Gauge invariant formulation of 3$\gamma$ decay of particle-antiparticle   bound states
  We construct the gauge invariant three-photon decay amplitude of particle-antiparticle bound states modeled by the Dyson-Schwinger and Bethe-Salpeter equations. Application to the quark-antiquark ($q\bar{q}$) bound states is emphasized. An essential aspect of our formulation is that it applies to any underlying quantum field theoretic model of the $q\bar{q}$ system, and not just to models, like exact QCD, where the quark self-energy $\Sigma$ couples to the electromagnetic field solely via dressed quark propagators. In this way, applications to effective field theories and other QCD motivated models are envisioned. The three-photon decay amplitude is constructed by attaching currents to all possible places in the Feynman diagrams contributing to the dressed quark propagator. The gauge invariance of our construction is thus a direct consequence of respecting the underlying structure of the quantum field theory determining the dynamics. In the resultant expression for the three-photon decay amplitude, all the basic ingredients consisting of the bound state wave function, the final-state interaction $q\bar{q}$ $t$ matrix, the dressed quark propagator, and dressed quark currents, are determined by a universal Bethe-Salpeter kernel. 
B. Blankleider, A. N. Kvinikhidze, Z. K. Silagadze
hep-ph nucl-th