Neutron electric dipole moment from gauge/string duality

TL;DR

This paper calculates the neutron electric dipole moment using a holographic QCD model, revealing vector meson dominance and providing an estimate consistent with physical expectations.

Contribution

It introduces a holographic approach to compute nucleon electric dipole moments, incorporating vector meson dominance and detailed mode contributions.

Findings

Neutron EDM estimated at 1.8 x 10^{-16} θ e·cm.

Electric dipole moment of proton is opposite to neutron.

Vector meson dominance observed in form factor.

Abstract

We compute the electric dipole moment of nucleons in the large $N_c$ QCD model by Witten, Sakai and Sugimoto with $N_f=2$ degenerate massive flavors. Baryons in the model are instantonic solitons of an effective five-dimensional action describing the whole tower of mesonic fields. We find that the dipole electromagnetic form factor of the nucleons, induced by a finite topological $\theta$ angle, exhibits complete vector meson dominance. We are able to evaluate the contribution of each vector meson to the final result - a small number of modes are relevant to obtain an accurate estimate. Extrapolating the model parameters to real QCD data, the neutron electric dipole moment is evaluated to be $d_n = 1.8 \cdot 10^{-16}\, \theta\;e\cdot \mathrm{cm}$. The electric dipole moment of the proton is exactly the opposite.