particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, a lepton is an

elementary particle In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions ( quarks, leptons, an ...

half-integer spin In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks and ...

(

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

) that does not undergo

strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...

s. Two main classes of leptons exist: charged leptons (also known as the

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

-like leptons or muons), and neutral leptons (better known as

neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

s). Charged leptons can combine with other particles to form various

composite particle This is a list of known and hypothesized particles. Elementary particles Elementary particles are particles with no measurable internal structure; that is, it is unknown whether they are composed of other particles. They are the fundamental ob ...

s such as

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

s and

positronium Positronium (Ps) is a system consisting of an electron and its antimatter, anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two parti ...

, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the

. There are six types of leptons, known as '' flavours'', grouped in three ''

generations A generation is "all of the people born and living at about the same time, regarded collectively." Generation or generations may also refer to: Science and technology * Generation (particle physics), a division of the elementary particles * Gen ...

''. The first-generation leptons, also called ''electronic leptons'', comprise the

() and the

electron neutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name electron neutrino. It was first hypothesized by Wolfgang Pauli ...

(); the second are the ''muonic leptons'', comprising the

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 ...

() and the

muon neutrino The muon neutrino is an elementary particle which has the symbol () and zero electric charge. Together with the muon it forms the second generation of leptons, hence the name muon neutrino. It was discovered in 1962 by Leon Lederman, Melvin Schwar ...

(); and the third are the ''tauonic leptons'', comprising the

tau Tau (uppercase Τ, lowercase τ, or \boldsymbol\tau; el, ταυ ) is the 19th letter of the Greek alphabet, representing the voiceless dental or alveolar plosive . In the system of Greek numerals, it has a value of 300. The name in English ...

() and the tau neutrino (). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of

particle decay In particle physics, particle decay is the spontaneous process of one unstable subatomic particle transforming into multiple other particles. The particles created in this process (the ''final state'') must each be less massive than the original, ...

: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

, whereas muons and taus can only be produced in high energy collisions (such as those involving

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s and those carried out in

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...

s). Leptons have various

intrinsic properties In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...

, including

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

, and

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

. Unlike

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly o ...

s, however, leptons are not subject to the

, but they are subject to the other three

fundamental interaction In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and electrom ...

gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...

, the

weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, ...

, and to

electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...

, of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos. For every lepton flavor, there is a corresponding type of

antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...

, known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. According to certain theories, neutrinos may be their own antiparticle. It is not currently known whether this is the case. The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by

J. J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that c ...

. The next lepton to be observed was the

, discovered by

Carl D. Anderson Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936. ...

in 1936, which was classified as a

meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles ...

at the time. After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particles to be proposed. The first neutrino, the electron neutrino, was proposed by

Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics fo ...

in 1930 to explain certain characteristics of

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

. It was first observed in the

Cowan–Reines neutrino experiment The Cowan–Reines neutrino experiment was conducted by Washington University in St. Louis alumnus Clyde L. Cowan and Stevens Institute of Technology and New York University alumnus Frederick Reines in 1956. The experiment confirmed the existenc ...

conducted by

Clyde Cowan Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist, the co-discoverer of the neutrino along with Frederick Reines. The discovery was made in 1956 in the neutrino experiment. Frederick Reines received the Nobel Pr ...

and

Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in ...

in 1956. The muon neutrino was discovered in 1962 by

Leon M. Lederman Leon Max Lederman (July 15, 1922 – October 3, 2018) was an American experimental physicist who received the Nobel Prize in Physics in 1988, along with Melvin Schwartz and Jack Steinberger, for research on neutrinos. He also received the Wolf Pr ...

Melvin Schwartz Melvin Schwartz (; November 2, 1932 – August 28, 2006) was an American physicist. He shared the 1988 Nobel Prize in Physics with Leon M. Lederman and Jack Steinberger for their development of the neutrino beam method and their demonstration of ...

, and

Jack Steinberger Jack Steinberger (born Hans Jakob Steinberger; May 25, 1921December 12, 2020) was a German-born American physicist noted for his work with neutrinos, the subatomic particles considered to be elementary constituents of matter. He was a recipient ...

, and the tau discovered between 1974 and 1977 by

Martin Lewis Perl Martin Lewis Perl (June 24, 1927 – September 30, 2014) was an American chemical engineer and physicist who won the Nobel Prize in Physics in 1995 for his discovery of the tau lepton. Life and career Perl was born in New York City, New York. Hi ...

and his colleagues from the

Stanford Linear Accelerator Center SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...

and

Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory (LBNL), commonly referred to as the Berkeley Lab, is a United States Department of Energy National Labs, United States national laboratory that is owned by, and conducts scientific research on behalf of, t ...

. The tau neutrino remained elusive until July 2000, when the

DONUT A doughnut or donut () is a type of food made from leavened fried dough. It is popular in many countries and is prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and franc ...

collaboration from

Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy national laboratory specializing in high-energy particle physics. Since 2007, Fermilab has been operat ...

announced its discovery. Leptons are an important part of the

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 ...

. Electrons are one of the components of

s, alongside

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s and

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

Exotic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) ...

s with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as

Etymology

The name ''lepton'' comes from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''leptós'', "fine, small, thin" ( neuter nominative/accusative singular form: λεπτόν ''leptón''); the earliest attested form of the word is the

Mycenaean Greek Mycenaean Greek is the most ancient attested form of the Greek language, on the Greek mainland and Crete in Mycenaean Greece (16th to 12th centuries BC), before the hypothesised Dorian invasion, often cited as the ''terminus ad quem'' for the ...

, ''re-po-to'', written in

Linear B Linear B was a syllabic script used for writing in Mycenaean Greek, the earliest attested form of Greek. The script predates the Greek alphabet by several centuries. The oldest Mycenaean writing dates to about 1400 BC. It is descended from ...

syllabic script. ''Lepton'' was first used by physicist

Léon Rosenfeld Léon Rosenfeld (; 14 August 1904 in Charleroi – 23 March 1974) was a Belgium, Belgian physicist and Marxist. Rosenfeld was born into a Jewish secularism, secular Jews, Jewish family. He was a polyglot who knew eight or nine languages and ...

in 1948:

Following a suggestion of Prof. C. Møller, I adopt—as a pendant to "nucleon"—the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass.

Rosenfeld chose the name because the only known leptons at the time were electrons and muons, whose masses are small compared to nucleons—the mass of an electron ()C. Amsler et al. (2008)
Particle listings—
/ref> and the mass of a muon (with a value of )C. Amsler et al. (2008)
Particle listings—
/ref> are fractions of the mass of the "heavy" proton ().C. Amsler et al. (2008)
Particle listings—
/ref> However, the mass of the tau (discovered in the mid-1970s) ()C. Amsler et al. (2008)
Particle listings—
/ref> is nearly twice that of the proton and about 3,500 times that of the electron.

History

Feynman diagram of muon to electron decay

The first lepton identified was the electron, discovered by

J.J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that ...

and his team of British physicists in 1897. Then in 1930,

postulated the

to preserve

conservation of energy In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means th ...

conservation of momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

, and

conservation of angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed system ...

. Pauli theorized that an undetected particle was carrying away the difference between the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

, and

angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...

of the initial and observed final particles. The electron neutrino was simply called the neutrino, as it was not yet known that neutrinos came in different flavours (or different "generations"). Nearly 40 years after the discovery of the electron, the

was discovered by

in 1936. Due to its mass, it was initially categorized as a

rather than a lepton. It later became clear that the muon was much more similar to the electron than to mesons, as muons do not undergo the

, and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles—the leptons. In 1962,

, and

showed that more than one type of neutrino exists by first detecting interactions of the

neutrino, which earned them the 1988 Nobel Prize, although by then the different flavours of neutrino had already been theorized. The

was first detected in a series of experiments between 1974 and 1977 by

with his colleagues at the

SLAC SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departm ...

LBL group. Like the electron and the muon, it too was expected to have an associated neutrino. The first evidence for tau neutrinos came from the observation of "missing" energy and momentum in tau decay, analogous to the "missing" energy and momentum in beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the

collaboration at

, making it the second-to-latest particle of the

to have been directly observed,K. Kodama (2001) with

Higgs boson The Higgs boson, sometimes called the Higgs particle, is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field, one of the fields in particle physics theory. In the Stand ...

being discovered in 2012. Although all present data is consistent with three generations of leptons, some particle physicists are searching for a fourth generation. The current lower limit on the mass of such a fourth charged lepton is , while its associated neutrino would have a mass of at least .

Properties

Spin and chirality

Leptons are

particles. The spin-statistics theorem thus implies that they are

fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks an ...

s and thus that they are subject to the

Pauli exclusion principle In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulated ...

: no two leptons of the same species can be in the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down. A closely related property is

chirality Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from ...

, which in turn is closely related to a more easily visualized property called helicity. The helicity of a particle is the direction of its spin relative to its

; particles with spin in the same direction as their momentum are called ''right-handed'' and they are otherwise called ''left-handed''. When a particle is massless, the direction of its momentum relative to its spin is the same in every reference frame, whereas for massive particles it is possible to 'overtake' the particle by choosing a faster-moving

reference frame In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin (mathematics), origin, orientation (geometry), orientation, and scale (geometry), scale are specified by a set of reference point ...

; in the faster frame, the helicity is reversed. Chirality is a technical property, defined through transformation behaviour under the

Poincaré group The Poincaré group, named after Henri Poincaré (1906), was first defined by Hermann Minkowski (1908) as the group of Minkowski spacetime isometries. It is a ten-dimensional non-abelian Lie group that is of importance as a model in our und ...

, that does not change with reference frame. It is contrived to agree with helicity for massless particles, and is still well defined for particles with mass. In many

quantum field theories 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 ...

, such as

quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and spec ...

and

quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...

, left- and right-handed fermions are identical. However, the Standard Model's

treats left-handed and right-handed fermions differently: only left-handed fermions (and right-handed anti-fermions) participate in the weak interaction. This is an example of

parity violation In physics, a parity transformation (also called parity inversion) is the flip in the sign of ''one'' spatial coordinate. In three dimensions, it can also refer to the simultaneous flip in the sign of all three spatial coordinates (a point refle ...

explicitly written into the model. In the literature, left-handed fields are often denoted by a capital subscript (e.g. the normal electron: ) and right-handed fields are denoted by a capital subscript (e.g. a positron ). Right-handed neutrinos and left-handed anti-neutrinos have no possible interaction with other particles (''see''

sterile neutrino Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that are believed to interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term ''sterile neutrin ...

s) and so are not a functional part of the Standard Model, although their exclusion is not a strict requirement; they are sometimes listed in particle tables to emphasize that they would have no active role if included in the model. Even though electrically charged right-handed particles (electron, muon, or tau) do not engage in the weak interaction specifically, they can still interact electrically, and hence still participate in the combined electroweak force, although with different strengths ( _W).

Electromagnetic interaction

One of the most prominent properties of leptons is their

, . The electric charge determines the strength of their

electromagnetic interaction In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...

s. It determines the strength of the

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

generated by the particle (see

Coulomb's law Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventiona ...

) and how strongly the particle reacts to an external electric or magnetic field (see

Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...

). Each generation contains one lepton with

\; Q = -1 \, e \;

and one lepton with zero electric charge. The lepton with electric charge is commonly simply referred to as a ''charged lepton'' while a neutral lepton is called a ''neutrino''. For example, the first generation consists of the electron with a negative electric charge and the electrically neutral electron neutrino . In the language of quantum field theory, the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with the quantum of the electromagnetic field, the

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

. The

Feynman diagram In theoretical physics, a Feynman diagram is a pictorial representation of the mathematical expressions describing the behavior and interaction of subatomic particles. The scheme is named after American physicist Richard Feynman, who introduc ...

of the electron–photon interaction is shown on the right. Because leptons possess an intrinsic rotation in the form of their spin, charged leptons generate a magnetic field. The size of their

magnetic dipole moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...

is given by :

\;\mu = g\, \frac \;,

where is the mass of the lepton and is the so-called " factor" for the lepton. First-order quantum mechanical approximation predicts that the factor is 2 for all leptons. However, higher-order quantum effects caused by loops in Feynman diagrams introduce corrections to this value. These corrections, referred to as the ''

anomalous magnetic dipole moment In quantum electrodynamics, the anomalous magnetic moment of a particle is a contribution of effects of quantum mechanics, expressed by Feynman diagrams with loops, to the magnetic moment of that particle. (The ''magnetic moment'', also called '' ...

'', are very sensitive to the details of a quantum field theory model, and thus provide the opportunity for precision tests of the Standard Model. The theoretical and measured values for the ''electron'' anomalous magnetic dipole moment are within agreement within eight significant figures. The results for the ''muon'', however, are problematic, hinting at a small, persistent discrepancy between the Standard Model and experiment.

Weak interaction

In the Standard Model, the left-handed charged lepton and the left-handed neutrino are arranged in

doublet Doublet is a word derived from the Latin ''duplus'', "twofold, twice as much", and has been tested in measurements of the

and

lifetimes and of boson partial

decay width Decay may refer to: Science and technology * Bit decay, in computing * Software decay, in computing * Distance decay, in geography * Decay time (fall time), in electronics Biology * Decomposition of organic matter * Tooth decay (dental caries ...

s, particularly at the

Stanford Linear Collider SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...

(SLC) and Large Electron–Positron Collider (LEP) experiments. The decay rate (

\Gamma

) of muons through the process is approximately given by an expression of the form (see muon decay for more details) :

\Gamma \left ( \mu^- \rarr e^- + \bar +\nu_\mu \right ) \approx K_2\,  G_\text^2\,  m_\mu^5 ~,

where is some constant, and is the Fermi coupling constant. The decay rate of tau particles through the process is given by an expression of the same form :

\Gamma \left ( \tau^- \rarr e^- + \bar +\nu_\tau \right ) \approx K_3\, G_\text^2\, m_\tau^5 ~,

where is some other constant. Muon–tauon universality implies that On the other hand, electron–muon universality implies :

\Gamma \left( \tau^- \rarr e^- + \bar +\nu_\tau \right) = \Gamma \left( \tau^- \rarr \mu^- + \bar +\nu_\tau \right) ~.

This explains why the branching ratios for the electronic mode (17.82%) and muonic (17.39%) mode of tau decay are equal (within error). Universality also accounts for the ratio of muon and tau lifetimes. The lifetime

\Tau_\ell

of a lepton

\ell

(with

\ell

= "" or "") is related to the decay rate by :

\Tau_\ell = \frac\,

, where

\; B(x \rarr y) \;

denotes the branching ratios and

\;\Gamma(x \rarr y) \;

denotes the decay width, resonance width of the process

\; x \rarr y ~,

with and replaced by two different particles from "" or "" or "". The ratio of tau and muon lifetime is thus given by :

\frac = \frac\, \left(\frac\right)^5 ~.

Using values from the 2008 ''Review of Particle Physics'' for the branching ratios of the muon and tau yields a lifetime ratio of ~, comparable to the measured lifetime ratio of ~. The difference is due to and not ''actually'' being constants: They depend slightly on the mass of leptons involved. Recent tests of lepton universality in B meson, meson decays, performed by the LHCb, BaBar experiment, BaBar, and Belle experiment, Belle experiments, have shown consistent deviations from the Standard Model predictions. However the combined statistical and systematic significance is not yet high enough to claim an observation of new physics. In July 2021 results on lepton universality have been published testing W decays, previous measurements by the LEP had given a slight imbalance but the new measurement by the ATLAS experiment, ATLAS collaboration have twice the precision and give a ratio of

B(W\rarr \tau^-+\nu_\tau)/B( W\rarr \mu^-+\nu_\mu)=0.992\pm0.013

which agrees with the standard-model prediction of unity

Table of leptons

Notes

References

External links

* – The PDG compiles authoritative information on particle properties. * – a summary of leptons. {{Authority control Leptons, Elementary particles 1897 in science

Etymology

History

Properties

Spin and chirality

Electromagnetic interaction

Weak interaction

Table of leptons

See also

Notes

References

External links