We report the measurement of the inclusive cross sections for $e^{+}{e}^{-}\to \mathrm{nOCH}$ (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: $\mathcal{R}(3760)$, $\mathcal{R}(3780)$, and $\mathcal{R}(3810)$ with significances of $8.1\sigma$, $13.7\sigma$, and $8.8\sigma$, respectively. The $\mathcal{R}(3810)$ state is observed for the first time, while the $\mathcal{R}(3760)$ and $\mathcal{R}(3780)$ 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 $\mathcal{R}(3810)$ state has mass ($3805.7\pm 1.1\pm 2.7$) $[(3805.7\pm 1.1\pm 2.7)]\mathrm{MeV}/c^2$, total width ($11.6\pm 2.9\pm 1.9$) $[(11.5\pm 2.8\pm 1.9)]\mathrm{MeV}$, and an electronic width multiplied by the nOCH decay branching fraction of ($10.9\pm 3.8\pm 2.5$) $[(11.0\pm 3.4\pm 2.5)]\mathrm{eV}$. In addition, we measure the branching fractions $\mathcal{B}[\mathcal{R}(3760)\to \mathrm{nOCH}]=(25.2\pm 16.1\pm 30.4)\%[(6.4\pm 4.8\pm 7.7)\%]$ and $\mathcal{B}[\mathcal{R}(3780)\to \mathrm{nOCH}]=(12.3\pm 6.6\pm 8.3)\%[(10.4\pm 4.8\pm 7.0)\%]$ for the first time. The $\mathcal{R}(3760)$ state can be interpreted as an open-charm (OC) molecular state, but containing a simple four-quark state component. The $\mathcal{R}(3810)$ state can be interpreted as a hadrocharmonium state.

• Received 19 July 2023
• Revised 10 March 2024
• Accepted 29 March 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.191902

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Published by the American Physical Society