Observation of a Near-Threshold Structure in the K+ Recoil-Mass Spectra in e+e- →k+ (Ds- D∗0+ Ds∗- D0)

We report a study of the processes of e+e-→K+Ds-D∗0 and K+Ds∗-D0 based on e+e- annihilation samples collected with the BESIII detector operating at BEPCII at five center-of-mass energies ranging from 4.628 to 4.698 GeV with a total integrated luminosity of 3.7 fb-1. An excess of events over the known contributions of the conventional charmed mesons is observed near the Ds-D∗0 and Ds∗-D0 mass thresholds in the K+ recoil-mass spectrum for events collected at s=4.681 GeV. The structure matches a mass-dependent-width Breit-Wigner line shape, whose pole mass and width are determined as (3982.5-2.6+1.8±2.1) MeV/c2 and (12.8-4.4+5.3±3.0) MeV, respectively. The first uncertainties are statistical and the second are systematic. The significance of the resonance hypothesis is estimated to be 5.3 σ over the contributions only from the conventional charmed mesons. This is the first candidate for a charged hidden-charm tetraquark with strangeness, decaying into Ds-D∗0 and Ds∗-D0. However, the properties of the excess need further exploration with more statistics.