Magnetic and axial vector form factors as probes of orbital angular momentum in the proton

We have recently examined the static properties of the baryon octet (magnetic moments and axial vector coupling constants) in a generalized quark model in which the angular momentum of a polarized nucleon is partly spin <S_z> and partly orbital <L_z>. The orbital momentum was represented by the rotation of a flux tube connecting the three constituent quarks. The best fit is obtained with <S_z>=0.08+/-0.15, <L_z>=0.42+/-0.14. We now consider the consequences of this idea for the q² dependence of the magnetic and axial vector form factors. It is found that the isovector magnetic form factor G^isovec_M(q²) differs in shape from the axial form factor F_A(q²) by an amount that depends on the spatial distribution of orbital angular momentum. The model of a rigidly rotating flux tube leads to a relation between the magnetic, axial vector and matter radii, <r²>_mag=f_spin<r²>_axial+52f_orb<r²>_matt, where f_orb/f_spin=13<L_z>/G_A, f_spin+f_orb=1. The shape of F_A(q²) is found to be close to a dipole with M_A=0.92+/-0.06 GeV.