Towards a holographic dual of large-Nc QCD -: art. no. 041

We study N-f D6-brane probes in the supergravity background dual to N-c D4-branes compactifted on a circle with supersymmetry-breaking boundary conditions. In the limit in which the resulting Kaluza-Klein modes decouple, the gauge theory reduces to non-supersymmetric, four-dimensional QCD with N-c colours and N-f much less than N-c flavours. As expected, this decoupling is not fully realised within the supergravity/Born-Infeld approximation. For N-f = 1 and massless quarks, m(q) = 0, we exhibit spontaneous chiral symmetry breaking by a quark condensate, () 4 0, and find the associated massless 'pion' in the spectrum. The latter becomes massive for m(q) > 0, obeying the Gell-Mann-Oakes-Renner relation: M-pi(2) = -m(q) <(psi) over bar psi>/f(pi)(2). In the case N-f > 1 we provide a holographic version of the Vafa-Witten theorem, which states that the U(Nf) flavour symmetry cannot be spontaneously broken. Further, we find N-f(2) - 1 unexpectedly light pseudo-scalar mesons in the spectrum. We argue that these are not (pseudo-)Goldstone bosons and speculate on the string mechanism responsible for their lightness. We then study the theory at finite temperature and exhibit a phase transition associated with a discontinuity in <(psi) over bar psi> (T). D6/(D6) over bar pairs are also briefly discussed.