Observation of an Axial-Vector State in the Study of the Decay (Formula presented)

Using (Formula presented) (Formula presented) events collected with the BESIII detector at BEPCII, a partial wave analysis of the decay (Formula presented) is performed with the covariant tensor approach. In addition to the established states (Formula presented)) and (Formula presented), an axial-vector state with a mass near (Formula presented) is observed for the first time. Its mass and width are measured to be (Formula presented) and (Formula presented), respectively. The product branching fractions of (Formula presented) and (Formula presented) are determined to be (Formula presented) and (Formula presented), respectively. The branching fraction (Formula presented) is measured for the first time to be ((Formula presented). The first uncertainties are statistical and the second are systematic.