A covariant diquark-quark model of the nucleon in the Salpeter approach
  We develop a rather simple, formally covariant quark-diquark model of the nucleon. The nucleon is treated as a bound state of a constituent quark and a diquark interacting via a quark exchange. We include both scalar and axial-vector diquarks. The underlying Bethe-Salpeter equation is transformed into a pair of coupled Salpeter equations. The electromagnetic form factors of the nucleon are calculated in the Mandelstam formalism. We obtain a very good description of all electromagnetic form factors for momentum transfers up to -3 (GeV/c)^2. 
Volker Keiner