We study polarized inelastic electron-nucleon scattering at low momentum
transfer, in the Witten-Sakai-Sugimoto model of holographic QCD. We focus in
particular on resonance production contributions to the nucleon spin structure
functions. Our analysis includes both spin 3/2 and spin 1/2 low-lying
nucleon resonances with positive and negative parity. We determine, in turn,
the helicity amplitudes for nucleon-resonance transitions and the resonance
contributions to the neutron and proton generalized spin polarizabilities.
Extrapolating the model parameters to realistic QCD data, our analysis,
triggered by recent experimental results from Jefferson Lab, agrees with the
observation that the Δ(1232) resonance gives the dominant contribution
to the forward spin polarizabilities at low momentum transfer. The contribution
is negative and increases towards zero as the momentum transfer increases. As
expected, the contribution of the Δ(1232) to the longitudinal-transverse
polarizabilities is instead negligible. Our analysis shows that different spin
1/2 resonances give different contributions, in sign and magnitude, to the
generalized longitudinal-transverse spin polarizabilities. In the proton case
they globally give rise to a positive function which decreases towards zero as
the momentum transfer increases. In the neutron case, the net effect produces a
negative increasing function. These features are in qualitative agreement with
experimental data.Comment: 43 pages, 17 figures, 2 table