Meson spectra of asymptotically free gauge theories from holography

Abstract

Using holography, we study the low-lying mesonic spectrum of a range of asymptotically free gauge theories. First, we revisit a simple top-down holographic model of QCD-like dynamics with predictions in the M?-M? plane. The meson masses in this model are in very good agreement with lattice gauge theory calculations in the quenched approximation. We show that the key ingredient for the meson mass predictions is the running of the anomalous dimension of the quark condensate ?. This provides an explanation for the agreement of holographic and quenched lattice gauge theory calculations. We then study the “dynamic AdS/QCD model,” in which the gauge theory dynamics is included by a choice for the running of ?. We use the naive two-loop perturbative running of the gauge coupling extrapolated to the nonperturbative regime to estimate the running of ? across a number of theories. We consider models with quarks in the fundamental, adjoint, two-index symmetric and two-index antisymmetric representations. We display predictions for M?, M?, M? and the lightest glueball mass. Many of these theories, where the contribution to the running of ? is dominated by the gluons, give very similar spectra, which also match with lattice expectations for QCD. On the other hand, a significant difference between spectra in different holographic models is seen for theories where the quark content changes the gradient of the running of ? around the scale at which chiral symmetry breaking is triggered at ??1. For these walking theories, we see an enhancement of the ? mass and a suppression of the ? mass. Both phenomena are characteristic for walking behavior in the physical meson masses.

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