Measurement of the Rate of Formation of Pi-Mu Atoms in ${K_L}^0$ Decay

Hydrogenlike atoms consisting of a pion and a muon can be formed in ${K_L}^0\to \pi \mu \nu$ decays. In an intense, high-energy ${K_L}^0$ beam, 320 pi-mu atoms were detected and simultaneously the ${K_L}^0$ flux was monitored by recording ordinary ${K_L}^0\to \pi \mu \nu$ decays. The first measurement is reported of the branching ratio $R=\frac{\Gamma ({K_L}^0\to \mathrm{p}\mathrm{i}-\mathrm{m}\mathrm{u}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}+\nu )}{\Gamma ({K_L}^0\to \pi \mu \nu )}=(3.88\mathrm{}\pm 0.41)\times {10}^{-7\mathrm{}}$, using a subset of 155 atoms. This ratio may be sensitive to anomalous interactions between the pion and the muon. In the absence of such interactions, theory predicts $R=(4.43\mathrm{}\pm 0.12)\times {10}^{-7\mathrm{}}$.