Search for the decay B0 to a1 rho

We present a search for the rare B -meson decay B 0 → a ± 1 ρ ∓ with a ± 1 → π + π − π ± . We use (110 ± 1.2) × 10 6 Υ (4 S ) → BB decays collected with the B A B AR detector at the PEP-II asymmetric-energy B Factory at SLAC. We obtain an upper limit of 30 × 10 − 6 (90% C . L . ) for the branching fraction product B ( B 0 → a ± 1 ρ ∓ ) B ( a ± 1 → π + π − π ± ), where we assume that the a ± 1 decays exclusively to ρ 0 π ± .