A sphaleron is a rare process of the electroweak interaction of the Standard Model of particle physics. It converts three baryons to three antileptons, and related processes. This violates conservation of baryon number and lepton number, but the difference B-L is conserved.

A sphaleron is a type of instanton, so it is non-perturbative. This means that under normal conditions sphalerons are unobservably rare. However, they would have been more common at the higher temperatures of the early universe. In some theories of baryogenesis an imbalance of the number of leptons and antileptons is formed first by leptogenesis and sphalerons then convert this to an imbalance in the numbers of baryons and antibaryons.

See also

Instanton