Cross-section measurements of 𝑛bar⁢𝑝 →𝐾+⁢𝐾−⁢𝜋+⁡(𝜋0) via antineutrons produced by 𝐽/𝜓 →𝑝⁢𝜋−⁢𝑛bar decays

Based on a novel method for producing antineutrons via $J/\psi$ decays, we report a study of inelastic $\bar{n}p$ scattering into final states containing kaons. The analysis uses $(10087 \pm 44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector operating at the BEPCII storage ring. Antineutrons are produced via $J/\psi \to p\pi^-\bar{n}$ decays and tagged by the detected protons and pions, resulting in antineutron momenta ranging from $0$ to $1174~\mathrm{MeV}/c$, while the target protons are provided by the hydrogen in the beam-pipe material. The flux-averaged cross sections of the reactions $\bar{n}p \to K^+K^-\pi^+$ and $\bar{n}p \to K^+K^-\pi^+\pi^0$ over this antineutron momentum spectrum are measured to be $$\sigma(\bar{n}p \to K^+K^-\pi^+) = \left(0.53^{+0.15}_{-0.12}\,(\mathrm{stat}) \pm 0.08\,(\mathrm{syst})\right)\,\mathrm{mb},$$ and $$\sigma(\bar{n}p \to K^+K^-\pi^+\pi^0) = \left(1.09^{+0.36}_{-0.30}\,(\mathrm{stat}) \pm 0.31\,(\mathrm{syst})\right)\,\mathrm{mb},$$ respectively, where the first uncertainties are statistical and the second are systematic. Owing to the limited statistics, the contributions of intermediate states remain speculative. The observation of clean antineutron-proton scattering candidates demonstrates the potential of this approach for future investigations of antineutron-proton interactions.