Leptogenesis (physics)

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In the strict sense, leptogenesis is a process which creates leptons. Theories of leptogenesis try to explain how the Universe changed from a state with no leptons just after the Big Bang to a state containing many leptons (mostly electrons) today.

The equivalent problem for baryons is called baryogenesis. While there are no observational bounds on the relative number of leptons and anti-leptons, since leptons can be converted to neutrinos or anti-neutrinos and remain invisible, there is observed a significant imbalance in the number of baryons and anti-baryons.

It should be understood however that it is not possible to create only electrons (or only protons) without violating the conservation of the electric charge. In other words, the conservation of the electric charge requires an equal number of electrons and protons.

Baryogenesis and leptogenesis are also connected by a phenomenon that happens in the currently accepted model for the elementary interactions, the so-called standard model. Indeed, certain (non-perturbative) configurations of gauge fields that are called sphalerons 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, realizing a speculative possibility suggested by A. Sakharov in the sixties. The simplest version of the standard model, however, is quantitatively unable to realize this possibility.

A simple modification of the standard model that is instead able to realize the program of Sakharov is the one suggested by Fukugita and Yanagida. The standard model augmented with adding right handed neutrinos permits to implement the see-saw mechanism and to provide neutrinos with mass. At the same time, the augmented 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. Often, by an extension terms, the physicists use the word leptogenesis to denote the mechanism here described.