The discontinuities of conformal transitions and mass spectra of V-QCD

TL;DR

This paper investigates the mass spectra of mesons and glueballs in a holographic QCD model, revealing discontinuities at the conformal transition and the absence of a dilaton in the spectrum.

Contribution

It provides a detailed analysis of spectral behavior and correlation function discontinuities across the conformal transition in V-QCD models.

Findings

Spectra are discrete and gapped below the critical x_c.

Masses approach zero in the walking region due to Miransky scaling.

No dilaton appears in the spectrum at the conformal transition.

Abstract

Zero temperature spectra of mesons and glueballs are analyzed in a class of holographic bottom-up models for QCD in the Veneziano limit, N_c -> infinity, N_f -> infinity, with x = N_f/N_c fixed (V-QCD). The backreaction of flavor on color is fully included. It is found that spectra are discrete and gapped (modulo the pions) in the QCD regime, for x below the critical value x_c where the conformal transition takes place. The masses uniformly converge to zero in the walking region x -> x_c^- due to Miransky scaling. All the ratios of masses asymptote to non-zero constants as x -> x_c^- and therefore there is no "dilaton" in the spectrum. The S-parameter is computed and found to be of O(1) in units of N_f N_c in the walking regime, while it is always an increasing function of x. This indicates the presence of a subtle discontinuity of correlation functions across the conformal transition at x = x_c.