Neutral Decay Modes of theθ10andΛ0

Abstract

In a large multiplate cloud chamber, an event has been found which strongly indicates that the decay mode ${{\theta }_1}^0\to 2{\pi }^0$ exists, and therefore that the spin of the $K$ meson is even. Two showers are seen which, from their spatial angles and energies, must represent essentially the whole energy of the two ${\pi }^0\text{'}\mathrm{s}$ from a ${{\theta }_1}^0$ produced in association with a ${\Lambda }^0$. On the basis of this event, the fraction of ${{\theta }_1}^0$ decays going by the $2{\pi }^0$ mode is ∼0.06, and the probability that this fraction could be consistent with the value of $\frac{1}{3}$ predicted by $\Delta I=\frac{1}{2}$ selection rule is 3%. Furthermore, the chance that the observed ratio is ≥¼, a value possible with an admixture of $\Delta I=\frac{3}{2}$, is 10%. Our observation of single shower events attributable to ${{\theta }_1}^0\to 2{\pi }^0$ give even smaller probabilities that the fraction of neutral decays is as large as $\frac{1}{3}$ or ¼, but the necessity of a background subtraction makes these results less significant. On the other hand, the fraction of ${\Lambda }^0$ decays going by a neutral decay mode is quite consistent with the value of $\frac{1}{3}$ predicted by the $\Delta I=\frac{1}{2}$ rule. The values obtained for this fraction are 0.22±0.13 from the direct observation of showers (presumably from ${\Lambda }^0\to n+{\pi }^0$), and 0.33±0.06 from the number of ${\Lambda }^0\text{'}\mathrm{s}$ not decaying by the charged mode.