Measurement of Γ( D0 → K-μ+ν)/ Γ( D0→ μ+X using a D ∗ tag

The relative branching fraction R_K = Γ( D⁰ → K^-μ⁺v)/ Γ( D⁰ → μ^-X) has been measured by Fermilab experiment E653, using D⁰ mesons produced by 600 GeV π^-. The semimuonic decay sample of 232 events was identified by the decay chain D ^∗+ → D ⁰π ⁺, D ⁰ → μ ⁺h ^-X , where h is a hadron. The Kμv component was extracted from the joint distribution of these events in the D⁰ decay variables M_min and p_Th. We find R_K = 0.472 ± 0.051 (stat) ± 0.040 (sys), and use a world average of the D⁰ → Klv branching fraction to obtain the D⁰ inclusive semimuonic branching fraction B( D⁰ → μ⁺X) = (7.67 ± 1.13)%.