Zc(3900): What has been really seen?

The Zc±(3900)/Zc±(3885) resonant structure has been experimentally observed in the Y(4260)→J/ψππY(4260)→J/ψππ and Y(4260)→D¯⁎Dπ decays. This structure is intriguing since it is a prominent candidate of an exotic hadron. Yet, its nature is unclear so far. In this work, we simultaneously describe the D¯⁎D and J/ψπJ/ψπ invariant mass distributions in which the ZcZc peak is seen using amplitudes with exact unitarity. Two different scenarios are statistically acceptable, where the origin of the ZcZc state is different. They correspond to using energy dependent or independent D¯⁎DS  -wave interaction. In the first one, the ZcZc peak is due to a resonance with a mass around the DD¯⁎ threshold. In the second one, the ZcZc peak is produced by a virtual state which must have a hadronic molecular nature. In both cases the two observations, Zc±(3900) and Zc±(3885), are shown to have the same common origin, and a D¯⁎D bound state solution is not allowed. Precise measurements of the line shapes around the DD¯⁎ threshold are called for in order to understand the nature of this state.