A potential diquark in the proton

A non-relativistic, QCD-based, potential quark model for the proton and the neutron inevitably predicts a spin-0 diquark structure with an rms radius of the order of 0.35 fm or smaller. We prove this by solving the (S wave) hamiltonian by De Rujula et al. with variational methods. It is essential to include all quark interactions and to use realistic test wave-functions. The proton-delta mass difference, the magnetic moments and the nucleon charge radii can be reproduced only with wave functions that contain a mixture of quark-diquark and three-quark states. Approaches with just a quark-diquark component give incorrect magnetic moments, while those without diquarks lead to a too low proton-delta mass difference.