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

Abstract

Resonant structures in $B^0\to \ddot{I}{ˆ}^{\prime }\ddot{I}{€}^{-}{K}^{+}$ decays are analyzed by performing a four-dimensional fit of the decay amplitude, using $pp$ collision data corresponding to $3f{\mathrm{b}}^{-1}$ collected with the LHCb detector. The data cannot be described with $K^{+}\ddot{I}{€}^{-}$ resonances alone, which is confirmed with a model-independent approach. A highly significant $Z(4430{)}^{-}\to \ddot{I}{ˆ}^{\prime }\ddot{I}{€}^{-}$ component is required, thus confirming the existence of this state. The observed evolution of the $Z(4430{)}^{-}$ amplitude with the $\ddot{I}{ˆ}^{\prime }\ddot{I}{€}^{-}$ mass establishes the resonant nature of this particle. The mass and width measurements are substantially improved. The spin-parity is determined unambiguously to be $1^{+}$.

10 pages, 4 figures