Observation of a Fine Structure in $e^+e^- \to$ hadrons Production at the Nucleon-antinucleon Threshold

A study of hadron production at the nucleon-antinucleon threshold has been performed with the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider. A very fast rise with about 1 MeV width has been observed in the $e^+e^- \to p\bar p$ cross section. A sharp drop in the $e^+e^- \to 3(\pi^+\pi^-)$ cross section has been confirmed and found to have a less than 2 MeV width, in agreement with the observed fast rise of the $e^+e^- \to p\bar p$ cross section. For the first time a similar sharp drop is demonstrated in the $e^+e^- \to K^+K^-\pi^+\pi^-$ cross section. The behavior of the $e^+e^- \to 3(\pi^+\pi^-),~ K^+K^-\pi^+\pi^-$ cross sections cannot be explained by an interference of any resonance amplitude with continuum, therefore this phenomenon cannot be due to a narrow near-threshold resonance. No such structure has been observed in the $e^+e^- \to 2(\pi^+\pi^-)$ cross section.