Pion Form Factor in Improved Holographic QCD Backgrounds

TL;DR

This paper improves holographic QCD models to better predict the pion electromagnetic form factor, analyzing boundary conditions and interpolating backgrounds, but still finds discrepancies at high momentum transfer.

Contribution

It introduces an interpolated background model combining hard-wall and soft-wall features and examines the impact of boundary conditions on form factor calculations.

Findings

Using chiral symmetry-breaking fields minimally affects results.

Interpolated models better reproduce phenomenological features.

Form factor predictions remain shallower than experimental data at high Q^2.

Abstract

We extend a recent numerical calculation of the pion electromagnetic form factor F_\pi (Q^2) in holographic QCD to study two important issues regarding the behavior of fields in the bulk. First, we show that using a chiral symmetry-breaking field formally satisfying the boundary conditions of the ``soft-wall'' model changes numerical results very little from the earlier calculation that ignores these constraints. Second, we use a background field that interpolates between ``hard-wall'' and ``soft-wall'' models to obtain an improved model that reproduces the desirable phenomenological features of both. In all cases, F_\pi$ for large Q^2 is shallower than data, an effect that can be cured by relaxing the fit to one of the static observables, particularly the decay constant f_\pi.