Measurement of the muon decay parameter $\delta$

The muon decay parameter $\delta$ has been measured by the TWIST collaboration. We find $\delta =0.74964\pm 0.00066(\mathrm{\text{stat}}.)\pm 0.00112(\mathrm{\text{syst}}.)$, consistent with the standard model value of $3/4$. This result implies that the product $P_{\mu }\xi$ of the muon polarization in pion decay, $P_{\mu }$, and the muon decay parameter $\xi$ falls within the 90% confidence interval $0.9960<P_{\mu }\xi \le \xi <1.0040$. It also has implications for left-right-symmetric and other extensions of the standard model.

Figure 1

The difference between downstream and upstream tracks, for both data and MC, resulting in: top, the positron momentum change in the central stopping target, bottom, $\Delta \theta$ for a positron that passed through the central stopping target. The MC results were normalized to those of the data for the purposes of this figure.

Figure 2

Decay positron angular distributions from set B (solid curves) and the corresponding best fit distributions within the fiducial region (dashed curves) for selected momentum bins. ${\theta }_z=\pi -\theta$.

Figure 3

(a) The observed muon decay asymmetry from set B for all events within $0.50<|\cos \theta |<0.84$. (b) The same quantity for those events that fall within the fiducial region. (c) The difference between the data in panel (b) and the best fit MC spectrum. (d) The difference between the asymmetry calculated from the reconstructed MC events and as thrown by the MC, which illustrates the spectrometer response.