Holographic Models for QCD in the Veneziano Limit

TL;DR

This paper develops bottom-up holographic models to study QCD-like dynamics in the Veneziano limit, capturing phase transitions, conformal window, and walking behavior of the coupling.

Contribution

It introduces a new class of holographic models that incorporate backreaction of flavor and analyze phase structure in the Veneziano limit of QCD.

Findings

Identifies the conformal window for x >= x_c.

Discovers chiral symmetry breaking phase for x < x_c.

Demonstrates walking behavior of the coupling near x_c.

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.