Measurement of the $B^0 \to D^ *- D_s^ *+ $ and $D_s^+ \to \phi \pi^+$ branching fractions

The authors present measurements of the branching fractions {Beta}(B{sup 0} {yields} D*{sup -}D*{sub s}{sup +}) and {Beta}(D{sub s}{sup +} {yields} {phi}{pi}{sup +}), based on 123 x 10{sup 6} {Upsilon}(4S) {yields} B{bar B} decays collected by the BABAR detector at the PEP-II asymmetric-energy e{sup +}e{sup -} B factory. A partial reconstruction technique is used to measure {Beta}(B{sup 0} {yields} D*{sup -}D*{sub s}{sup +}) and the decay chain is fully reconstructed to measure the branching fraction product {Beta}(B{sup 0} {yields} D*{sup -} D*{sub s}{sup +}) x {Beta}(D{sub s}{sup +} {yields} {phi}{pi}{sup +}). Comparing these two measurements provides a model-independent determination of the D{sub s}{sup +} {yields} {phi}{pi}{sup +} branching fraction. They obtain {Beta}(B{sup 0} {yields} D*{sup -}D*{sub s}{sup +}) = (1.88 {+-} 0.09 {+-} 0.17)% and {Beta}(D{sub s}{sup +} {yields} {phi}{pi}{sup +}) = (4.81 {+-} 0.52 {+-} 0.38)%, where the first uncertainties are statistical and the second systematic.