Chiral symmetry breaking from five dimensional spaces

Based on the AdS/CFT correspondence we study the breaking of the chiral symmetry in QCD using a simple five dimensional model. The model gives definite predictions for the spectrum of vector mesons, their decay constants and interactions as a function of one parameter related to the quark condensate. We calculate the coefficients $L_i$ of the low-energy QCD chiral lagrangian, as well as other physical quantities for the pions. All the predictions are shown to be in good agreement with the experimental data. We also show that they are robust under modifications of the 5D metric in the IR, and that some of them arise as a consequence of the higher-dimensional gauge symmetry. For example, at the tree-level, we find $M_\rho \simeq \sqrt{3} g_{\rho\pi\pi} F_\pi$, $F_\rho\simeq \sqrt{3} F_\pi$ and BR($a_1\to \pi\gamma)=0$.