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Orthopositronium is a short-lived bound state of an electron and its antiparticle, the positron, with both particles in the same quantum state. It is the lightest and longest-lived of the positronium states. Orthopositronium is formed when a positron is captured by an electron in a singlet state, resulting in the formation of a positronium atom in the ortho state. This state has a total angular momentum of 1 and a total spin of 1, making it a triplet state.

The lifetime of orthopositronium is approximately 142 nanoseconds, during which it can undergo several processes. The primary decay mode is the annihilation of the electron and positron, which can occur through two main channels: two gamma rays or three gamma rays. The two-gamma decay is the dominant process, accounting for about 90% of the decays. The three-gamma decay occurs when the orthopositronium is in an excited state and decays to the ground state, emitting an additional gamma ray.

Orthopositronium is of interest in various fields, including particle physics, astrophysics, and material science. In particle physics, the study of orthopositronium provides insights into the behavior of electron-positron pairs and the fundamental forces governing their interactions. In astrophysics, orthopositronium is thought to play a role in the formation and evolution of certain types of stars and nebulae. In material science, orthopositronium is used as a probe to study the electronic structure and properties of materials, particularly in the study of semiconductors and insulators.

The formation and study of orthopositronium require specialized experimental techniques, such as the use of positron beams and high-resolution spectrometers. Researchers continue to investigate the properties and behavior of orthopositronium to deepen our understanding of the fundamental particles and forces that govern the universe.