Measurement of the rate of formation of pi-mu atoms in KL0 decay

Hydrogenlike atoms consisting of a pion and a muon can be formed in K/sub L//sup 0/..--> pi mu nu.. decays. In an experiment at Fermilab, 320 pi-mu atoms were detected and simultaneously the K/sub L//sup 0/ flux was monitored by recording ordinary K/sub L//sup 0/..--> pi mu nu.. decays. The first measurement of the branching ratio R = GAMMA(K/sub L//sup 0/..-->..pi-mu atom+..nu..)/GAMMA(K/sub L//sup 0/..--> pi mu nu..) = (3.90 +- 0.39) x 10/sup -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) x 10/sup -7/.