Two-photon decay of para-positronium within a composite approach

The decay of the para-positronium into two photons is studied in the framework of a composite Quantum Field Theoretical approach. This amounts to the evaluation of the electron-positron dressing together with the Weinberg compositeness condition for the positronium and the triangle-shaped diagram with virtual electrons circulating in it, leading to the two-photon final state. An important role is played by the positronium-electron-positron vertex, which is linked to the wave function of the para-positronium. We show how possible choices for the vertex function affect the γγ decay rate. Outlooks to other decay channels and to other positronia are presented.