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.