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In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles
mediate Mediate may refer to: * "Mediate" (song), by INXS * Domenic Mediate (born 1982), professional soccer player *Rocco Mediate (born 1962), professional golfer *A common misspelling of the website Mediaite Mediaite is a news website focusing on pol ...
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 ...
; the respective symbols are , , and . The  bosons have either a positive or negative electric charge of 1
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
and are each other's
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 ...
s. The  boson is electrically neutral and is its own antiparticle. The three particles each have a
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 ...
of 1. The  bosons have a magnetic moment, but the has none. All three of these particles are very short-lived, with a
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of about . Their experimental discovery was pivotal in establishing what is now called the Standard Model of particle physics. The  bosons are named after the ''weak'' force. The
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate caus ...
Steven Weinberg Steven Weinberg (; May 3, 1933 – July 23, 2021) was an American theoretical physicist and Nobel laureate in physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interact ...
named the additional particle the " particle", — The electroweak unification paper. and later gave the explanation that it was the last additional particle needed by the model. The  bosons had already been named, and the  bosons were named for having ''zero'' electric charge. The two  bosons are verified mediators of neutrino absorption and emission. During these processes, the  boson charge induces electron or positron emission or absorption, thus causing
nuclear transmutation Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed. A transmut ...
. The  boson mediates the transfer of momentum, spin and energy when neutrinos scatter '' elastically'' from matter (a process which conserves charge). Such behavior is almost as common as inelastic neutrino interactions and may be observed in
bubble chamber A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the ...
s upon irradiation with neutrino beams. The  boson is not involved in the absorption or emission of electrons or positrons. Whenever an electron is observed as a new free particle, suddenly moving with kinetic energy, it is inferred to be a result of a neutrino interacting with the electron (with the momentum transfer via the Z boson) since this behavior happens more often when the neutrino beam is present. In this process, the neutrino simply strikes the electron (via exchange of a boson) and then scatters away from it, transferring some of the neutrino's momentum to the electron.


Basic properties

These bosons are among the heavyweights of the elementary particles. With masses of and , respectively, the and  bosons are almost 80 times as massive as the proton – heavier, even, than entire iron atoms. Their high masses limit the range of the weak interaction. By way of contrast, the photon is the force carrier of the electromagnetic force and has zero mass, consistent with the infinite range of
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 ...
; the hypothetical graviton is also expected to have zero mass. (Although
gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bind q ...
s are also presumed to have zero mass, the range of the color force is limited for different reasons; ''see
color confinement In quantum chromodynamics (QCD), color confinement, often simply called confinement, is the phenomenon that color-charged particles (such as quarks and gluons) cannot be isolated, and therefore cannot be directly observed in normal conditions ...
''.) All three bosons have particle spin ''s'' = 1. The emission of a or boson either lowers or raises the electric charge of the emitting particle by one unit, and also alters the spin by one unit. At the same time, the emission or absorption of a  boson can change the type of the particle – for example changing a strange quark into an
up quark The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quark ...
. The neutral Z boson cannot change the electric charge of any particle, nor can it change any other of the so-called "
charges Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...
" (such as strangeness,
baryon number In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as ::B = \frac\left(n_\text - n_\bar\right), where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Baryon ...
,
charm Charm may refer to: Social science * Charisma, a person or thing's pronounced ability to attract others * Superficial charm, flattery, telling people what they want to hear Science and technology * Charm quark, a type of elementary particle * Ch ...
, etc.). The emission or absorption of a  boson can only change the spin, momentum, and energy of the other particle. (See also ''
Weak neutral current Weak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the Z boson. The discovery of weak neutral currents was a significant step towar ...
''.)


Relations to the weak nuclear force

The and bosons are carrier particles that mediate the weak nuclear force, much as the photon is the carrier particle for the electromagnetic force.


W bosons

The bosons are best known for their role in
nuclear decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
. Consider, for example, the
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 e ...
of
cobalt-60 Cobalt-60 (60Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2713 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisoto ...
. : → + + This reaction does not involve the whole cobalt-60
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
, but affects only one of its 33 neutrons. The neutron is converted into a proton while also emitting an electron (called a
beta particle A beta particle, also called beta ray or beta radiation (symbol β), is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β� ...
in this context) and an electron antineutrino: : Again, the neutron is not an elementary particle but a composite of an
up quark The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a significant constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quark ...
and two
down quark The down quark or d quark (symbol: d) is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons (one up quark, two down quarks) and protons (two up ...
s (). It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (). At the most fundamental level, then, the weak force changes the
flavour Flavor or flavour is either the sensory perception of taste or smell, or a flavoring in food that produces such perception. Flavor or flavour may also refer to: Science * Flavors (programming language), an early object-oriented extension to Li ...
of a single quark: : which is immediately followed by decay of the itself: :


Z bosons

The is its own antiparticle. Thus, all of its
flavour quantum numbers In particle physics, flavour or flavor refers to the ''species'' of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with ''flavour quantum numbers'' th ...
and
charges Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...
are zero. The exchange of a  boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of spin and/or momentum. boson interactions involving neutrinos have distinct signatures: They provide the only known mechanism for elastic scattering of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via  boson exchange) as inelastically (via W boson exchange). Weak neutral currents via boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in the
Gargamelle Gargamelle was a heavy liquid bubble chamber detector in operation at CERN between 1970 and 1979. It was designed to detect neutrinos and antineutrinos, which were produced with a beam from the Proton Synchrotron (PS) between 1970 and 1976, b ...
bubble chamber A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the ...
at
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
.


Predictions of the W, W and Z bosons

Following the success of quantum electrodynamics in the 1950s, attempts were undertaken to formulate a similar theory of the weak nuclear force. This culminated around 1968 in a unified theory of electromagnetism and weak interactions by Sheldon Glashow,
Steven Weinberg Steven Weinberg (; May 3, 1933 – July 23, 2021) was an American theoretical physicist and Nobel laureate in physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interact ...
, and
Abdus Salam Mohammad Abdus Salam Salam adopted the forename "Mohammad" in 1974 in response to the anti-Ahmadiyya decrees in Pakistan, similarly he grew his beard. (; ; 29 January 192621 November 1996) was a Punjabis, Punjabi Pakistani theoretical physici ...
, for which they shared the 1979
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. (see also
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
on Wikipedia)
Their
electroweak theory In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
postulated not only the bosons necessary to explain beta decay, but also a new  boson that had never been observed. The fact that the and bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by an SU(2) gauge theory, but the bosons in a gauge theory must be massless. As a case in point, the photon is massless because electromagnetism is described by a U(1) gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the and in the process. The
Higgs mechanism In the Standard Model of particle physics, the Higgs mechanism is essential to explain the generation mechanism of the property " mass" for gauge bosons. Without the Higgs mechanism, all bosons (one of the two classes of particles, the other b ...
, first put forward by the
1964 PRL symmetry breaking papers The 1964 ''PRL'' symmetry breaking papers were written by three teams who proposed related but different approaches to explain how mass could arise in local gauge theories. These three papers were written by: Robert Brout and François Englert; P ...
, fulfills this role. It requires the existence of another particle, the Higgs boson, which has since been found at the
Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle collider. It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundre ...
. Of the four components of a Goldstone boson created by the Higgs field, three are absorbed by the and bosons to form their longitudinal components, and the remainder appears as the spin 0 Higgs boson. The combination of the SU(2) gauge theory of the weak interaction, the electromagnetic interaction, and the Higgs mechanism is known as the Glashow–Weinberg–Salam model. Today it is widely accepted as one of the pillars of the Standard Model of particle physics, particularly given the 2012 discovery of the Higgs boson by the CMS and
ATLAS An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographi ...
experiments. The model predicts that and bosons have the following masses: :\begin m_ &= \tfracvg \\ m_ &= \tfrac v\sqrt \end where g is the SU(2) gauge coupling, g' is the U(1) gauge coupling, and v is the Higgs
vacuum expectation value In quantum field theory the vacuum expectation value (also called condensate or simply VEV) of an operator is its average or expectation value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle ...
.


Discovery

Unlike beta decay, the observation of neutral current interactions that involve particles requires huge investments 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 and
detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s, such as are available in only a few high-energy physics laboratories in the world (and then only after 1983). This is because  bosons behave in somewhat the same manner as photons, but do not become important until the energy of the interaction is comparable with the relatively huge mass of the  boson. The discovery of the and bosons was considered a major success for CERN. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge Gargamelle bubble chamber photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a neutrino interacting with the electron by the exchange of an unseen boson. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the electron by the interaction. The discovery of the and bosons themselves had to wait for the construction of a particle accelerator powerful enough to produce them. The first such machine that became available was the
Super Proton Synchrotron The Super Proton Synchrotron (SPS) is a particle accelerator of the synchrotron type at CERN. It is housed in a circular tunnel, in circumference, straddling the border of France and Switzerland near Geneva, Switzerland. History The SPS was de ...
, where unambiguous signals of W bosons were seen in January 1983 during a series of experiments made possible by Carlo Rubbia and Simon van der Meer. The actual experiments were called UA1 (led by Rubbia) and UA2 (led by Pierre Darriulat), and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end ( stochastic cooling). UA1 and UA2 found the boson a few months later, in May 1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in Physics, a most unusual step for the conservative Nobel Foundation. The and bosons, together with the photon (), comprise the four
gauge boson In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles, whose interactions are described by a gauge theory, interact with each other by the exchange of gauge ...
s of the
electroweak interaction In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
.


2022 unexpected measurement of W boson mass

Before 2022, measurements of the W boson mass appeared to be consistent with the Standard Model. For example, in 2021, experimental measurements of the W boson mass were assessed to converge around 80,379 ± 12 MeV. However, in April 2022, a new analysis of data that was obtained by the
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 opera ...
Tevatron collider before its closure in 2011 determined the mass of the W boson to be 80,433 ± 9 MeV, which is seven standard deviations above that predicted by the Standard Model, meaning that if the model is correct there should only be a one-trillionth chance that such a large mass would arise by non-systematic observational error. According to Ashutosh Kotwal of
Duke University Duke University is a private research university in Durham, North Carolina. Founded by Methodists and Quakers in the present-day city of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco and electric power industrialist Jame ...
and the leader of the Collider Detector at Fermilab collaboration, the lower beam luminosity used reduced the chance that events of interest would be obscured by other collisions and that the use of proton-antiproton collisions simplifies the process of quark-antiquark annihilation, which then decayed to give a
lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neut ...
and a neutrino. The team deliberately encrypted its data and withheld any preliminary results from themselves until the analysis was complete, to prevent "confirmation bias" bending their interpretation of the data. Kotwal described it as the 'largest crack in this beautiful theory', speculating that it might be the 'first clear evidence' of other forces or particles not accounted for by the Standard Model, and which might be accounted for by theories such as supersymmetry. The Nobel-winning theoretical physicist
Frank Wilczek Frank Anthony Wilczek (; born May 15, 1951) is an American theoretical physicist, mathematician and Nobel laureate. He is currently the Herman Feshbach Professor of Physics at the Massachusetts Institute of Technology (MIT), Founding Direct ...
described the result as a 'monumental piece of work'. Besides being inconsistent with the Standard Model, the new measurement is also inconsistent with previous measurements such as ATLAS. This suggests that either the old or the new measurements, despite all precautions, have an unexpected systematic error, such as an undetected quirk in the equipment. Future experiments with the LHC may help determine which set of measurements, if either, are the correct ones. Fermilab Deputy Director
Joseph Lykken Joseph David Lykken ( ; born June 17, 1957) is a theoretical physicist at the Fermi National Accelerator Laboratory. From July 1, 2014 to Sept 6, 2022 he was the Deputy Director of Fermilab. He is currently leading the Fermilab Quantum Institute. ...
reiterated that "...the (new) measurement needs to be confirmed by another experiment before it can be interpreted fully." Matthias Schott, of the
University of Mainz The Johannes Gutenberg University Mainz (german: Johannes Gutenberg-Universität Mainz) is a public research university in Mainz, Rhineland Palatinate, Germany, named after the printer Johannes Gutenberg since 1946. With approximately 32,000 stu ...
, commented that "I do not think we have to discuss which new physics could explain the discrepancy between CDF ollider Detector at Fermilaband the Standard Model – we first have to understand why the CDF measurement is in strong tension with all ther measurements.


Decay

The and bosons decay to
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 and ...
pairs but neither the nor the  bosons have sufficient energy to decay into the highest-mass
top quark The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the
coupling constant In physics, a coupling constant or gauge coupling parameter (or, more simply, a coupling), is a number that determines the strength of the force exerted in an interaction. Originally, the coupling constant related the force acting between two ...
s.


W bosons

bosons can decay to a
lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neut ...
and antilepton (one of them charged and another neutral) or to a
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 ...
and antiquark of complementary types (with opposite electric charges and ). The
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 ...
of the W boson to a quark–antiquark pair is proportional to the corresponding squared CKM matrix element and the number of quark colours, The decay widths for the W boson are then proportional to: : Here, , , denote the three flavours of
lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neut ...
s (more exactly, the positive charged
antilepton In particle physics, a lepton is an elementary particle of half-integer spin (spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutra ...
s). , , denote the three flavours of neutrinos. The other particles, starting with and , all denote
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 and antiquarks (factor is applied). The various \, V_ \, denote the corresponding CKM matrix coefficients. Unitarity of the CKM matrix implies that ~ , V_\text, ^2 + , V_\text, ^2 + , V_\text, ^2 ~ = ~ ~, V_\text, ^2 + , V_\text, ^2 + , V_\text, ^2 = 1 ~, thus each of two quark rows Therefore, the leptonic
branching ratio In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...
s of the W boson are approximately \, B( \mathrm^ \mathrm_\mathrm) = \,\, B(\mathrm^ \mathrm_\mathrm) = \,\, B(\mathrm^ \mathrm_\mathrm) = \, The hadronic branching ratio is dominated by the CKM-favored and final states. The sum of the
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...
ic branching ratios has been measured experimentally to be , with


boson

bosons decay into a fermion and its antiparticle. As the  boson is a mixture of the pre- symmetry-breaking and bosons (see weak mixing angle), each vertex factor includes a factor ~ T_3 - Q \sin^2 \,\theta_\mathsf ~, where \, T_3 \, is the third component of the
weak isospin In particle physics, weak isospin is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol or , with the third component written as or . It can ...
of the fermion (the "charge" for the weak force), \, Q \, is the electric charge of the fermion (in units of the
elementary charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
), and \; \theta_\mathsf \; is the weak mixing angle. Because the weak isospin (\, T_3 \,) is different for fermions of different
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 i ...
, either left-handed or right-handed, the coupling is different as well. The ''relative'' strengths of each coupling can be estimated by considering that the
decay rate Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
s include the square of these factors, and all possible diagrams (e.g. sum over quark families, and left and right contributions). The results tabulated below are just estimates, since they only include tree-level interaction diagrams in the Fermi theory. : ::To keep the notation compact, the table uses ~ x = \sin^2 \,\theta_\text ~. ::* The impossible decay into a
top quark The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling y_ is very close to unity; in the Standard ...
-antiquark pair is left out of the table. ::Subheadings and denote the
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 i ...
or "handedness" of the fermions. In 2018, the CMS collaboration observed the first exclusive decay of the  boson to a meson and a
lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neut ...
-antilepton pair.


See also

* * List of particles * *
Weak charge In nuclear physics and atomic physics, weak charge refers to the Standard Model weak interaction coupling of a particle to the Z boson. For example, for any given nuclear isotope, the total weak charge is approximately −0.99 per neutron, and + ...
* * : analogous pair of bosons predicted by the Grand Unified Theory *


Footnotes


References


External links

*
The Review of Particle Physics
the ultimate source of information on particle properties.
The W and Z particles: a personal recollection
by Pierre Darriulat
When CERN saw the end of the alphabet
by Daniel Denegri

{{Authority control Bosons Elementary particles Electroweak theory Gauge bosons Standard Model Force carriers Subatomic particles with spin 1 de:W-Boson