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

Abstract

Zero temperature spectra of mesons and glueballs are analyzed in a class of holographic bottom-up models for QCD in the Veneziano limit, Nc → ∞, Nf → ∞, with x = Nf/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 xc where the conformal transition takes place. The masses uniformly converge to zero in the walking region x → xc- 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 fNc 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 = xc.