Cross Section Measurements of $\bar{n}p\to K^{+}{K}^{-}\ddot{I}{€}^{+}(\ddot{I}{€}^0)$ via Antineutrons Produced by $J/\ddot{I}ˆ\to p\ddot{I}{€}^{-}\bar{n}$ Decays

Abstract

Based on a novel method for producing antineutrons via $J/\ddot{I}ˆ$ decays, we report a study of $\bar{n}p$ inelastic scattering into final states containing kaons. The analysis uses $(10087\pm 44)\times 10^6$ $J/\ddot{I}ˆ$ events collected at the BESIII detector operating at the BEPCII storage ring. Antineutrons are produced via $J/\ddot{I}ˆ\to p\ddot{I}{€}^{-}\bar{n}$ decays and tagged by the detected protons and pions, resulting in antineutron momenta ranging from 0 to 1174~MeV/$c$, while target protons are provided by the hydrogen in the beam-pipe material. The flux-averaged cross sections of the reactions $\bar{n}p\to K^{+}{K}^{-}\ddot{I}{€}^{+}$ and $\bar{n}p\to K^{+}{K}^{-}\ddot{I}{€}^{+}\ddot{I}{€}^0$ over this antineutron momentum spectrum are measured to be $0.53_{-0.12}^{+0.15}\pm 0.08$~mb and $1.09_{-0.30}^{+0.36}\pm 0.31$~mb, respectively, where the first uncertainties are statistical and the second are systematic. Due to limited statistics, the intermediate states in these processes are not quantitatively investigated. The observation of clean antineutron-proton scattering candidates indicates the potential of this approach for future investigations of antineutron-proton interactions.