Are there diquarks in the nucleon?

This work is devoted to the study of diquark correlations inside the nucleon. We analyze some matrix elements which encode information about the nonperturbative forces, in different color antitriplet diquark channels. We suggest a lattice calculation to check the quark-diquark picture and clarify the role of instanton-mediated interactions. We study in detail the physical properties of the 0(+) diquark, using the random instanton liquid model. We find that instanton forces are sufficiently strong to form a diquark bound state, with a mass of similar to 500 MeV, which is compatible with earlier estimates. We also compute its electromagnetic form factor and find that the diquark is a broad object, with a size comparable with that of the proton.