Using $e^+e^-$ collision data corresponding to an integrated luminosity of 44 fb$^{ -1}$ collected with the BESIII detector at the BEPCII collider, we report the first measurement of Born cross sections and effective form factors for $e^+e^-\to\Sigma^-\bar\Sigma^+$ at centre-of-mass energies between 3.51 and 4.95 GeV. With a fit to the $\sqrt{ s}$-dependent dressed cross sections, the decay $\psi(3770)\to\Sigma^-\bar\Sigma^+$ is observed for the first time with a significance of 5.5$\sigma$, including systematic uncertainties. Upper limits at the 90\% confidence level on the product of the branching fraction and the electronic partial width are provided for other possible charmonium(-like) states. The ratios of Born cross sections for the $\Sigma$ isospin-triplet states are determined and can be used to test the vector-meson-dominance model.
Categories: hep-ex