First Observation of a Three-Resonance Structure in 𝑒+⁢𝑒−→Nonopen Charm Hadrons

We report the measurement of the inclusive cross sections for 𝑒+⁢𝑒−→nOCH (where nOCH denotes non-open charm hadrons) with improved precision at center-of-mass (c.m.) energies from 3.645 to 3.871 GeV. We observe three resonances: ℛ⁡(3760), ℛ⁡(3780), and ℛ⁡(3810) with significances of 8.1⁢𝜎, 13.7⁢𝜎, and 8.8⁢𝜎, respectively. The ℛ⁡(3810) state is observed for the first time, while the ℛ⁡(3760) and ℛ⁡(3780) states are observed for the first time in the nOCH cross sections. Two sets of resonance parameters describe the energy-dependent line shape of the cross sections well. In set I [set II], the ℛ⁡(3810) state has mass (3805.7±1.1±2.7) [(3805.7±1.1±2.7)]  MeV/𝑐2, total width (11.6±2.9±1.9) [(11.5±2.8±1.9)]  MeV, and an electronic width multiplied by the nOCH decay branching fraction of (10.9±3.8±2.5) [(11.0±3.4±2.5)]  eV. In addition, we measure the branching fractions ℬ⁡[ℛ⁡(3760)→nOCH]=(25.2±16.1±30.4)%[(6.4±4.8±7.7)%] and ℬ⁡[ℛ⁡(3780)→nOCH]=(12.3±6.6±8.3)%[(10.4±4.8±7.0)%] for the first time. The ℛ⁡(3760) state can be interpreted as an open-charm (OC) molecular state, but containing a simple four-quark state component. The ℛ⁡(3810) state can be interpreted as a hadrocharmonium state.