LCSR application to radiative tau decay

The non-perturbative QCD effects involved in radiative tau decay (τ̄ → π̄ ν<inf>τ</inf>γ) are encoded in two form factors (FFs); vector (F<inf>A</inf>) and axial-vector (F<inf>A</inf>). These FFs are the same as those involved in radiative pion decay with the crucial difference that the square of the momentum transferred, C, between the π - γ system is positive. This makes them timelike and can result in the production of real hadronic resonances. We calculate the analytical form for these FFs using the method of light-cone sum rules (LCSR) and present the decay width and the invariant mass spectrum of the π - γ system. The structure-dependent parameter γ (i.e., F<inf>A</inf>/F<inf>V</inf> at zero momentum transfer) is found to be in good agreement with the experimental determination.