Resonance contributions to nucleon spin structure in Holographic QCD

TL;DR

This paper uses holographic QCD to analyze resonance contributions to nucleon spin structure functions, finding that the $232$ resonance dominates low-momentum transfer spin polarizabilities, aligning qualitatively with experimental data.

Contribution

It applies the Witten-Sakai-Sugimoto holographic model to compute resonance contributions to nucleon spin structure functions, including helicity amplitudes and polarizabilities, with results consistent with recent Jefferson Lab experiments.

Findings

The $232$ resonance dominates the forward spin polarizabilities at low momentum transfer.

Resonance contributions tend to zero as momentum transfer increases.

Results qualitatively agree with experimental data on helicity amplitudes.

Abstract

We study polarized inelastic electron-nucleon scattering at low momentum transfer in the Witten-Sakai-Sugimoto model of holographic QCD, focusing 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 $\Delta(1232)$ resonance gives the dominant contribution to the forward spin polarizabilities at low momentum transfer. The contribution is negative and tends to zero as the momentum transfer increases. As expected, the contribution of the $\Delta(1232)$ to the longitudinal-transverse polarizabilities is instead negligible. The latter, for both nucleons, turn out the be negative functions with zero asymptote. The holographic results, at least for the proton where enough data are available, are in qualitative agreement with the resonance contributions to the spin polarizabilities extracted from experimental data on the helicity amplitudes.