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In particle physics, the W and Z bosons are
vector boson In particle physics, a vector boson is a boson whose spin equals one. The vector bosons that are regarded as elementary particles in the Standard Model are the gauge bosons, the force carriers of fundamental interactions: the photon of electromag ...
s 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; 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 antiparticles. 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 of about . Their experimental discovery was pivotal in establishing what is now called 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 ...
of particle physics. The  bosons are named after the ''weak'' force. The physicist Steven Weinberg 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 transmutatio ...
. 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 chambers 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 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 ...
– 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; 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 Color Force is an American independent film and television production company founded in 2007 by producer and film executive Nina Jacobson after her 2006 termination as president of Disney's Buena Vista Motion Pictures Group. Its films includ ...
is limited for different reasons; ''see color confinement''.) All three bosons have
particle spin Spin is a conserved quantity carried by elementary particles, and thus by composite particles (hadrons) and atomic nuclei. Spin is one of two types of angular momentum in quantum mechanics, the other being ''orbital angular momentum''. The orbita ...
''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 The strange quark or s quark (from its symbol, s) is the third lightest of all quarks, a type of elementary particle. Strange quarks are found in subatomic particles called hadrons. Examples of hadrons containing strange quarks include kaons ( ...
into an 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 In particle physics, strangeness ("''S''") is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions which occur in a short period of time. The strangeness of a parti ...
, baryon number,
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 towa ...
''.)


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. Consider, for example, the beta decay of cobalt-60. : → + + 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 Nucle ...
, 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 and two down quarks (). 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 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 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 ...
. 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 bubble chamber 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, and Abdus Salam, for which they shared the 1979 Nobel Prize in Physics. (see also Nobel Prize in Physics on Wikipedia) Their electroweak theory 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 In physics, a gauge theory is a type of field theory in which the Lagrangian (and hence the dynamics of the system itself) does not change (is invariant) under local transformations according to certain smooth families of operations (Lie groups) ...
, 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, 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 theory, gauge theories. These three papers were written by: Robert Brout and Franço ...
, fulfills this role. It requires the existence of another particle, the
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 ...
, 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 hundred ...
. 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 In particle physics, the electroweak interaction or electroweak force is the unified field theory, unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two force ...
. 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 CMS may refer to: Computing * Call management system * CMS-2 (programming language), used by the United States Navy * Code Morphing Software, a technology used by Transmeta * Collection management system for a museum collection * Color managem ...
and ATLAS 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.


Discovery

Unlike beta decay, the observation of neutral current interactions that involve particles requires huge investments in particle accelerators and detectors, 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 Simon van der Meer (24 November 19254 March 2011) was a Dutch Accelerator physics, particle accelerator physicist who shared the Nobel Prize in Physics in 1984 with Carlo Rubbia for contributions to the CERN project which led to the discovery o ...
. The actual experiments were called UA1 (led by Rubbia) and
UA2 UA, U-A, Ua, uA, or ua may refer to: Arts and entertainment Gaming * ''Unearthed Arcana'', a Dungeons & Dragons sourcebook * ''Unknown Armies'', a role playing game * ''Urban Assault'', a first-person shooter and real-time strategy computer gam ...
(led by
Pierre Darriulat Pierre Darriulat (born 17 February 1938) is a French experimental particle physicist. As staff member at CERN, he contributed in several prestigious experiments. He was the spokesperson of the UA2 collaboration from 1981 to 1986, during which t ...
), and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end (
stochastic cooling Stochastic cooling is a form of particle beam cooling. It is used in some particle accelerators and storage rings to control the emittance of the particle beams in the machine. This process uses the electrical signals that the individual charg ...
). 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 Nobel Foundation ( sv, Nobelstiftelsen) is a private institution founded on 29 June 1900 to manage the finances and administration of the Nobel Prizes. The foundation is based on the last will of Alfred Nobel, the inventor of dynamite. It ...
. 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.


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 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 Ashutosh Vijay Kotwal (born December 20, 1965) is an American particle physicist of Indian origin. He is the Fritz London Professor of Physics at Duke University, and conducts research in particle physics related to W bosons and the Higgs boson ...
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 James ...
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 neutr ...
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 In a supersymmetric theory the equations for force and the equations for matter are identical. In theoretical and mathematical physics, any theory with this property has the principle of supersymmetry (SUSY). Dozens of supersymmetric theories e ...
. The Nobel-winning theoretical physicist Frank Wilczek 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, 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 Ther may refer to: *''Thér.'', taxonomic author abbreviation of Irénée Thériot (1859–1947), French bryologist *Agroha Mound, archaeological site in Agroha, Hisar district, India *Therapy *Therapeutic drugs See also *''Ther Thiruvizha'', 1968 ...
.


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 an ...
pairs but neither the nor the  bosons have sufficient energy to decay into the highest-mass top quark. Neglecting phase space effects and higher order corrections, simple estimates of their
branching fraction 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 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 neutr ...
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 of the W boson to a quark–antiquark pair is proportional to the corresponding squared CKM matrix element and the number of quark
colours Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associa ...
, 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 neutr ...
s (more exactly, the positive charged antileptons). , , 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 In quantum physics, unitarity is the condition that the time evolution of a quantum state according to the Schrödinger equation is mathematically represented by a unitary operator. This is typically taken as an axiom or basic postulate of quantu ...
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 ratios 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 hadronic 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 The weak mixing angle or Weinberg angle is a parameter in the Weinberg– Salam theory of the electroweak interaction, part of the Standard Model of particle physics, and is usually denoted as . It is the angle by which spontaneous symmetry bre ...
), each vertex factor includes a factor ~ T_3 - Q \sin^2 \,\theta_\mathsf ~, where \, T_3 \, is the third component of the weak isospin 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 The weak mixing angle or Weinberg angle is a parameter in the Weinberg– Salam theory of the electroweak interaction, part of the Standard Model of particle physics, and is usually denoted as . It is the angle by which spontaneous symmetry bre ...
. 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 ...
, 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 In particle physics, Fermi's interaction (also the Fermi theory of beta decay or the Fermi four-fermion interaction) is an explanation of the beta decay, proposed by Enrico Fermi in 1933. The theory posits four fermions directly interacting ...
. : ::To keep the notation compact, the table uses ~ x = \sin^2 \,\theta_\text ~. ::* The impossible decay into a top quark-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 ...
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 neutr ...
-antilepton pair.


See also

* *
List of particles 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 ...
* *
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