On the spectra of scalar mesons from HQCD models

We determine the holographic spectra of scalar mesons from the fluctuations of the embedding of flavor D-brane probes in HQCD models. The models we consider include a generalization of the Sakai Sugimoto model at zero temperature and at the ''high-temperature intermediate phase", where the system is in a deconfining phase while admitting chiral symmetry breaking and a non-critical 6d model at zero temperature. All these models are based on backgrounds associated with near extremal N _c D4 branes and a set of N _f << N _c flavor probe branes that admit geometrical chiral symmetry breaking. We point out that the spectra of these models include a 0 ^- branch which does not show up in nature. At zero temperature we found that the masses of the mesons M _n depend on the ''constituent quark mass" parameter m ^c _q and on the excitation number n as M _n ² ∼ m ^c _q and M ² _n ∼ n ^1.7 for the ten dimensional case and as M _n ∼ m ^c _q and M _n ∼ n ^0.75 in the non-critical case. At the high temperature intermediate phase we detect a decrease of the masses of low spin mesons as a function of the temperature similar to holographic vector mesons and to lattice calculations.