Precision measurement of the $e^{+}{e}^{-}\to \hat{I}{›}_c^{+}\bar{\hat{I}}{›}_c^{-}$ cross section near threshold

Abstract

The Born cross section of the $e^{+}{e}^{-}\to \hat{I}{›}_c^{+}\bar{\hat{I}}{›}_c^{-}$ process is measured with unprecedented precision using data collected with the BESIII detector at $\sqrt{s}=4574.5$, $45580.0$, $4590.0$ and $4599.5$ $\mathrm{MeV}$. The non-zero cross section near the $\hat{I}{›}_c^{+}\bar{\hat{I}}{›}_c^{-}$ production threshold is discerned. At center-of-mass energies $\sqrt{s}=4574.5$ and $4599.5$ $\mathrm{MeV}$, the higher statistics data enable us to measure the $\hat{I}{›}_c$ polar angle distributions. From these, the ratio between the $\hat{I}{›}_c$ electric and magnetic form factors ($|G_E/G_M|$) is measured for the first time. They are found to be $1.14\pm 0.14\pm 0.07$ and $1.23\pm 0.05\pm 0.03$ respectively, where the first uncertainties are statistical and the second are systematic.