A measurement of the B0 --> J/psi pi+pi- branching fraction

We present a measurement of the branching fraction for the decay of the neutral B meson into the final state J/{psi}{pi}{sup +}{pi}{sup -}. The data set contains approximately 56 million B{bar B} pairs produced at the {Upsilon}(4S) resonance and recorded with the BABAR detector at the PEP-II asymmetric-energy e{sup +}e{sup -} storage ring. The result of this analysis is {Beta}(B {sup 0} {yields} J/{psi} {pi}{sup +}{pi}{sup -}) = (4.6 {+-} 0.7 {+-} 0.6) x 10{sup -5}, where the first error is statistical and the second is systematic. In addition we measure {Beta}(B{sup 0} {yields} J?{psi}{rho}{sup 0}) = (1.6 {+-} 0.6 {+-} 0.4 ) x 10{sup -5}.