Holographic Models for QCD in the Veneziano Limit

Abstract

We construct a class of bottom-up holographic models with physics comparable to the one expected from QCD in the Veneziano limit of large N_f and N_c with fixed x = N_f/N_c. The models capture the holographic dynamics of the dilaton (dual to the YM coupling) and a tachyon (dual to the chiral condensate), and are parametrized by the real parameter x, which can take values within the range 0 <= x < 11/2. We analyze the saddle point solutions, and draw the phase diagram at zero temperature and density. The backreaction of flavor on the glue is fully included. We find the conformal window for x >= x_c, and the QCD-like phase with chiral symmetry breaking at x < x_c, where the critical value x_c lies close to four. We also find Miransky scaling as x -> x_c as well as Efimov-like saddle points. By calculating the holographic beta-functions, we demonstrate the "walking" behavior of the coupling in the region near and below x_c.