lepton

TheInfoList

In
particle physics Particle physics (also known as high energy physics) is a branch of that studies the nature of the particles that constitute and . Although the word ' can refer to various types of very small objects (e.g. , gas particles, or even household d ...
, a lepton is an
elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s (s, s, s, and s), which generally are " particles" and " particles", as well ...
of
half-integer spin In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, spin 3/2, etc. These particles obey the Pauli exclusion principle. Fermions include all quarks and leptons, as we ...
(
spin Spin or spinning may refer to: Businesses * or South Pacific Island Network * , an American scooter-sharing system * , a chain of table tennis lounges Computing * , 's tool for formal verification of distributed software systems * , a Mach-like ...
) that does not undergo
strong interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
s. Two main classes of leptons exist: charged leptons (also known as the
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

-like leptons or muons), and neutral leptons (better known as
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion (an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστ ...

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 obj ...
s such as
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

s and
positronium Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge Electric charg ...

, while neutrinos rarely interact with anything, and are consequently rarely observed. The best known of all leptons is the
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

. There are six types of leptons, known as ''
flavours Flavor (American English), flavour (British English; American and British English spelling differences#-our, -or, see spelling differences), or taste is the perceptual impression of food or other chemical substance, substances, and is determined ...
'', grouped in three '' generations''. The leptons, also called ''electronic leptons'', comprise the
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...

() and the
electron neutrino The electron neutrino () is a subatomic lepton In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowle ...

(); the second are the ''muonic leptons'', comprising the
muon The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπι ...

() and the
muon neutrino The muon neutrino is a lepton, an elementary subatomic particle which has the symbol and no electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Ther ...

(); and the third are the ''tauonic leptons'', comprising the
tau Tau (uppercase Τ, lowercase τ; el, ταυ ) is the 19th letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late ninth or early eighth century BC. It is derived from the earlier Phoenician ...
() and the
tau neutrino The tau neutrino or tauon neutrino is a subatomic elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ ( ...

(). 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: 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 ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

, whereas muons and taus can only be produced in high energy collisions (such as those involving
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approx ...
s and those carried out in
particle accelerator , a synchrotron collider type particle accelerator at Fermi National Accelerator Laboratory (Fermilab), Batavia, Illinois, USA. Shut down in 2011, until 2007 it was the most powerful particle accelerator in the world, accelerating protons to an en ...
s). Leptons have various intrinsic properties, including
electric charge Electric charge is the physical property A physical property is any property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of th ...
,
spin Spin or spinning may refer to: Businesses * or South Pacific Island Network * , an American scooter-sharing system * , a chain of table tennis lounges Computing * , 's tool for formal verification of distributed software systems * , a Mach-like ...
, and
mass Mass is the quantity Quantity is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value ...
. Unlike
quark A quark () is a type of and a fundamental constituent of . Quarks combine to form s called s, the most stable of which are s and s, the components of . All commonly observable matter is composed of up quarks, down quarks and s. Owing to a phe ...

s, however, leptons are not subject to the
strong interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
, but they are subject to the other three
fundamental interaction In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through Sp ...
s:
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is th ...

, the
weak interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
, and to
electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagnet ...

, 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 s (left) and antiparticles (right). From top to bottom; electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classica ...
, 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. The next lepton to be observed was the
muon The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπι ...

, discovered by Carl D. Anderson in 1936, which was classified as a
meson In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...

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 Quantum mechanics is a fundamental theory in physics that provides a description of the physica ...

in 1930 to explain certain characteristics of
beta decay (the accompanying antineutrino is omitted). The inset shows beta decay of a free neutron. Neither of these depictions shows the intermediate Virtual particle, virtual boson. In nuclear physics, beta decay (''β''-decay) is a type of radioactive ...

. It was first observed in the
Cowan–Reines neutrino experimentThe 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 existence ...
conducted by
Clyde Cowan Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist, the co-discoverer of the neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion (an elementary particle In particle physics Particle ...

and
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area o ...

in 1956. The muon neutrino was discovered in 1962 by ,
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 Accelerator neutrino, beam method an ...
, and
Jack Steinberger Jack Steinberger (born Hans Jakob Steinberger; May 25, 1921December 12, 2020) was a German-born American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge ...
, and the tau discovered between 1974 and 1977 by
Martin Lewis Perl Martin Lewis Perl (June 24, 1927 – September 30, 2014) was an Americans, American chemical engineer and physicist who won the Nobel Prize in Physics in 1995 for his Discovery (observation), discovery of the tau lepton. Life and career Perl was ...
and his colleagues from the
Stanford Linear Accelerator Center SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy The United States Department of Energy (DOE) is a cabinet Cabinet or The Cabinet may refer to: Furnitur ...
and
Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory (LBNL), commonly referred to as Berkeley Lab, is a United States national laboratory that conducts scientific research on behalf of the Department of EnergyA Ministry of Energy or Department of Energy is a ...
. The
tau neutrino The tau neutrino or tauon neutrino is a subatomic elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ ( ...

remained elusive until July 2000, when the
DONUT A doughnut or donut ( IPA: ) is a type of 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, supermarket A supermarket is a self-serv ...

collaboration from
Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy partic ...

announced its discovery. Leptons are an important part of the
Standard Model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsi ...

. Electrons are one of the components of
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

s, alongside
proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approximately one atomic mass unit, are collecti ...

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 behav ...

s.
Exotic atomAn 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) o ...
s with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as
positronium Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge Electric charg ...

.

# Etymology

The name ''lepton'' comes from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
''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 Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēnikḗ'') is an independent branch of the Indo-European languages, Indo-European family of lan ...
, ''re-po-to'', written in
Linear B Linear B is a syllabic script that was used for writing Mycenaean Greek, the earliest attested form of Greek. The script predates the Greek alphabet The Greek alphabet has been used to write the Greek language since the late ninth or ...
syllabic script. ''Lepton'' was first used by physicist Léon Rosenfeld 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

The first lepton identified was the electron, discovered by J.J. Thomson and his team of British physicists in 1897. Then in 1930,
Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics Quantum mechanics is a fundamental theory in physics that provides a description of the physica ...

postulated the
electron neutrino The electron neutrino () is a subatomic lepton In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowle ...

to preserve conservation of energy, conservation of momentum, and conservation of angular momentum in
beta decay (the accompanying antineutrino is omitted). The inset shows beta decay of a free neutron. Neither of these depictions shows the intermediate Virtual particle, virtual boson. In nuclear physics, beta decay (''β''-decay) is a type of radioactive ...

. Pauli theorized that an undetected particle was carrying away the difference between the energy, momentum, and angular momentum 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
muon The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπι ...

was discovered by Carl D. Anderson in 1936. Due to its mass, it was initially categorized as a
meson In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...

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
strong interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
, and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles—the leptons. In 1962, ,
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 Accelerator neutrino, beam method an ...
, and
Jack Steinberger Jack Steinberger (born Hans Jakob Steinberger; May 25, 1921December 12, 2020) was a German-born American physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge ...
showed that more than one type of neutrino exists by first detecting interactions of the
muon The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπι ...

neutrino, which earned them the Nobel Prize in Physics, 1988 Nobel Prize, although by then the different flavours of neutrino had already been theorized. The tau (particle), tau was first detected in a series of experiments between 1974 and 1977 by
Martin Lewis Perl Martin Lewis Perl (June 24, 1927 – September 30, 2014) was an Americans, American chemical engineer and physicist who won the Nobel Prize in Physics in 1995 for his Discovery (observation), discovery of the tau lepton. Life and career Perl was ...
with his colleagues at the SLAC Lawrence Berkeley National Laboratory, 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
DONUT A doughnut or donut ( IPA: ) is a type of 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, supermarket A supermarket is a self-serv ...

collaboration at
Fermilab Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a United States Department of Energy United States Department of Energy National Labs, national laboratory specializing in high-energy partic ...

, making it the second-to-latest particle of the
Standard Model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsi ...

to have been directly observed,K. Kodama (2001) with Higgs boson 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
spin Spin or spinning may refer to: Businesses * or South Pacific Island Network * , an American scooter-sharing system * , a chain of table tennis lounges Computing * , 's tool for formal verification of distributed software systems * , a Mach-like ...
particles. The spin-statistics theorem thus implies that they are fermions and thus that they are subject to the Pauli exclusion principle: 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 (physics), chirality, which in turn is closely related to a more easily visualized property called helicity (particle physics), helicity. The helicity of a particle is the direction of its spin relative to its momentum; 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 Lorentz transformation, reference frame; in the faster frame, the helicity is reversed. Chirality is a technical property, defined through transformation behaviour under the Poincaré group, 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, such as quantum electrodynamics and quantum chromodynamics, left- and right-handed fermions are identical. However, the Standard Model's Weak interaction treats left-handed and right-handed fermions differently: Only left-handed fermions (and right-handed anti-fermions) participate in the
weak interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
. This is an example of parity violation 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 neutrinos) 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 Electroweak unification, combined electro-weak force, although with different strengths (Weak hypercharge, W).

## Electromagnetic interaction

One of the most prominent properties of leptons is their
electric charge Electric charge is the physical property A physical property is any property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of th ...
, . The electric charge determines the strength of their electromagnetic interactions. It determines the strength of the electric field generated by the particle (see Coulomb's law) and how strongly the particle reacts to an external electric or magnetic field (see Lorentz force). 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. The Feynman diagram 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 is given by :$\;\mu = g\, \frac \;,$ where is the mass of the lepton and is the so-called g-factor (physics), " 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'', 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, Muon g-2, 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 (physics), doublet that transforms in the spinor representation ( = ) of the weak isospin SU(2) gauge symmetry. This means that these particles are eigenstates of the isospin projection 3 with eigenvalues and respectively. In the meantime, the right-handed charged lepton transforms as a weak isospin scalar ( = 0) and thus does not participate in the
weak interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
, while there is no evidence that a right-handed neutrino exists at all. The Higgs mechanism recombines the gauge fields of the weak isospin SU(2) and the weak hypercharge U(1) symmetries to three massive vector bosons (, , ) mediating the
weak interaction In nuclear physics Nuclear physics is the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
, and one massless vector boson, the photon, responsible for the electromagnetic interaction. The electric charge can be calculated from the isospin projection 3 and weak hypercharge W through the Gell-Mann–Nishijima formula, : = 3 +  W To recover the observed electric charges for all particles, the left-handed weak isospin doublet must thus have W = −1, while the right-handed isospin scalar e must have W = −2. The interaction of the leptons with the massive weak interaction vector bosons is shown in the figure on the right.

## Mass

In the
Standard Model The Standard Model of particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsi ...

, each lepton starts out with no intrinsic mass. The charged leptons (i.e. the electron, muon, and tau) obtain an effective mass through interaction with the Higgs field, but the neutrinos remain massless. For technical reasons, the masslessness of the neutrinos implies that there is no mixing of the different generations of charged leptons as CKM matrix, there is for quarks. The zero mass of neutrino is in close agreement with current direct experimental observations of the mass. However, it is known from indirect experiments—most prominently from observed neutrino oscillations—that neutrinos have to have a nonzero mass, probably less than . This implies the existence of physics beyond the Standard Model. The currently most favoured extension is the so-called seesaw mechanism, which would explain both why the left-handed neutrinos are so light compared to the corresponding charged leptons, and why we have not yet seen any right-handed neutrinos.

## Lepton flavor quantum numbers

The members of each generation's weak isospin doublet (physics), doublet are assigned lepton number, leptonic numbers that are conserved under the Standard Model.B. R. Martin, G. Shaw (1992) Electrons and electron neutrinos have an ''electronic number'' of , while muons and muon neutrinos have a ''muonic number'' of , while tau particles and tau neutrinos have a ''tauonic number'' of . The antileptons have their respective generation's leptonic numbers of . Conservation of the leptonic numbers means that the number of leptons of the same type remains the same, when particles interact. This implies that leptons and antileptons must be created in pairs of a single generation. For example, the following processes are allowed under conservation of leptonic numbers: :, :, but not these: :, :, :. However, neutrino oscillations are known to violate the conservation of the individual leptonic numbers. Such a violation is considered to be smoking gun evidence for physics beyond the Standard Model. A much stronger conservation law is the conservation of the total number of leptons ( ), conserved even in the case of neutrino oscillations, but even it is still violated by a tiny amount by the chiral anomaly.

# Universality

The coupling of leptons to all types of gauge boson are flavour-independent: The interaction between leptons and a gauge boson measures the same for each lepton. This property is called lepton universality and has been tested in measurements of the
muon The muon (; from the Greek letter mu (μ) used to represent it) is an elementary particle In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπι ...

and tau lepton, tau mean lifetime, lifetimes and of boson partial decay widths, particularly at the Stanford Linear Collider (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) : $\Gamma \left \left( \mu^- \rarr e^- + \bar +\nu_\mu \right \right) \approx K_2\, G_\text^2\, m_\mu^5 ~,$ 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 : $\Gamma \left \left( \tau^- \rarr e^- + \bar +\nu_\tau \right \right) \approx K_3\, G_\text^2\, m_\tau^5 ~,$ where is some other constant. Muon–tauon universality implies that On the other hand, electron–muon universality implies : $\Gamma \left\left( \tau^- \rarr e^- + \bar +\nu_\tau \right\right) = \Gamma \left\left( \tau^- \rarr \mu^- + \bar +\nu_\tau \right\right) ~.$ 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 $\; B\left(x \rarr y\right) \;$ denotes the branching ratios and $\;\Gamma\left(x \rarr y\right) \;$ 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 : $\frac = \frac\, \left\left(\frac\right\right)^5 ~.$ 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 $B\left(W\rarr \tau^-+\nu_\tau\right)/B\left( W\rarr \mu^-+\nu_\mu\right)=0.992\pm0.013$ which agrees with the standard-model prediction of unity

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