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

TL;DR

This paper investigates the spectra of mesons and glueballs in a holographic QCD model, revealing discrete, gapped spectra with Miransky scaling near the conformal transition, and finds the S-parameter to be of order one in the walking regime.

Contribution

It provides a detailed analysis of the zero temperature spectra in V-QCD models, including the behavior near the conformal transition and the absence of a dilaton in the spectrum.

Findings

Spectra are discrete and gapped below the critical x_c.

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

The S-parameter remains of order one in the walking regime.

Abstract

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 = N_f/N_c 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 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. The ratio of masses all 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 the walking regime.