Dynamics of Baryons from String Theory and Vector Dominance

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.