Leptogenesis (physics)

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 dominance of leptons over antileptons. The analogous mechanism for baryons is called baryogenesis.

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, universal 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. (Observations of the primordial helium-4 abundance place an upper limit on any lepton asymmetry residing in the neutrino sector, which is not very stringent though.)

It should be understood however that in the currently accepted model for the elementary interactions, the so-called Standard Model, it is not possible to create only "standalone" leptons as these processes are bound by conservation laws such as the conservation of electric charge.

Leptogenesis theories employ sub-disciplines of physics such as quantum field theory, and statistical physics, to describe such possible mechanisms. Baryogenesis and leptogenesis are also connected by a phenomenon that happens in the Standard Model that allows to convert baryon number and lepton number into each other. This makes leptogenesis a possible scenario of baryogenesis, as the lepton asymmetry can partly be converted into a baryon asymmetry. The (non-perturbative) quantum Adler–Bell–Jackiw anomaly can result in sphalerons, which can convert leptons into baryons and vice versa. This means that the Standard Model is in principle able to provide a mechanism to create baryons and leptons, according to conditions suggested by Andrei Sakharov in 1967.

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