O(Nc) and USp(Nc) QCD from string theory

We propose a holographic dual of large Nc quantum chromodynamics (QCD) with the gauge groups O(Nc) and USp(Nc) and Nf flavors of massless quarks. This is constructed by adding O6-planes to an intersecting D4-D8 system in type IIA superstring theory. The holographic dual description is formulated in Witten's D4-brane background with D8-branes and O6-planes embedded in it as probes. The D4-brane background gives rise to a smooth interpolation of D8-D8-bar pairs and an O6-O6-bar pair. We show that the resultant brane configuration explains geometrically the flavor symmetry breaking patterns in O(Nc) and USp(Nc) QCD, which are caused by quark bilinear condensates. We next discuss that baryons can be realized as D4-D4-bar pairs wrapped on S4, which intersect with the O6-plane. By analyzing the tachyons on it, we reproduce the stability conditions of the baryons that are expected from the gauge theory viewpoint. The stable baryon configurations are classified systematically using K-theory. We also give a similar analysis of the flux tubes and again reproduce the results that are consistent with QCD. (author)