Measurement of the rate of formation of pi-mu atoms inKL0decay

Hydrogenlike atoms consisting of a pion and a muon can be formed in $K_L^0$→πμν decays. In an experiment at Fermilab, 320 pi-mu atoms were detected and simultaneously the $K_L^0$ flux was monitored by recording ordinary $K_L^0$→πμν decays. The first measurement of the branching ratio R=Γ($K_L^0$→pi-mu atom+ν)/Γ($K_L^0$→πμν)=(3.90 ±0.39)×${10}^{\mathrm{-}7}$ is reported, based on 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=(4.31±0.08)×${10}^{\mathrm{-}7}$..AE