Observation of a fine structure in e⁺e⁻ → 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. The very fast rise with about 1 MeV width has been observed in the e⁺e⁻→pp¯ cross section. A sharp drop in the e⁺e⁻→3(π⁺π⁻)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⁻→pp¯ cross section. For the first time a similar sharp drop is demonstrated in the e⁺e⁻→K⁺K⁻π⁺π⁻ cross section. The behavior of the e⁺e⁻→3(π⁺π⁻),K⁺K⁻π⁺π⁻ 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⁻→2(π⁺π⁻)cross section.