Parapositronium

Parapositronium (p-Ps) is the singlet bound state of an electron and a positron, collectively known as positronium. In its ground state, the system has orbital angular momentum L = 0 and total spin S = 0, giving total angular momentum J = 0 and quantum numbers J^PC = 0^-+. This state is distinct from orthopositronium (o-Ps), which has S = 1 and J^PC = 1^--.

Parapositronium decays predominantly into two gamma photons. In the rest frame of the bound state, the two photons are emitted in nearly opposite directions with energies of 511 keV each, corresponding to the rest-mass energy of the electron and positron. The decay is a quick electromagnetic process, with a lifetime in vacuum of about 125 picoseconds.

Formation and observation: Positronium forms when a positron implanted into matter slows down and captures an electron. The two spin configurations—para- and ortho-positronium—have different lifetimes and decay modes. Para­positronium primarily annihilates into two photons, while orthopositronium decays mainly into three photons and has a longer vacuum lifetime, about 142 nanoseconds. In materials, interactions such as pick-off can alter observed lifetimes and decay rates.

Significance and applications: Parapositronium provides a precise test of bound-state quantum electrodynamics and enables high-resolution studies of annihilation processes. In materials science, positronium lifetimes, including the relative formation of p-Ps and o-Ps, are used in positron annihilation lifetime spectroscopy to probe microstructure and free-volume holes in solids.