Role of the \Delta resonance in the population of a four-nucleon state in the 56Fe \rightarrow 54Fe reaction at relativistic energies

The 54Fe nucleus was populated from a 56Fe beam impinging on a Be target with an energy of E/A=500 M eV. The internal decay via \gamma-ray emission of the 10+ metastable state was observed. As the structure of this isomeric state has to involve at least four unpaired nucleons, it cannot be populated in a simple two-neutron removal reaction from the 56Fe ground state. The isomeric state was produced in the low-momentum/energy tail of the parallel momentum/energy distribution of 54Fe, suggesting that it was populated via the decay of the \Delta ^0 resonance into a proton. This process allows the population of four-nucleon states, such as the observed isomer. Therefore, it is concluded that the observation of this 10+ metastable state in 54Fe is a consequence of the quark structure of the nucleons.