Observation of a resonance-like structure in the pi^+- psi' mass distribution in exclusive B-->K pi^+- psi' decays

A distinct peak is observed in the pi^+/- psi’ invariant mass distribution near 4.43 GeV in B->K pi^+/- psi’ decays. A fit using a Breit Wigner resonance shape yields a peak mass and width of 4433±4(stat)±2(syst) MeV and Gamma = 45^+18_-13(stat) ^+30_-13 MeV. The product branching fraction is determined to be Bf(B–>KZ(4430))xBf(Z(4430)–>pi^+psi’) = (4.1±1.0(stat)±1.4(syst))x10^{-5), where Z(4430) is used to denote the observed structure. The statistical significance of the observed peak is 6.5sigma. These results are obtained from a 605 fb-1 data sample that contains 657 million BBbar pairs collected near the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric energy e+e- collider.