Measurement of the branching ratio of the decay D⁰ -> π^-μ^+ν relative to D⁰ -> K^-μ^+ν

We present a new measurement of the branching ratio of the Cabibbo suppressed decay D^0\to \pi^-\mu^+\nu relative to the Cabibbo favored decay D^0\to K^-\mu^+\nu and an improved measurement of the ratio |\frac{f_+^{\pi}(0)}{f_+^{K}(0)}|. Our results are 0.074 \pm 0.008 \pm 0.007 for the branching ratio and 0.85 \pm 0.04 \pm 0.04 \pm 0.01 for the form factor ratio, respectively.