NUCLEON POLARIZABILITIES FOR VIRTUAL PHOTONS

We generalize the sum rules for the nucleon electric plus magnetic pol arizability Sigma = alpha+beta and for the nucleon spin-polarizability gamma, to virtual photons with Q(2) > 0. The dominant low energy ampl itudes are represented in our calculation by one-pion-loop graphs of r elativistic baryon chiral perturbation theory and the Delta(1232)-reso nance excitation, For the proton we find good agreement of the calcula ted Sigma(p) (Q(2)) with empirical values obtained from integrating up electroproduction data for Q(2) < 0.4 GeV2. The proton spin-polarizab ility gamma(p)(Q(2)) switches sign around Q(2) = 0.4 GeV2 and it joins smoothly the ''partonic'' curve, extracted from polarized deep-inelas tic scattering, around Q(2) = 0.7 GeV2. For the neutron our prediction s of Sigma(n)(Q(2)) and gamma(n)(Q(2)) agree reasonably well at (Q(2) = 0 with existing determinations, Upcoming (polarized) electroproducti on experiments will be able to test the generalized polarizability sum rules investigated here. (C) 1998 Elsevier Science B.V.