Resonance states below pion-nucleon threshold and their consequences for nuclear systems

Regular sequences of narrow peaks have been observed in the missing mass spectra in the reactions pp --> p pi^+ X and pd --> ppX_1 below pion-production threshold. They are interpreted in the literature as manifestations of supernarrow light dibaryons, or nucleon resonances, or light pions forming resonance states with the nucleon in its ground state. We discuss how existence of such exotic states would affect properties of nuclear systems. We show that the neutron star structure is drastically changed in all three cases. We find that in the presence of dibaryons or nucleon resonances the maximal possible mass of a neutron star would be smaller than the observational limit. Presence of light pions does not contradict the observed neutron star masses. Light pions allow for the existence of extended nuclear objects of arbitrary size, bound by strong and electromagnetic forces.