  We investigate an asymmetry in the angular distribution of hard elastic
proton-neutron scattering with respect to 90deg center of mass scattering
angle. We demonstrate that the magnitude of the angular asymmetry is related to
the helicity-isospin symmetry of the quark wave function of the nucleon. Our
estimate of the asymmetry within the quark-interchange model of hard scattering
demonstrates that the quark wave function of a nucleon based on the exact SU(6)
symmetry predicts an angular asymmetry opposite to that of experimental
observations. On the other hand the quark wave function based on the diquark
picture of the nucleon produces an asymmetry consistent with the data.
Comparison with the data allowed us to extract the relative sign and the
magnitude of the vector and scalar diquark components of the quark wave
function of the nucleon. These two quantities are essential in constraining QCD
models of a nucleon. Overall, our conclusion is that the angular asymmetry of a
hard elastic scattering of baryons provides a new venue in probing quark-gluon
structure of baryons and should be considered as an important observable in
constraining the theoretical models.
