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, the Weinberg compositeness condition for the positronium, and the triangle-shaped diagram with virtual electrons circulating in it, leading to the final two-photon 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 other positronia are presented.