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In
physical cosmology, leptogenesis is the generic term for hypothetical physical processes that produced an
asymmetry
Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
between
leptons and antileptons in the
very early universe
The chronology of the universe describes the history and future of the universe according to Big Bang cosmology.
Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with ...
, resulting in the present-day dominance of leptons over antileptons. In the currently accepted
Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
,
lepton number is nearly conserved at temperatures below the
TeV
TEV may refer to:
* Transient Earth Voltage: a term for voltages appearing on the metal work of switchgear due to internal partial discharges
* TeV, or teraelectronvolt or trillion electron volt, a measure of energy
* Total Enterprise Value, a ...
scale, but
tunneling processes can change this number; at higher temperature it may change through interactions with
sphalerons, particle-like entities.
[Kuzmin, V. A., Rubakov, V. A., & Shaposhnikov, M. E. (1985). On anomalous electroweak baryon-number non-conservation in the early universe. Physics Letters B, 155(1-2), 36-42.] 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
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...
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,
muon
A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wi ...
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.
Leptogenesis theories employ sub-disciplines of
physics such as
quantum field theory
In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...
, 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.
See also
*
References
{{reflist
Further reading
LeptogenesisWilfried Buchmüller,
Scholarpedia, 9(3):11471.
doi:10.4249/scholarpedia.11471
External links
Planck satellite cosmic recipe
Physical cosmology
Particle physics