Dynamics of Baryons from String Theory and Vector Dominance

TL;DR

This paper models baryons using a holographic string theory approach, showing they are small solitons that interact with an infinite tower of mesons, leading to vector-dominated electromagnetic form factors and realistic nucleon properties.

Contribution

It introduces an effective field for baryons in holographic QCD, deriving their interactions with mesons and electromagnetic fields from string theory principles.

Findings

Baryons are small solitons with a unit Pontryagin number.

The effective action reproduces nucleon-meson interactions consistent with large N_c expansion.

Electromagnetic interactions are mediated by an infinite tower of vector mesons, explaining vector dominance.

Abstract

We consider a holographic model of QCD from string theory, a la Sakai and Sugimoto, and study baryons. In this model, mesons are collectively realized as a five-dimensional \$U(N_F)=U(1)\times SU(N_F)$ Yang-Mills field and baryons are classically identified as $SU(N_F)$ solitons with a unit Pontryagin number and $N_c$ electric charges. The soliton is shown to be very small in the large 't Hooft coupling limit, allowing us to introduce an effective field ${\cal B}$. Its coupling to the mesons are dictated by the soliton structure, and consists of a direct magnetic coupling to the $SU(N_F)$ field strength as well as a minimal coupling to the $U(N_F)$ gauge field. Upon the dimensional reduction, this effective action reproduces all interaction terms between nucleons and an infinite tower of mesons in a manner consistent with the large $N_c$ expansion. We further find that all electromagnetic interactions, as inferred from the same effective action via a holographic prescription, are mediated by an infinite tower of vector mesons, rendering the baryon electromagnetic form factors completely vector-dominated as well. We estimate nucleon-meson couplings and also the anomalous magnetic moments, which compare well with nature.