Results and Discussion

Fitting the sample yields , which can be interpreted as twice the helicity of the tau neutrino. The fit to the channels gives and .

The sources of systematic error which have been considered in the analysis include -pair selection efficiency and background, decay identification efficiency and background, and resolution, uncertainties in the measurements of the beam energy and polarization, uncertainty in the value of [18], and the effect of fixing the parameter. Table summarizes the systematic errors in our analysis. To investigate these errors, each parameter used in the fitting program is modified in turn by its uncertainty, and the fit is redone to obtain new values of , and . The systematic errors assigned to the effect of fixing are taken to be the differences from the fitted values of and when is a free parameter. Any correlation between these input parameters is taken into account when combining the errors. Each error listed in Table is a combination of several related contributions, and for the listed errors we assume they are not correlated. They are combined in quadrature to give the overall systematic error listed in the last row of the table.

The large uncertainty assigned due to the fixed parameter may be partly redundant with the systematic error introduced by the resolution in x. Both of these result from the preliminary parameterizations of this resolution for the lepton channels, but we have taken a conservative approach and quoted the errors separately. Another large contribution to the systematic uncertainty on is the radiative correction, which affects the electron channel more strongly. The uncertainty in is dominated by the selection efficiency and the interference with the decay channels N (, , etc...).

Including systematic errors, the SLD preliminary values for and the Michel parameters and are

The results are consistent with the Standard Model V-A predictions of -1, 1, . These measurements provide an interesting cross check with other experimental results [16], [17], [19] since this analysis does not rely on spin correlations and is the first measurement to be performed with polarized beams. These results demonstrate the power of polarized beams for probing deviations from the Standard Model in the weak couplings.