Measurement of $P_{\mu }\xi$ in polarized muon decay

The quantity $P_{\mu }^{\pi }\xi$, where $\xi$ is one of the muon decay parameters and $P_{\mu }^{\pi }$ is the degree of muon polarization in pion decay, has been measured. The value $P_{\mu }^{\pi }\xi =1.0003\pm 0.0006(\mathrm{stat}.)\pm 0.0038(\mathrm{syst}.)$ was obtained. This result agrees with previous measurements but is over a factor of 2 more precise. It also agrees with the Standard Model prediction for $P_{\mu }^{\pi }\xi$ and thus leads to restrictions on left-right symmetric models.

Figure 1

Location of the last beam line quadrupole, beam monitor, gas degrader, trigger scintillator, and the TWIST solenoid.

Figure 2

Beam measurements projected to the center of the beam monitor at each 1 mm by 1 mm bin in $y$ versus $x$. (a) Muon beam intensity profile. (b) Mean angle in $x$ (${\overline{\theta }}_x$).

Figure 3

Side view of the upstream half of the TWIST spectrometer planar chambers and support structure.

Figure 4

Sections of the 2-dimensional end point fit function of the muon decay spectrum for the bins within the fiducial region containing the smallest upstream and downstream angles. The data are shown as a solid line, the matching simulation set is shown as a dashed line, and the end point fit functions are shown as smooth curves.

Figure 5

Extrapolation to zero decay time of relative muon polarization, estimated using the decay positron integral asymmetry described in the text. The extrapolation function is fit to data: with an exponential shown as a dashed line, and as a Gaussian shown as a solid line.

Figure 6

The top panel shows the fit asymmetry versus positron momentum, $A(p)$, along with the contributions to the fit $A(p)$ from the $\xi$ and $\xi \delta$ terms. The bottom panel shows the difference between the data and fit, $\Delta A(p)$.

Figure 7

The top panel shows the manifest LRS model 90% confidence limits on $\zeta$ and $m_2$ ($g_L/g_R=1$) from measurements of $P_{\mu }^{\pi }\xi$. The bottom panel shows the same limits in the general LRS model case.