Level Order of Quark Systems: The Puzzle of the Roper Resonance, and Related Questions

The problem of ordering of radial <span class="it">versus</span> orbital excitations is reviewed. It is shown that the current quark models cannot explain the location of the Roper resonance which is slightly lower than the lowest negative-parity excitations. We also study some related spectral problems, such as the dependence of the energies on the quark masses, and the possibility of bound states in simple chromelectric models. Abstract Published by the Jagiellonian University 2026 authors