orthopositronium

Orthopositronium is the triplet spin state of the bound system formed by an electron and a positron, known as positronium (Ps). In this state the two leptons have parallel spins (total spin S = 1) and zero orbital angular momentum (L = 0), corresponding to the 1^3S1 level. Its J^PC quantum numbers are 1^{--}. Because of its quantum numbers, orthopositronium decays predominantly by annihilating into three photons.

The annihilation energy equals twice the electron rest mass, 2 m_e c^2, about 1022 keV, shared among the three photons. The decay rate is governed by quantum electrodynamics (QED) and by the bound-state nature of the system. In vacuum, the mean lifetime of orthopositronium is about 142 nanoseconds, and its decay rate is approximately 7.0 × 10^6 s^-1. In matter, the observed lifetime can be shortened by processes such as pickoff annihilation and spin-exchange interactions with surrounding electrons, making the effective lifetime sensitive to the environment and temperature.

Orthopositronium contrasts with parapositronium, the singlet S = 0 ground state, which decays primarily into two photons with a much shorter lifetime of about 125 picoseconds. The different decay channels arise from conservation laws including charge conjugation and angular momentum.

Positronium states, including orthopositronium, provide a testing ground for bound-state QED and precision measurements of annihilation processes. Throughout the late 20th century, measurements of orthopositronium decay rates motivated refinements in theoretical calculations and experimental techniques, contributing to the broader validation of QED in bound systems.