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

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 $e^{+}{e}^{-}\to {\gamma }_{\mathrm{ISR}}{\mu }^{+}{\mu }^{-}$ is 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 ${\mathrm{\Gamma }}_{ee}=(0.12_{-0.08}^{+0.13})\mathrm{eV}$, which is of the same order of magnitude as theoretical calculations.

Figure 1

The colored curves are energy-dependent cross sections of the process $e^{+}{e}^{-}\to \gamma J/\psi \to \gamma {\mu }^{+}{\mu }^{-}$ including (green and blue curves) and not including (red curve) the direct production of $e^{+}{e}^{-}\to {\chi }_{c1}$ (see text for more details). The gray curve denotes the signal strength in the hypothetical case of no interference. The location of the ${\chi }_{c1}$ mass is indicated by the vertical line. The black dots with error bars represent ${\sigma }_{\text{ISR BG}}^{\mathrm{MC}}+({\sigma }_{{\chi }_{c1}}+{\sigma }_{\mathrm{int}}{)}^{\mathrm{data}}$ at the ${\chi }_{c1}$ scan data samples. The numbers next to the four data points indicate the statistical significances associated with the ${\chi }_{c1}$ production.

Figure 2

One-dimensional projections of the two-dimensional fit to the $M_{{\mu }^{+}{\mu }^{-}}$ and $|\cos {\theta }_{\mu }|$ distributions from the ${\chi }_{c1}$ scan samples. The black dots with error bars are from data, the gray histograms are the irreducible background predicted by the corrected MC simulation. The red curve is the best fit result, the red dotted (blue dashed) curve is the signal (background) contribution. The region between $0.8<|\cos {\theta }_{\mu }|<0.86$ corresponds to the gap between the barrel and end cap modules of the EMC.

Figure 3

The 68.3% C.L. contour of ${\mathrm{\Gamma }}_{ee}$ and $\varphi$ on a distribution of log-likelihood ($-\ln L$) values. The distribution of $-\ln L$ in a larger parameter space region is shown in Supplemental Material [21].