Longitudinal Polarization of Low-Energy Muons fromKμ3+Decay

The longitudinal polarization of the muon from ${K_{\mu 3}}^{+}$ decay has been studied in the muon energy interval 6-27 MeV using a hydrogen bubble chamber. The muon polarization is determined from a measurement of the angular and momentum distribution of the decay positrons. The result is $〈P_{\mu }〉=+0.33\mathrm{}\pm 0.19$ and yields a value for the form factor ratio $\xi \equiv \frac{f_{-}}{f_{+}}={{1.2}_{-1.8\mathrm{}}}^{+2.4\mathrm{}}$. Combining this with previous experiment to resolve the various ambiguities gives a best value of $\xi$ based on polarization measurements alone of $\xi =+0.2\mathrm{}\pm 0.8$.