Kμ30Decay Spectrum

An experiment has been performed to observe the energy spectrum of the ${K_{\mu 3}}^0$ decay (${K_2}^0\to \pi +\mu +\nu$). The observed spectrum indicates that the decay interaction is vector, rather than scalar or tensor, in agreement with the $V-A$ theory of weak interactions. This conclusion is obtained even when energy-dependent form factors are considered. The two form factors ($f_{+}$ and $f_{-}$) of the vector interaction are found to be consistent with several possible theoretical models for their behavior. If we assume constant form factors, their ratio is found to be $\xi \equiv \frac{f_{-}}{f_{+}}=1.2\mathrm{}\pm 0.8$. If we assume a $J=1\mathrm{}$ intermediate $K-\pi$ state to predominate in the decay, we find its mass to be $540_{-70\mathrm{}}^{}{}_{}{}^{+140\mathrm{}}$ MeV; if $J=0\mathrm{}$, we find a mass of $570_{-70\mathrm{}}^{}{}_{}{}^{+160\mathrm{}}$ MeV. The data consist of 1371 events obtained from a neutral-beam spark-chamber detection system in which the charged decay products are observed in a magnetic field. The $\mu$ is identified by its ability to pass through several interaction lengths of material into a set of range chambers. A branching ratio of $\lesssim {10}^{-4\mathrm{}}$ is obtained for neutral-current ${K_2}^0$ decays.