Observation of the Resonant Character of the Z(4430)− State

Abstract

Resonant structures in B0→ψ′π−K+ decays are analyzed by performing a four-dimensional fit of the decay amplitude, using pp collision data corresponding to 3 fb−1 collected with the LHCb detector. The data cannot be described with K+π− resonances alone, which is confirmed with a model-independent approach. A highly significant Z(4430)−→ψ′π− component is required, thus confirming the existence of this state. The observed evolution of the Z(4430)− amplitude with the ψ′π− mass establishes the resonant nature of this particle. The mass and width measurements are substantially improved. The spin parity is determined unambiguously to be 1+.