Δ→Nγ* Coulomb quadrupole amplitude in PQCD -: art. no. 014006

We present a leading-order PQCD factorization formula for the helicity-flip Delta-->Ngamma(*) matrix element G(0) (Coulomb quadrupole amplitude C2), taking into account the transverse momenta of the quarks and the contribution from the gluons. In the large Q(2) limit, its scaling behavior acquires a double-logarithmic correction log(2)(Q(2)/Lambda(2)) compared with the standard scaling analysis, due to the contribution from the orbital motion of the small-x partons. Based on this and on the latest JLab experimental results of the C2-M1 ratio R-SM at Q(2)=3-4 GeV2, we make a phenomenological prediction for the latter at higher values of Q(2).