Portions of this entry contributed by Dana Romero
Positronium is a temporary state in the decay of the positron. A positron in free space will quickly interact with an electron via the Coulomb force, and the positron and electron will for a short time form a bound state. This state is considered to be an atom in the sense that it is a hydrogen atom with the proton replaced by a positron.
However, quantum field interactions between the positron and electron cause positronium to be unstable, and the particles annihilate in seconds to form photons. Most commonly, two photons are produced. Surprisingly, a positron and electron can sometimes produce three photons upon decay, a phenomenon which is easiest to understand using Mandelstam coordinates.
Atom, Electron, Hydrogen Atom, Positron
References
Arya, A. P. Elementary Modern Physics. Reading, MA: Addison-Wesley, 1974.
Basdevant, J.-L. and Dalibard, J. "The Spectrum of Positronium." Ch. 17 in The Quantum Mechanics Solver: How to Apply Quantum Theory to Modern Physics. Berlin: Springer-Verlag, pp. 127-134, 2000.
Griffiths, D. J. "Positronium." §5.6 in Introduction to Elementary Particles. New York: Wiley, pp. 159-164, 1987.
