${\boldsymbol N}\boldsymbol{\bar N}$ exotics and antiprotonic atoms

Abstract

The BES Collaboration measurements of J/ψ radiative decays into \( p{\bar p}\) indicate a strong enhancement at \( p {\bar p}\) threshold. In a related experiment a peak in the invariant π + π − η′ mass is observed. It is shown that both structures may be related to a broad \( N{\bar N}\) quasi-bound state in the \(^{11}S_{0} \) wave. The existence of this state finds additional support from the antiprotonic atom level widths. It is also explained by a traditional model of \( N \bar{N} \) interactions based on G-parity transformation. The level widths in H, D and He antiprotonic atoms and the radiochemical studies of \( {\bar p}\) capture in nuclei indicate the existence of another near-threshold quasi-bound state in a P wave.