Measurement of the cross section for $e^{+}{e}^{-}\to \hat{I}›\bar{\hat{I}}›$ and evidence of the decay $\ddot{I}ˆ(3770)\to \hat{I}›\bar{\hat{I}}›$

Abstract

The Born cross section of the process $e^{+}{e}^{-}\to \hat{I}›\bar{\hat{I}}›$ is measured at 33 center-of-mass energies between 3.51 and 4.60 GeV using data corresponding to the total integrated luminosity of 20.0 fb${}^{-1}$ collected with the BESIII detector at the BEPCII collider. Describing the energy dependence of the cross section requires a contribution from the $\ddot{I}ˆ(3770)\to \hat{I}›\bar{\hat{I}}›$ decay, which is fitted with a significance of 4.6-4.9$\ddot{I}ƒ$ including the systematic uncertainty. The lower bound on its branching fraction is $2.4\times 10^{-6}$ at the 90% confidence level (C.L.), at least an order of magnitude larger than expected from predictions using a scaling based on observed electronic widths. This result indicates the importance of effects from vector charmonium(-like) states when interpreting data in terms of e.g., electromagnetic structure observables. The data do not allow for definite conclusions on the interplay with other vector charmonium(-like) states, and we set 90% C.L.upper limits for the products of their electronic widths and the branching fractions.

8 pages, 4 Figures, 1 Supplemental Material with 2 pages, consistent with paper published in Phys. Rev. D 104, L091104 (2021)