Using (771.6 +/- 10.6)x106 B over line B meson pairs recorded by the Belle experiment at the KEKB e+e- collider, we report the branching fractions B( over line B0 -> D*+pi-)=(2.62 +/- 0.02 +/- 0.09)x10-3 and B( over line B0 -> D*+K-)=(2.22 +/- 0.06 +/- 0.08)x10-4; the quoted uncertainties are statistical and systematic, respectively. A measurement of the ratio of these branching fractions is also presented, RK/pi=B( over line B -> D*+K-)/B( over line B -> D*+pi-)=(8.41 +/- 0.24 +/- 0.13)x10-2, where systematic uncertainties due to the D*+ meson reconstruction cancel out. Furthermore, we report a new QCD factorization test based on the measured ratios for over line B -> D*+h- and over line B -> D*+l-nu decays at squared momentum transfer values equivalent to the mass of the h=pi or K hadron. The parameters |a1(h)| are measured to be |a1(pi)|=0.884 +/- 0.004 +/- 0.003 +/- 0.016 and |a1(K)|=0.913 +/- 0.019 +/- 0.008 +/- 0.013, where the last uncertainties account for all external inputs. These values are approximately 15% lower than those expected from theoretical predictions. Subsequently, flavor SU(3) symmetry is tested by measuring the ratios for pions and kaons, |a1(K)|2/|a1(pi)|2=1.066 +/- 0.042 +/- 0.018 +/- 0.023, as well as for different particle species. The ratio is consistent with unity and therefore no evidence for SU(3) symmetry breaking effects is found at the 5% precision level.
