Sphalerons and Baryogenesis
Since a sphaleron may convert baryons to anti-leptons and anti-baryons to leptons, if the density of sphalerons was at some stage high enough, they would wipe off any baryon asymmetry (i.e. any net excess of baryons or anti-baryons). This has two important implications in any theory of baryogenesis within the Standard Model
- Any baryon net excess existing before the electroweak symmetry breaking was wiped off because sphalerons were abundant due to the high temperatures existing in the universe.
- A baryon net excess can be created during the electroweak symmetry breaking can be later preserved only if this phase transition was first-order. This is because in a second-order phase transition sphalerons would wipe off any baryon asymmetry as it is created, while in a first-order phase transition sphalerons would wipe off baryon asymmetry only in the unbroken phase.
In some theories of baryogenesis an imbalance of the number of leptons and antileptons is formed first by leptogenesis and sphaleron transitions then convert this to an imbalance in the numbers of baryons and antibaryons.
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