A mass analysis of the whole particle spectrum, combined with the hypothesis that hadrons are solid-phase and shell-structured, indicates that the proton and the neutron are made of three shells. Mesons states corresponding to shell 1 (pion) and 2 (kaon) are available, but no shells 1 and 2 baryons have been seen. This may be due to the fact that mesons and baryons are built on different lattice systems, the baryonic one being less cohesive. One of the interpretations of the shape of the p-p elastic scattering dsigma/dt also relies on a three-layered proton.
12.40.Yx Hadron mass models and calculations
12.90.+b Miscellaneous theoretical ideas and models