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Concept
Leptogenesis
Summary
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In physical cosmology, leptogenesis is the generic term for hypothetical physical processes that produced an asymmetry between leptons and antileptons in the very early universe, resulting in the present-day dominance of leptons over antileptons. In the currently accepted Standard Model, lepton number is nearly conserved at temperatures below the TeV scale, but tunneling processes can change this number; at higher temperature it may change through interactions with sphalerons, particle-like entities. In both cases, the process involved is related to the weak nuclear force, and is an example of chiral anomaly.
Such processes could have hypothetically created leptons in the early universe. In these processes baryon number is also non-conserved, and thus baryons should have been created along with leptons. Such non-conservation of baryon number is indeed assumed to have happened in the early universe, and is known as baryogenesis. However, in some theoretical models, it is suggested that leptogenesis also occurred prior to baryogenesis; thus the term leptogenesis is often used to imply the non-conservation of leptons without corresponding non-conservation of baryons. In the standard model, the difference between the lepton number and the baryon number is precisely conserved, so that leptogenesis without baryogenesis is impossible. Thus such leptogenesis implies extensions to the standard model.
The lepton and baryon asymmetries affect the much better understood Big Bang nucleosynthesis at later times, during which light atomic nuclei began to form. Successful synthesis of the light elements requires that there be an imbalance in the number of baryons and antibaryons to one part in a billion when the universe is a few minutes old. An asymmetry in the number of leptons and antileptons is not mandatory for Big Bang nucleosynthesis. However, charge conservation suggests that any asymmetry in the charged leptons and antileptons (electrons, muons and tau particles) should be of the same order of magnitude as the baryon asymmetry.
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