Exploring the threshold behavior and implications on the nature of Y(4260) and Zc(3900)

By assuming that Y(4260) either is a D-1(D) over bar molecular state or has sizeable couplings with D-0(D) over bar* and D'(1)(D) over bar, we investigate several decay modes of Y(4260). Under the special kinematic configurations, triangle singularity may occur in the rescattering amplitude, which will significantly change the threshold behavior. Obvious threshold enhancements or narrow cusp structures appear quite naturally without introducing a genuine resonance. We emphasize that the radiative decay modes may be useful for studying D-(*())(D) over bar S-wave scattering.