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

A bstractZero 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/Nc fixed(V-QCD).The back reaction 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\to {x_{{{c^{-}}}}} $ due to Miransky scaling. All the ratios of masses asymptote to non-zero constants as $ x\to {x_{{{c^{-}}}}} $ and therefore there is no “dilaton” in the spectrum. The S-parameter is computed and found to be of $ \mathcal{O}(1) $ in units of NfNc 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.