A simple model to explain narrow nucleon resonances below the πN threshold

Abstract

Several experiments suggest the existence of narrow excited states in the nucleon below the πN threshold. Taking all information together a series of states emerges at masses of 940, 966, 985, 1003, m_4, 1044, m_6, 1094 MeV above the nucleon ground state at 940 MeV with m_4 and m_6 missing. A model based on the excitation of collective states of the quark condensate is proposed explaining these states as multiple production of a ``genuine'' Goldstone Boson with a mass of \approx 20 MeV. The model also accounts in a natural way for the non-observation of m_4 and m_6. It suggests an explanation of the mass gap problem of the hadronic spectrum and a confinement mechanism.