Ioannis Iatrakis

An AdS/QCD model from tachyon condensation: II

A simple holographic model is presented and analyzed that describes chiral symmetry breaking and the physics of the meson sector in QCD. This is a bottom-up model that incorporates string theory ingredients like tachyon condensation which is expected to be the main manifestation of chiral symmetry breaking in the holographic context. As a model for glue the Kuperstein-Sonnenschein background is used. The structure of the flavor vacuum is analyzed in the quenched approximation. Chiral symmetry breaking is shown at zero temperature. Above the deconfinement transition chiral symmetry is restored. A complete holographic renormalization is performed and the chiral condensate is calculated for different quark masses both at zero and non-zero temperatures. The 0++, 0−+, 1++, 1−− meson trajectories are analyzed and their masses and decay constants are computed. The asymptotic trajectories are linear. The model has one phenomenological parameter beyond those of QCD that affects the 1++, 0−+ sectors. Fitting this parameter we obtain very good agreement with data. The model improves in several ways the popular hard-wall and soft wall bottom-up models.

AdS/QCD model from an effective action for open string tachyons

We construct a new, simple phenomenological model along the lines of AdS/QCD. The essential new ingredient is the brane-antibrane effective action including the open string tachyon proposed by Sen. Chiral symmetry breaking happens because of tachyon dynamics. We fit a large number of low-spin meson masses at the 10%-15% level. The only free parameters involved in the fits correspond to the overall QCD-scale and the quark masses. Several aspects of previous models are qualitatively improved.

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

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 ratios 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.

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

The Chern-Simons diffusion rate in improved holographic QCD

x\to {x_{{{c^{-}}}}}

Inverse magnetic catalysis from improved holographic QCD in the Veneziano limit

due to Miransky scaling. All the ratios of masses asymptote to non-zero constants as

The anomalous transport of axial charge: topological vs non-topological fluctuations

x\to {x_{{{c^{-}}}}}

Thermodynamics and CP-odd transport in holographic QCD with finite magnetic field

and therefore there is no “dilaton” in the spectrum. The S-parameter is computed and found to be of

Axial current generation by P-odd domains in QCD matter

\mathcal{O}(1)

Spectral functions in V-QCD with matter: masses, susceptibilities, diffusion and conductivity

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.