Semiclassical projection of hedgehog models with quarks

Delta-->Ngamma in large-N(c) QCD

The decay Delta(+)-->pgamma is studied in the 1/N(c) expansion of QCD. The ratio of the helicity amplitudes is determined to be A(3/2)/A(1/2)=sqrt[3]+O(1/N(2)(c)). Equivalently, the ratio E2/M1 of the multipole amplitudes is predicted to be order 1/N(2)(c).

Moments of the neutron charge form factor and the N --> Delta quadrupole transition

Recent data allow a new parametrization of the neutron charge form factor GnE. A parameter-free quark-model relation between GnE and the N-->Delta quadrupole form factor G(N-->Delta)C2 is used to predict G(N-->Delta)C2 from GnE data. In particular, <r2n> is related to N-->Delta quadrupole moment Q(N-->Delta), while <r4n> connects to the N-->Delta quadrupole transition radius <r2(N-->Delta)>. From the latter we derive an experimental value for the charge radius of the light constituent quarks r(gamma(q)) = 0.8 fm. Finally, the C2/M1 ratio in pion electroproduction is predicted from the elastic neutron form factor data.