#### $\eta$ PARAMETER

($\mathit V−\mathit A$) theory predicts $\eta$ = 0.
VALUE EVTS DOCUMENT ID TECN CHG  COMMENT
 $\bf{ 0.057 \pm0.034}$ OUR AVERAGE
$0.071$ $\pm0.037$ $\pm0.005$ 30M
 2005
CNTR + $7 - 53$ MeV ${{\mathit e}^{+}}$
$0.011$ $\pm0.081$ $\pm0.026$ 5.3M 1
 1985 B
CNTR + 9$-$53 MeV ${{\mathit e}^{+}}$
$-0.12$ $\pm0.21$ 6346
 1969
HBC + 1.6$-$6.8 MeV ${{\mathit e}^{+}}$
• • We do not use the following data for averages, fits, limits, etc. • •
$-0.0021$ $\pm0.0070$ $\pm0.0010$ 30M 2
 2005
CNTR + $7 - 53$ MeV ${{\mathit e}^{+}}$
$-0.012$ $\pm0.015$ $\pm0.003$ 5.3M 2
 1985 B
CNTR + 9$-$53 MeV ${{\mathit e}^{+}}$
$-0.007$ $\pm0.013$ 5.3M 3
 1985 B
FIT + 9$-$53 MeV ${{\mathit e}^{+}}$
$-0.7$ $\pm0.5$ 170k 4
 1968
ASPK + 25$-$53 MeV ${{\mathit e}^{+}}$
$-0.7$ $\pm0.6$ 280k 4
 1967
ASPK + 25$-$53 MeV ${{\mathit e}^{+}}$
$0.05$ $\pm0.5$ 800k 4
 1966
ASPK + 20$-$53 MeV ${{\mathit e}^{+}}$
$-2.0$ $\pm0.9$ 9213 5
 1960
HBC + Whole spectrum
 1 Previously we used the global fit result from BURKARD 1985B in OUR AVERAGE, we now only include their actual measurement.
 2 $\alpha$ = $\alpha {{}^\prime}$ = 0 assumed.
 3 Global fit to all measured parameters. The fit correlation coefficients are given in BURKARD 1985B.
 4 ${{\mathit \rho}}$ constrained = 0.75.
 5 Two parameter fit to ${{\mathit \rho}}$ and ${{\mathit \eta}}$ ; PLANO 1960 discounts value for ${{\mathit \eta}}$ .
References:
 DANNEBERG 2005
PRL 94 021802 Muon Decay: Measurement of the Transverse Polarization of the Decay Positrons and its Implications for the Fermi Coupling Constant and Time Reversal Invariance
 BURKARD 1985B
PL 160B 343 Muon Decay: Measurement of the Transverse Positron Polarization and General Analysis
 Also
PR D24 2004 Measurement of the Positron Longitudinal Polarization in Muon Decay
 Also
PL 129B 260 Does the Positron from Muon Decay have Transverse Polarization?
 DERENZO 1969
PR 181 1854 Measurement of the Low Energy End of the ${{\mathit \mu}^{+}}$ Decay Spectrum
 FRYBERGER 1968
PR 166 1379 Measurement of the Muon Decay Spectrum with a Wire Spark Chamber Spectrometer
 SHERWOOD 1967
PR 156 1475 Measurement of the Muon Decay Spectrum with a Wire Spark Chamber Spectrometer
 PEOPLES 1966
Nevis 147 unpub. Positron Spectrum from Muon Decay.
 PLANO 1960
PR 119 1400 Momentum and Asymmetry Spectrum of ${{\mathit \mu}}$ Meson Decay