Observation of a Resonancelike Structure in the π+-ψ' Mass Distribution in Exclusive B→Kπ+-ψ' Decays

A distinct peak is observed in the π^±ψ^' invariant mass distribution near 4.43 GeV in B→Kπ^±ψ^' decays. A fit using a Breit-Wigner resonance shape yields a peak mass and width of M=4433±4(stat)±2(syst)MeV and Γ=45_-13⁺¹⁸(stat)_-13⁺³⁰(syst)MeV. The product branching fraction is determined to be B(B⁰→K^∓Z^±(4430))×B(Z^±(4430)→π^±ψ^')=(4.1±1.0(stat)±1.4(syst))×10^-5, where Z^±(4430) is used to denote the observed structure. The statistical significance of the observed peak is 6.5σ. These results are obtained from a 605fb^-1 data sample that contains 657×10⁶ B Bmacr pairs collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider.