Leptogenesis (physics)
Encyclopedia
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 baryon
s is called baryogenesis
.
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 law
s 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. The next step after leptogenesis is the much better understood Big Bang nucleosynthesis
, 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 symmetry in the charged leptons and antileptons (electron
s, muon
s 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.)
Baryogenesis and leptogenesis are also connected by a phenomenon that happens in the Standard Model
. Indeed, certain (non-perturbative) configurations of gauge fields
, called sphaleron
s, 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 Andrei 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 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. Often, by an extension terms, the physicists use the word leptogenesis to denote the mechanism here described.
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
, leptogenesis is the generic term for hypothetical physical processes that produced an asymmetry
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
between leptons and antileptons in the very early universe
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
, resulting in the dominance of leptons over antileptons. The analogous mechanism for baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s is called baryogenesis
Baryogenesis
In physical cosmology, baryogenesis is the generic term for hypothetical physical processes that produced an asymmetry between baryons and antibaryons in the very early universe, resulting in the substantial amounts of residual matter that make up the universe today.Baryogenesis theories employ...
.
It should be understood however that in the currently accepted model for the elementary interactions, the so called Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
, it is not possible to create only "standalone" leptons as these processes are bound by conservation law
Conservation law
In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves....
s such as the conservation of electric charge.
| + | → | + | (Pair creation) | ||||
| → | + | + | (Muon decay) | ||||
| → | + | + | (Beta decay Beta decay In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a... ) |
||||
Leptogenesis theories employ sub-disciplines of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
such as quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
, and statistical physics
Statistical physics
Statistical physics is the branch of physics that uses methods of probability theory and statistics, and particularly the mathematical tools for dealing with large populations and approximations, in solving physical problems. It can describe a wide variety of fields with an inherently stochastic...
, to describe such possible mechanisms. The next step after leptogenesis is the much better understood Big Bang nucleosynthesis
Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis refers to the production of nuclei other than those of H-1 during the early phases of the universe...
, 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 symmetry in the charged leptons and antileptons (electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s, muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s and tau particles) should be of the same order of magnitude as the baryon asymmetry. (Observations of the primordial Helium-4
Helium-4
Helium-4 is a non-radioactive isotope of helium. It is by far the most abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is the same as an alpha particle, consisting of two protons and two neutrons. Alpha decay of heavy...
abundance place an upper limit on any lepton asymmetry residing in the neutrino sector which is not very stringent though.)
Baryogenesis and leptogenesis are also connected by a phenomenon that happens in the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
. Indeed, certain (non-perturbative) configurations of gauge fields
Gauge theory
In physics, gauge invariance is the property of a field theory in which different configurations of the underlying fundamental but unobservable fields result in identical observable quantities. A theory with such a property is called a gauge theory...
, called sphaleron
Sphaleron
A sphaleron is a static solution to the electroweak field equations of the Standard Model of particle physics, and it is involved in processes that violate baryon and lepton number. Such processes cannot be represented by Feynman diagrams, and are therefore called non-perturbative...
s, 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 Andrei Sakharov
Andrei Sakharov
Andrei Dmitrievich Sakharov was a Soviet nuclear physicist, dissident and human rights activist. He earned renown as the designer of the Soviet Union's Third Idea, a codename for Soviet development of thermonuclear weapons. Sakharov was an advocate of civil liberties and civil reforms in the...
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 M. Fukugita and T. Yanagida
The Standard Model is extended by adding right-handed neutrinos, permitting implementation of the see-saw mechanism
Seesaw mechanism
In theoretical physics, the seesaw mechanism is a mechanism within grand unification theory, and in particular in theories of neutrino masses and neutrino oscillation, where it can be used to explain the smallness of observed neutrino masses relative to those of quarks and leptons.There are several...
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. Often, by an extension terms, the physicists use the word leptogenesis to denote the mechanism here described.