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

Abstract

Resonant structures in B 0 → ψ ′ π − K + decays are analyzed by performing a four-dimensional fit of the decay amplitude, using p p 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 + .