Observation of B --> J/ψK1(1270)

We report the first observation of the exclusive decay process B-->J/ψK₁(1270) using a sample of 11.2M BBbar meson pairs collected in the Belle detector at the KEKB asymmetric energy e⁺e^- collider. We measure branching fractions of B[B⁰-->J/ψK⁰₁(1270)] = (1.30+/-0.34+/-0.32)×10^-3 and B[B⁺-->J/ψK⁺₁(1270)] = (1.80+/-0.34+/-0.39)×10^-3, where the first error is statistical and the second is systematic. These modes constitute approximately 15% of the total number of B-->J/ψX decays. No evidence is seen for B-->J/ψK₁(1400) and we set an upper limit for this branching fraction.