anti-de Sitter (AdS) space. In particular, the hard wall model was used in [@BoschiFilho:2002ta; @BoschiFilho:2002vd] as a tool for calculating masses of glueballs. Another AdS/QCD model, the soft wall, where the square of the mass grow linearly with the radial excitation number was introduced in ref. [@Karch:2006pv]. In this case, the background involves AdS space and a scalar field that acts effectively as a smooth infrared cutoff. A recent recent review of AdS/QCD with a wide list of references can be found in [@Brodsky:2014yha].

AdS/QCD models provide also a tool for calculating another important property of hadrons: the decay constant. The decay of mesons is represented as a transition from the initial state to the hadronic vacuum. For a meson at radial excitation level $n$ with mass $m_n$ the decay constant $  f_n $ is defined as: $  \langle 0 \vert \, J_\mu (0)  \,  \vert n \rangle = \epsilon_\mu f_n m_n $, where $ J_\mu $ is the gauge current and $ \epsilon_\mu $ the polarization. Expressing the two point correlator of gauge currents as a sum over transition matrix elements, one finds a holographic expression for decay constants [@Karch:2006pv; @Grigoryan:2007my].

A problem of the original formulations of the 