Positronium is the electromagnetic bound state of an electron and a Positron. Their equal masses give reduced mass , half the hydrogenic Rydberg binding scale and approximately twice the electron-proton separation of hydrogen. Its spin singlet and triplet differ in hyperfine energy; annihilation also affects their physical lifetimes and splitting.
For the dimensionless-spin Hamiltonian , the two aligned-spin states remain at . The two states of total spin projection zero mix and have the displayed energies. The parameter compares off-diagonal Zeeman mixing in the singlet-triplet basis with half the zero-field splitting. A crossover is gradual, not a level-crossing phase transition.
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Positronium is an exotic atom-like system composed of an electron and its antiparticle, a positron. The electron carries a negative electric charge, while the positron has a positive electric charge. This unique pairing occurs because of the attraction between the two opposite charges, allowing them to bind together.