0.00066$ $\pm0.00112$ 6G
 2005
TWST + Surface ${{\mathit \mu}^{+}}$
3
 1969
$0.752$ $\pm0.009$ 490k
 1968
ASPK + 25$-$53 MeV ${{\mathit e}^{+}}$
$0.782$ $\pm0.031$
 1961
$0.78$ $\pm0.05$ 8354
 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
 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