Observation of the resonant character of the $Z(4430)^-$ state

Understand

Resonant structures in $B^0\to\psi'\pi^-K^+$ decays are analyzed by performing a four-dimensional fit of the decay amplitude, using $pp$ collision data corresponding to $\rm 3 fb^{-1}$ collected with the LHCb detector.

• The data cannot be described with $K^+\pi^-$ resonances alone, which is confirmed with a model-independent approach.
• A highly significant $Z(4430)^-\to\psi'\pi^-$ component is required, thus confirming the existence of this state.
• The observed evolution of the $Z(4430)^-$ amplitude with the $\psi'\pi^-$ mass establishes the resonant nature of this particle.