|Unsolved problem in physics:
Why does the observable universe have more matter than antimatter?
(more unsolved problems in 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 present-day dominance of leptons over antileptons. In the currently accepted Standard Model, lepton number is conserved; it is not possible to create leptons directly without corresponding antileptons. Leptogenesis can therefore only take place in theories of physics beyond 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. 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.
Leptogenesis theories employ sub-disciplines of physics such as quantum field theory, and statistical physics, to describe such possible mechanisms. Baryogenesis, the generation of a baryon–antibaryon asymmetry, and leptogenesis can be connected by processes that convert baryon number and lepton number into each other. The (non-perturbative) quantum Adler–Bell–Jackiw anomaly can result in sphalerons, which can convert leptons into baryons and vice versa. Thus, the Standard Model is in principle able to provide a mechanism to create baryons and leptons.
A simple modification of the Standard Model that is instead able to realize the program of Sakharov is the one suggested by M. Fukugita and T. Yanagida. The Standard Model is extended by adding right-handed neutrinos, permitting implementation of the see-saw mechanism and providing the neutrinos with mass. At the same time, the extended model is able to spontaneously generate leptons from the decays of right-handed neutrinos. Finally, the sphalerons are able to convert the spontaneously generated lepton asymmetry into the observed baryonic asymmetry. Due to its popularity, this entire process is sometimes referred to simply as leptogenesis.
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