V-QCD: Spectra, the dilaton and the S-parameter

Zero temperature spectra of mesons and glueballs are analyzed in a class of holographic bottom-up models for QCD (named V-QCD), as a function of x=NfNc with the full back-reaction 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. The ratios of masses all asymptote to non-zero constants as x→xc and therefore there is no “dilaton” in the spectrum. The S-parameter is computed and found to be of O(1) in the walking regime.