First Observation of the Direct Production of the ${\chi }_{c1}$ in $e^{+}{e}^{-}$ Annihilation

Abstract

We study the direct production of the $J^{PC}=1^{++}$ charmonium state ${\chi }_{c1}(1P)$ in electron-positron annihilation by carrying out an energy scan around the mass of the ${\chi }_{c1}(1P)$. The data were collected with the BESIII detector at the BEPCII collider. An interference pattern between the signal process $e^{+}{e}^{-}\to {\chi }_{c1}(1P)\to \gamma J/\psi \to \gamma {\mu }^{+}{\mu }^{-}$ and the background processes $e^{+}{e}^{-}\to {\gamma }_{\mathrm{ISR}}J/\psi \to {\gamma }_{\mathrm{ISR}}{\mu }^{+}{\mu }^{-}$ and $e^{+}{e}^{-}\to {\gamma }_{\mathrm{ISR}}{\mu }^{+}{\mu }^{-}$ are observed by combining all the data samples. The ${\chi }_{c1}(1P)$ signal is observed with a significance of 5.1$\sigma$. This is the first observation of a $C$-even state directly produced in $e^{+}{e}^{-}$ annihilation. The electronic width of the ${\chi }_{c1}(1P)$ resonance is determined to be ${\Gamma }_{ee}=(0.12_{-0.08}^{+0.13})\mathrm{eV}$, which is of the same order of magnitude as theoretical calculations.

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