Lensing function relation in Hadrons

Nontrivial model-induced relations between parton distribution functions can be helpful to interpret, at least qualitatively, the mechanisms at work in scattering processes off the nucleon. However great care is needed in the transition between qualitative and quantitative interpretation of the results. One example is given by the relations between transverse distortions in the distribution of quarks in impact-parameter space and analogous distortions in transverse-momentum space due to T-odd effects, that have been found in the context of various model calculations. We discuss the origin of such relations pointing out the very specific conditions under which they are realised. These conditions are typically verified only in relatively simple models that describe hadrons as two-body bound systems and involve a helicity-conserving coupling between the gauge boson and the spectator system.