View source for W and Z bosons

{{Short description|Bosons that mediate the weak interaction}}
{{Infobox particle
| <!-- bgcolor =red -- no longer valid? still documented -->
| name = {{SubatomicParticle|W boson+-}} and
{{SubatomicParticle|Z boson0}} Bosons
| image = 
| caption = 
| num_types = 
| composition = [[Elementary particle]]
| statistics = [[Boson]]ic
| group = [[Gauge boson]] <!-- "group=" -- no longer valid? still in template documentation. -->
| generation =
| interaction = [[Weak interaction]]
| theorized = [[Sheldon Glashow|Glashow]], [[Steven Weinberg|Weinberg]], [[Abdus Salam|Salam]] (1968)
| discovered = [[UA1]] and [[UA2]] collaborations, [[CERN]], 1983
| symbol = 
| mass = W:&nbsp;{{val|80.377|0.012|ul=GeV/c2}} (2022)<ref name="PDG2018W">
{{cite journal
 |last1=Tanabashi |first1=M. |display-authors=etal
 |collaboration=Particle Data Group
 |title=Review of Particle Physics
 |journal=Physical Review D
 |year=2018
 |volume=98 |issue=3 |page=030001
 |bibcode=2018PhRvD..98c0001T
 |doi=10.1103/PhysRevD.98.030001 |doi-access=free
 |url=https://pdglive.lbl.gov/Particle.action?node=S043
|hdl=10044/1/68623
 |hdl-access=free
 }}
</ref><ref>R. L. Workman et al. (Particle Data Group), [https://pdg.lbl.gov/2022/reviews/rpp2022-rev-w-mass.pdf "Mass and Width of the W Boson"], Prog. Theor. Exp. Phys. 2022, 083C01 (2022).</ref><br>Z:&nbsp;{{val|91.1876|0.0021|u=GeV/c2}}<ref name="PDG2018Z">
{{cite journal
 |last1=Tanabashi |first1=M. |display-authors=etal
 |collaboration=Particle Data Group
 |title=Review of Particle Physics
 |journal=Physical Review D
 |year=2018
 |volume=98 |issue=3 |page=030001
 |bibcode=2018PhRvD..98c0001T
 |doi=10.1103/PhysRevD.98.030001 |doi-access=free
 |url=https://pdglive.lbl.gov/Particle.action?node=S044
|hdl=10044/1/68623
 |hdl-access=free
 }}</ref>
|width = W:&nbsp;{{val|2.085|0.042|ul=GeV}}<ref name="PDG2018W"/><br>Z:&nbsp;{{val|2.4952|0.0023|u=GeV}}<ref name="PDG2018Z"/>
| decay_time = 
| decay_particle = 
| electric_charge = W:&nbsp;±1&nbsp;[[elementary charge|''e'']]<br>Z:&nbsp;0&nbsp;''e''
| colour_charge = 
| weak_isospin = W:&nbsp;±1<br>Z:&nbsp;0
| weak_hypercharge = 0
| spin = 1&nbsp;[[reduced Planck constant|''ħ'']]
| num_spin_states = 
}}
{{Standard model of particle physics}}
In [[particle physics]], the '''W and Z bosons''' are [[vector boson]]s that are together known as the '''weak bosons''' or more generally as the '''intermediate vector bosons'''. These [[elementary particle]]s [[force carrier|mediate]] the [[weak interaction]]; the respective symbols are {{SubatomicParticle|W boson+}}, {{SubatomicParticle|W boson-}}, and {{SubatomicParticle|Z boson0}}. The {{SubatomicParticle|W boson+-}}&nbsp;bosons have either a positive or negative [[electric charge]] of 1 [[elementary charge]] and are each other's [[antiparticle]]s. The {{SubatomicParticle|Z boson0}}&nbsp;boson is electrically [[neutral particle|neutral]] and is its own antiparticle. The three particles each have a [[spin (physics)|spin]] of 1. The {{SubatomicParticle|W boson+-}}&nbsp;bosons have a magnetic moment, but the {{SubatomicParticle|Z boson0}} has none. All three of these particles are very short-lived, with a [[half-life]] of about {{val|3|e=-25|u=s}}. Their experimental discovery was pivotal in establishing what is now called the [[Standard Model]] of [[particle physics]].

The {{SubatomicParticle|W boson}}&nbsp;bosons are named after the ''weak'' force. The [[physicist]] [[Steven Weinberg]] named the additional particle the "{{SubatomicParticle|Z boson}}&nbsp;particle",<ref name="Ref_">{{cite journal |last1=Weinberg |first1=Steven |author-link=Steven Weinberg |title=A Model of Leptons |journal=Physical Review Letters |volume=19 |issue=21 |year=1967 |pages=1264–1266 |bibcode=1967PhRvL..19.1264W |doi=10.1103/physrevlett.19.1264 |url=http://astrophysics.fic.uni.lodz.pl/100yrs/pdf/12/066.pdf |archive-url=https://web.archive.org/web/20120112142352/http://astrophysics.fic.uni.lodz.pl/100yrs/pdf/12/066.pdf |url-status=dead |archive-date=January 12, 2012}} — The electroweak unification paper.</ref> and later gave the explanation that it was the last additional particle needed by the model. The {{SubatomicParticle|W boson}}&nbsp;bosons had already been named, and the {{SubatomicParticle|Z boson}}&nbsp;bosons were named for having ''zero'' electric charge.<ref name="SW1993">{{cite book |last=Weinberg |first=Steven |title=Dreams of a Final Theory: The search for the fundamental laws of nature |year=1993 |publisher=Vintage Press |page=[https://archive.org/details/dreamsoffinalthe00wein/page/94 94] |isbn=978-0-09-922391-7 |url=https://archive.org/details/dreamsoffinalthe00wein |url-access=limited}}</ref>

The two {{SubatomicParticle|W boson}}&nbsp;bosons are verified mediators of [[neutrino]] absorption and emission. During these processes, the {{SubatomicParticle|W boson+-}}&nbsp;boson charge induces electron or positron emission or absorption, thus causing [[nuclear transmutation]].

The {{SubatomicParticle|Z boson}}&nbsp;boson mediates the transfer of momentum, spin and energy when neutrinos scatter ''[[elastic scattering|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]]s upon irradiation with neutrino beams. The {{SubatomicParticle|Z boson}}&nbsp;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&nbsp;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.{{efn|Because neutrinos are neither affected by the [[strong force]] nor the [[electromagnetic force]], and because the [[gravitational force]] between subatomic particles is negligible, by [[abductive reasoning|deduction (technically, ''abduction'')]], such an interaction can only happen via the weak force. Since such an electron is not created from a nucleon (the nucleus left behind remains the same as before) and the departing electron is unchanged, except for the impulse imparted by the neutrino, this force interaction between the neutrino and the electron must be mediated by an electromagnetically neutral, weak force [[boson]]. Thus, since no other neutrino-interacting neutral force carrier is known, the observed interaction must have occurred by exchange of a {{SubatomicParticle|Z boson0}}&nbsp;boson.}}

==Basic properties==
These bosons are among the heavyweights of the elementary particles. With [[mass]]es of {{val|80.4|u=GeV/c2}} and {{val|91.2|u=GeV/c2}}, respectively, the {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;bosons are almost 80&nbsp;times as massive as the [[proton]] – heavier, even, than entire [[iron]] [[atom]]s.

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]]s are also presumed to have zero mass, the range of the [[strong nuclear force]] is limited for different reasons; ''see [[Color confinement]]''.)

All three bosons have [[spin (physics)|particle spin]] ''s''&nbsp;=&nbsp;1. The emission of a {{SubatomicParticle|W boson+}} or {{SubatomicParticle|W boson-}}&nbsp;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 {{SubatomicParticle|W boson+-}}&nbsp;boson can change the type of the particle – for example changing a [[strange quark]] into an [[up quark]]. The neutral Z&nbsp;boson cannot change the electric charge of any particle, nor can it change any other of the so-called "[[charge (physics)|charges]]" (such as [[strangeness]], [[baryon number]], [[charm (quantum number)|charm]], etc.). The emission or absorption of a {{SubatomicParticle|Z boson0}}&nbsp;boson can only change the spin, momentum, and energy of the other particle. (See also ''[[Weak neutral current]]''.)

==Relations to the weak nuclear force==
[[File:Beta Negative Decay.svg|thumb|right|280px|The [[Feynman diagram]] for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate {{SubatomicParticle|W boson-}}&nbsp;boson]]
The {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;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 {{SubatomicParticle|W boson+-}}&nbsp;bosons are best known for their role in [[nuclear decay]]. Consider, for example, the [[beta decay]] of [[cobalt-60]].
: {{nuclide|link=yes|Cobalt|60}} → {{nuclide|link=yes|Nickel|60}}<sup>+</sup> + {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}

This reaction does not involve the whole cobalt-60 [[atomic nucleus|nucleus]], but affects only one of its 33&nbsp;neutrons. The neutron is converted into a proton while also emitting an electron (often called a [[beta particle]] in this context) and an electron antineutrino:
: {{SubatomicParticle|link=yes|Neutron0}} → {{SubatomicParticle|link=yes|Proton+}} + {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}

Again, the neutron is not an elementary particle but a composite of an [[up quark]] and two [[down quark]]s ({{SubatomicParticle|Up quark}}{{SubatomicParticle|Down quark}}{{SubatomicParticle|Down quark}}). It is one of the down quarks that interacts in beta decay, turning into an up quark to form a proton ({{SubatomicParticle|Up quark}}{{SubatomicParticle|Up quark}}{{SubatomicParticle|Down quark}}). At the most fundamental level, then, the weak force changes the [[flavour (particle physics)|flavour]] of a single quark:
: {{SubatomicParticle|link=yes|Down quark}} → {{SubatomicParticle|link=yes|Up quark}} + {{SubatomicParticle|W boson-}}
which is immediately followed by decay of the {{SubatomicParticle|W boson-}} itself:
: {{SubatomicParticle|W boson-}} → {{SubatomicParticle|link=yes|Electron}} + {{SubatomicParticle|link=yes|Electron antineutrino}}

===Z bosons <span class="anchor" id="Z boson"></span>===
The {{SubatomicParticle|Z boson0}}&nbsp;boson is [[truly neutral particle|its own antiparticle]]. Thus, all of its [[flavour quantum numbers]] and [[charge (physics)|charges]] are zero. The exchange of a {{SubatomicParticle|Z boson}}&nbsp;boson between particles, called a [[neutral current]] interaction, therefore leaves the interacting particles unaffected, except for a transfer of spin and/or [[momentum]].{{efn|However, see [[Flavor-changing neutral current]] for a conjecture that a rare {{SubatomicParticle|Z boson}} exchange might cause flavor change.}}

{{SubatomicParticle|Z boson}}&nbsp;boson interactions involving [[neutrino]]s 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 {{SubatomicParticle|Z boson}}&nbsp;boson exchange) as inelastically (via W&nbsp;boson exchange).{{efn|name=Lopes|The first prediction of {{SubatomicParticle|Z boson}}&nbsp;bosons was made by Brazilian physicist [[José Leite Lopes]] in 1958,<ref name=Lopes1999>
{{cite journal
 |last=Lopes |first=J. Leite
 |date=September 1999
 |title=Forty years of the first attempt at the electroweak unification and of the prediction of the weak neutral boson
 |journal=Brazilian Journal of Physics
 |issn=0103-9733
 |volume=29 |issue=3 |pages=574–578
 |bibcode=1999BrJPh..29..574L
 |doi=10.1590/S0103-97331999000300024
 |doi-access=free
}}
</ref> by devising an equation which showed the analogy of the weak nuclear interactions with electromagnetism. Steve Weinberg, Sheldon Glashow, and Abdus Salam later used these results to develop the electroweak unification,<ref name=NobelPhys1979>
{{cite web
 |title=The Nobel Prize in Physics 1979
 |publisher=[[Nobel Foundation]]
 |url=https://www.nobelprize.org/prizes/physics/1979/summary/
}}
</ref> in 1973.}} Weak neutral currents via {{SubatomicParticle|Z boson}}&nbsp;boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in the [[Gargamelle]] [[bubble chamber]] at [[CERN]].<ref>
{{cite web
 |title=The discovery of the weak neutral currents
 |date=3 October 2004
 |publisher=CERN Courier
 |url=https://cerncourier.com/a/the-discovery-of-the-weak-neutral-currents/
 |url-status=live
 |archive-url=https://web.archive.org/web/20170307052419/http://cerncourier.com/cws/article/cern/29168/
 |archive-date=2017-03-07
}}
</ref>

==Predictions of the W{{sup|+}}, W{{sup|−}} and Z{{sup|0}} bosons==
[[File:Kaon-box-diagram.svg|thumb|right|A [[Feynman diagram]] showing the exchange of a pair of {{SubatomicParticle|W boson}}&nbsp;bosons. This is one of the leading terms contributing to neutral [[Kaon]] oscillation.]]
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]].<ref name=NobelPhys1979/><ref name=Lopes group=lower-alpha/> Their [[electroweak theory]] postulated not only the {{SubatomicParticle|W boson}}&nbsp;bosons necessary to explain beta decay, but also a new {{SubatomicParticle|Z boson}}&nbsp;boson that had never been observed.

The fact that the {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by an [[special unitary group#The group SU(2)|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 [[unitary group|U(1)]] gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}} in the process. The [[Higgs mechanism]], first put forward by the [[1964 PRL symmetry breaking papers]], fulfills this role. It requires the existence of another particle, the [[Higgs boson]], which has since been found at the [[Large Hadron Collider]]. Of the four components of a [[Goldstone boson]] created by the Higgs field, three are absorbed by the {{SubatomicParticle|W boson+}}, {{SubatomicParticle|Z boson0}}, and {{SubatomicParticle|W boson-}}&nbsp;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 [[Compact Muon Solenoid|CMS]] and [[ATLAS experiment|ATLAS]] experiments.

The model predicts that {{SubatomicParticle|W boson+-}} and {{SubatomicParticle|Z boson0}}&nbsp;bosons have the following masses:
: <math>\begin{align}
m_{\text{W}^\pm} &= \tfrac{1}{2}vg \\
m_{\text{Z}^0} &= \tfrac{1}{2} v\sqrt{g^2+{g'}^2}
\end{align}</math>
where <math>g</math> is the SU(2) gauge coupling, <math>g'</math> is the U(1) gauge coupling, and <math>v</math> is the Higgs [[vacuum expectation value]].

==Discovery==
[[File:CERN-20060225-24.jpg|thumb|The [[Gargamelle]] [[bubble chamber]], now exhibited at CERN]]
Unlike beta decay, the observation of neutral current interactions that involve particles {{em|other than neutrinos}} requires huge investments in [[particle accelerator]]s and [[particle detector]]s, such as are available in only a few [[high-energy physics]] laboratories in the world (and then only after 1983). This is because {{SubatomicParticle|Z boson}}&nbsp;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 {{SubatomicParticle|Z boson}}&nbsp;boson.

The discovery of the {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;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 produced by neutrino interactions and observed events where a neutrino interacted but did not produce a corresponding lepton. This is a hallmark of a neutral current interaction and is interpreted as a neutrino exchanging an unseen {{SubatomicParticle|Z boson}}&nbsp;boson with a proton or neutron in the bubble chamber. The neutrino is otherwise undetectable, so the only observable effect is the momentum imparted to the proton or neutron by the interaction.

The discovery of the {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;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]], where unambiguous signals of {{SubatomicParticle|W boson}}&nbsp;bosons were seen in January&nbsp;1983 during a series of experiments made possible by [[Carlo Rubbia]] and [[Simon van der Meer]]. The actual experiments were called [[UA1 experiment|UA1]] (led by Rubbia) and [[UA2 experiment|UA2]] (led by [[Pierre Darriulat]]),<ref name=Ref_b>
{{cite web
 |title=The UA2 Collaboration collection
 |url=http://library.web.cern.ch/library/Archives/isad/isaua2.html
 |access-date=2009-06-22
 |url-status=dead
 |archive-url=https://web.archive.org/web/20130604172721/http://library.web.cern.ch/library/Archives/isad/isaua2.html
 |archive-date=2013-06-04
}}
</ref> 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 {{SubatomicParticle|Z boson}}&nbsp;boson a few months later, in May&nbsp;1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in Physics, a most unusual step for the conservative [[Nobel Prize|Nobel Foundation]].
<ref name=NobelPhys1984>
{{cite press release
 |title=Nobel Prize in Physics 1984
 |publisher=Nobel Foundation
 |url=https://www.nobelprize.org/prizes/physics/1984/summary/
}}</ref>

The {{SubatomicParticle|W boson+}}, {{SubatomicParticle|W boson-}}, and {{SubatomicParticle|Z boson0}}&nbsp;bosons, together with the photon ({{SubatomicParticle|Photon}}), comprise the four [[gauge boson]]s of the [[electroweak interaction]].

==Measurements of W boson mass==
In May 2024, the [[Particle Data Group]] estimated the World Average mass for the W boson to be 80369.2 ± 13.3 MeV, based on experiments to date.<ref name=":1" />   

As of 2021, experimental measurements of the W boson mass had been similarly assessed to converge around {{val|80379|12|u=MeV}},<ref>P.A. Zyla et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2020, 083C01 (2021) and 2021 update. https://pdg.lbl.gov/2021/reviews/rpp2021-rev-w-mass.pdf</ref> all consistent with one another and with the Standard Model. 

In April 2022, a new analysis of historical data from  the [[Fermilab]] [[Tevatron]] collider before its closure in 2011 determined the mass of the W boson to be {{val|80433|9|u=MeV}}, which was seven standard deviations above that predicted by the Standard Model.<ref name="abc">{{cite news |author=Weule, Genelle |date=8 April 2022 |title=Standard Model of physics challenged by most precise measurement of W boson particle yet |url=https://www.abc.net.au/news/science/2022-04-08/standard-model-of-physics-challenged-by-w-boson-measurement/100964330 |accessdate=9 April 2022 |publisher=Australian Broadcasting Corporation}}</ref> Besides being inconsistent with the Standard Model, the new measurement was also inconsistent with previous measurements such as ATLAS. This suggests that either the old or the new measurements had an unexpected systematic error, such as an undetected quirk in the equipment.<ref name="quanta">{{cite news |author=Wood, Charlie |date=7 April 2022 |title=Newly Measured Particle Seems Heavy Enough to Break Known Physics |url=https://www.quantamagazine.org/fermilab-says-particle-is-heavy-enough-to-break-the-standard-model-20220407/ |accessdate=9 April 2022 |work=[[Quanta Magazine]]}}</ref> This led to careful reevaluation of this data analysis and other historical measurement, as well as the planning of future measurements to confirm the potential new result. Fermilab Deputy Director [[Joseph Lykken]] reiterated that "... the (new) measurement needs to be confirmed by another experiment before it can be interpreted fully."<ref>{{cite news |title=CDF collaboration at Fermilab announces most precise ever measurement of W boson mass to be in tension with the Standard Model |author=Marc, Tracy |work=[[Fermilab]] |date=7 April 2022 |accessdate=8 April 2022 |url=https://news.fnal.gov/2022/04/cdf-collaboration-at-fermilab-announces-most-precise-ever-measurement-of-w-boson-mass/}}</ref><ref>{{cite web |last=Schott |first=Matthias |date=2022-04-07 |title=Do we have finally found new physics with the latest W boson mass measurement? |website=Physics, Life and all the Rest |url=https://non-trivial-solution.blogspot.com/2022/04/do-we-have-finally-found-new-physics.html |access-date=2022-04-09}}</ref> 

In 2023, an improved ATLAS experiment measured the W boson mass at {{val|80360|16|u=MeV}}, aligning with predictions from the Standard Model.<ref>{{cite web |last1=Ouellette |first1=Jennifer |title=New value for W boson mass dims 2022 hints of physics beyond Standard Model |website=Ars Technica |date=24 March 2023 |url=https://arstechnica.com/science/2023/03/new-value-for-w-boson-mass-dims-2022-hints-of-physics-beyond-standard-model/ |access-date=26 March 2023}}</ref><ref name="atlas2023">{{cite web |title=Improved W boson Mass Measurement using $\sqrt{s}=7$ TeV Proton-Proton Collisions with the ATLAS Detector |url=https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2023-004/ |website=ATLAS experiment |publisher=CERN |date=22 March 2023 |access-date=26 March 2023}}</ref>

The Particle Data Group convened a working group on the Tevatron measurement of W boson mass, including W-mass experts from all hadron collider experiments to date, to understand the discrepancy.<ref name=":0">S. Navas et al.(Particle Data Group), Phys. Rev. D110, 030001 (2024)</ref> In May 2024 they concluded that the CDF measurement was an outlier, and the best estimate of the mass came from leaving out that measurement from the meta-analysis. "The corresponding value of the W boson mass is mW = 80369.2 ± 13.3 MeV, which we quote as the World Average."<ref name=":0" /><ref>M. Grünewald (University Coll. Dublin) and A. Gurtu (CERN; TIFR Mumbai) (PDG April 2024) ''Mass and Width of the W Boson''; https://pdg.lbl.gov/2024/reviews/rpp2024-rev-w-mass.pdf</ref><ref name=":1">{{Cite journal |date=18 Aug 2023 |title=Compatibility and combination of world W-boson mass measurements |arxiv=2308.09417 |last1=Amoroso |first1=Simone |last2=Andari |first2=Nansi |last3=Barter |first3=William |last4=Bendavid |first4=Josh |last5=Boonekamp |first5=Maarten |last6=Farry |first6=Stephen |last7=Gruenewald |first7=Martin |last8=Hays |first8=Chris |last9=Hunter |first9=Ross |last10=Kretzschmar |first10=Jan |last11=Lupton |first11=Oliver |last12=Pili |first12=Martina |author13=Miguel Ramos Pernas |last14=Tuchming |first14=Boris |last15=Vesterinen |first15=Mika |last16=Vicini |first16=Alessandro |last17=Wang |first17=Chen |last18=Xu |first18=Menglin |journal=European Physical Journal C |volume=84 |issue=5 |page=451 |doi=10.1140/epjc/s10052-024-12532-z |bibcode=2024EPJC...84..451L }}</ref> 

In September 2024, the CMS experiment measured the W boson mass at 80 360.2 ± 9.9 MeV.  This was the most precise measurement to date, obtained from observations of a large number of <math>\mathrm{W}\to\mu\nu</math> decays.<ref>{{Cite journal |last=CMS collaboration |first= |date=17 September 2024 |title=Measurement of the W boson mass in proton-proton collisions at √s=13 TeV |url=https://cms-results.web.cern.ch/cms-results/public-results/preliminary-results/SMP-23-002/index.html |journal=CMS document server}}</ref><ref>{{Cite web |title=CMS delivers the best-precision measurement of the W boson mass at the LHC {{!}} CMS Experiment |url=https://cms.cern/news/cms-delivers-best-precision-measurement-w-boson-mass-lhc |access-date=2024-09-20 |website=cms.cern}}</ref><ref>{{Cite web |date=2024-09-17 |title=New results from the CMS experiment put W boson mass mystery to rest {{!}} symmetry magazine |url=https://www.symmetrymagazine.org/article/new-results-from-the-cms-experiment-put-w-boson-mass-mystery-to-rest?language_content_entity=und |access-date=2024-09-20 |website=www.symmetrymagazine.org |language=en}}</ref>

==Decay==
The {{SubatomicParticle|W boson}} and {{SubatomicParticle|Z boson}}&nbsp;bosons decay to [[fermion]] pairs but neither the {{SubatomicParticle|W boson}} nor the {{SubatomicParticle|Z boson}}&nbsp;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]]s can be calculated from the [[coupling constant]]s.

===W bosons===
'''{{SubatomicParticle|W boson}}&nbsp;bosons''' can decay to a [[lepton]] and antilepton (one of them charged and another neutral){{efn|name="leptonic"|Specifically:<br>{{nowrap|{{SubatomicParticle|W boson-}} → charged lepton + antineutrino}}<br>{{nowrap|{{SubatomicParticle|W boson+}} → charged antilepton + neutrino}}}} or to a [[quark]] and antiquark [[quark#Electric charge|of complementary types]] (with opposite electric charges {{sfrac|±|1|3}} and {{sfrac|∓|2|3}}). The [[decay width]] of the W&nbsp;boson to a quark–antiquark pair is proportional to the corresponding squared [[Cabibbo–Kobayashi–Maskawa matrix|CKM matrix]] element and the number of quark [[color charge|colours]], {{nowrap|{{mvar|N}}{{sub|C}} {{=}} 3 .}} The decay widths for the W{{sup|+}}&nbsp;boson are then proportional to:
: {| class="wikitable" style="text-align:center;"
!colspan="2" width="100"|Leptons
!colspan="6" width="100"|Quarks
|-
| {{SubatomicParticle|Electron+}}{{SubatomicParticle|Electron neutrino}}
| 1
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Down antiquark}}
| 3 <math>|V_\text{ud}|^2</math>
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Strange antiquark}}
| 3 <math>|V_\text{us}|^2</math>
| {{SubatomicParticle|Up quark}}{{SubatomicParticle|Bottom antiquark}}
| 3 <math>|V_\text{ub}|^2</math>
|-
| {{SubatomicParticle|Muon+}}{{SubatomicParticle|Muon neutrino}}
| 1
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Down antiquark}}
| 3 <math>|V_\text{cd}|^2</math>
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Strange antiquark}}
| 3 <math>|V_\text{cs}|^2</math>
| {{SubatomicParticle|Charm quark}}{{SubatomicParticle|Bottom antiquark}}
| 3 <math>|V_\text{cb}|^2</math>
|-
| {{SubatomicParticle|Tauon+}}{{SubatomicParticle|Tauon neutrino}}
| 1
|colspan="6"|''Energy conservation forbids decay to'' {{SubatomicParticle|Top quark}}.
|-
|}

Here, {{SubatomicParticle|Electron+}}, {{SubatomicParticle|Muon+}}, {{SubatomicParticle|Tauon+}} denote the three flavours of [[lepton]]s (more exactly, the positive charged [[antiparticle|antilepton]]s). {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tauon neutrino}} denote the three flavours of neutrinos. The other particles, starting with {{SubatomicParticle|Up quark}} and {{SubatomicParticle|Down antiquark}}, all denote [[quark]]s and antiquarks (factor {{mvar|N}}{{sub|C}} is applied). The various <math>\, V_{ij} \,</math> denote the corresponding [[Cabibbo–Kobayashi–Maskawa matrix|CKM matrix]] coefficients.{{efn|Every entry in the lepton column can also be written as three decays, e.g. for the first row, as {{SubatomicParticle|electron+}}{{SubatomicParticle|neutrino}}<sub>1</sub>, {{SubatomicParticle|electron+}}{{SubatomicParticle|neutrino}}<sub>2</sub>, {{SubatomicParticle|electron+}}{{SubatomicParticle|neutrino}}<sub>3</sub>, for every neutrino mass eigenstate, with decay widths proportional to <math>\, |U_\text{e1}|^2 \, ,</math><math>\, |U_\text{e2}|^2\, ,</math><math>\, |U_\text{e3}|^2 \,</math> ([[Pontecorvo–Maki–Nakagawa–Sakata matrix|PMNS matrix]] elements), but experiments at present that measure the decays can't discriminate between neutrino mass eigenstates: They measure total decay width of the sum of all three processes.}}

[[Unitary matrix|Unitarity]] of the CKM matrix implies that
<math>~ |V_\text{ud}|^2 + |V_\text{us}|^2 + |V_\text{ub}|^2 ~ = </math>
<math>~|V_\text{cd}|^2 + |V_\text{cs}|^2 + |V_\text{cb}|^2 = 1 ~,</math> thus each of two quark rows {{nowrap|sums to 3.}} Therefore, the leptonic [[branching ratio]]s of the {{SubatomicParticle|W boson}}&nbsp;boson are approximately <math>\, B( \mathrm{e}^{+} \mathrm{\nu}_\mathrm{e}) = \,</math><math>\, B(\mathrm{\mu}^{+} \mathrm{\nu}_\mathrm{\mu}) = \,</math><math>\, B(\mathrm{\tau}^{+} \mathrm{\nu}_\mathrm{\tau}) = \,</math> {{sfrac|1|9}}. The hadronic branching ratio is dominated by the CKM-favored {{SubatomicParticle|Up quark}}{{SubatomicParticle|down antiquark}} and {{SubatomicParticle|Charm quark}}{{SubatomicParticle|strange antiquark}} final states. The sum of the [[hadron]]ic branching ratios has been measured experimentally to be {{val|67.60|0.27|s=%}}, with {{nowrap|<math>\, B( \ell^{+} \mathrm{\nu}_\ell ) = \,</math> {{val|10.80|0.09|s=%}}.}}<ref name="Beringer2012">
{{cite journal
 |last1=Beringer |first1=J. |display-authors=etal
 |collaboration=[[Particle Data Group]]
 |year=2012
 |series=2012 Review of Particle Physics
 |title=Gauge and Higgs bosons
 |journal=[[Physical Review D]]
 |volume=86 |issue=1
 |page=1
 |bibcode=2012PhRvD..86a0001B
 |doi=10.1103/PhysRevD.86.010001
 |doi-access=free
 |url=https://pdg.lbl.gov/2013/listings/rpp2013-list-w-boson.pdf
 |access-date=2013-10-21
 |url-status=live
 |archive-url=https://web.archive.org/web/20170220033253/http://pdg.lbl.gov/2013/listings/rpp2013-list-w-boson.pdf
 |archive-date=2017-02-20
}}
</ref>

===Z<sup>0</sup> boson===
{{See also|Weak charge}}
'''{{SubatomicParticle|Z boson}}&nbsp;bosons''' decay into a fermion and its antiparticle. As the {{SubatomicParticle|Z boson0}}&nbsp;boson is a mixture of the pre-[[spontaneous symmetry breaking|symmetry-breaking]] {{SubatomicParticle|W boson0}} and {{SubatomicParticle|B boson0}}&nbsp;bosons (see [[Weinberg angle|weak mixing angle]]), each [[Feynman diagram|vertex factor]] includes a factor <math>~ T_3 - Q \sin^2 \,\theta_\mathsf{W} ~,</math> where <math>\, T_3 \,</math> is the third component of the [[weak isospin]] of the fermion (the "charge" for the weak force), <math>\, Q \,</math> is the [[electric charge]] of the fermion (in units of the [[elementary charge]]), and <math>\; \theta_\mathsf{w} \;</math> is the [[Weinberg angle|weak mixing angle]]. Because the weak isospin <math>(\, T_3 \,)</math> is different for fermions of different [[chirality (physics)|chirality]], either [[Standard Model (mathematical formulation)#Right handed singlets, left handed doublets|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]]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]].

: {| class="wikitable" style="text-align:center;"
!colspan=2 | Particles
!colspan=2 | [[Weak isospin]] <math>(\, T_3 \,)</math>
!rowspan=2 | Relative factor
!colspan=2 | Branching ratio
|-
! Name
! Symbols
! {{sc|left}}
! {{sc|right}}
! Predicted for {{mvar|x}}&nbsp;=&nbsp;0.23
! Experimental measurements<ref name="Ref_e">
{{cite web
 |last1=Amsler |first1=C. |display-authors=etal
 |collaboration=Particle Data Group
 |year=2010
 |title=PL B667, 1 (2008), and 2009 partial update for the 2010 edition
 |url=https://pdg.lbl.gov/2009/tables/rpp2009-sum-gauge-higgs-bosons.pdf
 |access-date=2010-05-19
 |url-status=live
 |archive-url=https://web.archive.org/web/20110605110249/http://pdg.lbl.gov/2009/tables/rpp2009-sum-gauge-higgs-bosons.pdf
 |archive-date=2011-06-05
}}
</ref>
|-
| align="left" | '''Neutrinos''' (all)
| {{SubatomicParticle|Electron neutrino}}, {{SubatomicParticle|Muon neutrino}}, {{SubatomicParticle|Tauon neutrino}}
| {{sfrac|1|2}}
| 0&nbsp;{{efn|name=wronghanded}}
| {{nowrap|3 ({{sfrac|1|2}}){{sup|2}}}}
| {{val|20.5|s=%}}
| {{val|20.00|0.06|s=%}}
|-
| align="left" | '''Charged leptons''' (all)
| {{SubatomicParticle|Electron}}, {{SubatomicParticle|Muon}}, {{SubatomicParticle|Tauon}}
|colspan=2 |
| {{nowrap|3 (&minus;{{sfrac|1|2}} + {{mvar|x}}){{sup|2}} }}{{nowrap|+ 3{{mvar|x}}{{sup|2}}}}
| {{val|10.2|s=%}}
| {{val|10.097|0.003|s=%}}
|-
| align="right" | Electron
| {{SubatomicParticle|Electron}}
| &minus;{{sfrac|1|2}} + {{mvar|x}}
| {{mvar|x}}
| {{nowrap|(&minus;{{sfrac|1|2}} + {{mvar|x}}){{sup|2}} }}{{nowrap|+ {{mvar|x}}{{sup|2}}}}
| {{val|3.4|s=%}}
| {{val|3.363|0.004|s=%}}
|-
| align="right" | Muon
| {{SubatomicParticle|Muon}}
| &minus;{{sfrac|1|2}} + {{mvar|x}}
| {{mvar|x}}
| {{nowrap|(&minus;{{sfrac|1|2}} + {{mvar|x}}){{sup|2}} }}{{nowrap|+ {{mvar|x}}{{sup|2}}}}
| {{val|3.4|s=%}}
| {{val|3.366|0.007|s=%}}
|-
| align="right" | Tau
| {{SubatomicParticle|Tauon}}
| {{nowrap|&minus;{{sfrac|1|2}} + {{mvar|x}}}}
| {{mvar|x}}
| {{nowrap|(&minus;{{sfrac|1|2}} + {{mvar|x}}){{sup|2}} }}{{nowrap|+ {{mvar|x}}{{sup|2}}}}
| {{val|3.4|s=%}}
| {{val|3.367|0.008|s=%}}
|-
| align="left" | '''Hadrons'''
|colspan=4|
| {{val|69.2|s=%}}
| {{val|69.91|0.06|s=%}}
|-
| align="right" | Down-type quarks
| {{SubatomicParticle|Down quark}}, {{SubatomicParticle|Strange quark}}, {{SubatomicParticle|Bottom quark}}
| {{nowrap|&minus;{{sfrac|1|2}} + {{sfrac|1|3}}{{mvar|x}}}}
| {{sfrac|1|3}}{{mvar|x}}
| {{nowrap|3 (&minus;{{sfrac|1|2}} + {{sfrac|1|3}}{{mvar|x}}){{sup|2}} }}{{nowrap|+ 3 ({{sfrac|1|3}}{{mvar|x}}){{sup|2}}}}
| {{val|15.2|s=%}}
| {{val|15.6|0.4|s=%}}
|-
| align="right" | Up-type quarks<br>({{nowrap|'''*''' except}} {{SubatomicParticle|Top quark}})
| {{SubatomicParticle|Up quark}}, {{SubatomicParticle|Charm quark}}
| {{sfrac|1|2}} − {{sfrac|2|3}}{{mvar|x}}
| &minus;{{sfrac|2|3}}{{mvar|x}}
| {{nowrap|3 ({{sfrac|1|2}} − {{sfrac|2|3}}{{mvar|x}}){{sup|2}} }}{{nowrap|+ 3 (&minus;{{sfrac|2|3}}{{mvar|x}}){{sup|2}}}}
| {{val|11.8|s=%}}
| {{val|11.6|0.6|s=%}}
|}
:: To keep the notation compact, the table uses <math>~ x = \sin^2 \,\theta_\text{W} ~.</math>
:: '''<nowiki>*</nowiki>''' The impossible decay into a [[top quark]]–antiquark pair is left out of the table.{{efn| The mass of the {{SubatomicParticle|Top quark}} quark plus a {{SubatomicParticle|top antiquark}} is greater than the mass of the {{SubatomicParticle|Z boson}}&nbsp;boson, so it does not have sufficient energy to decay into a {{SubatomicParticle|Top quark}}{{SubatomicParticle|Top antiquark}} quark pair.}}
:: Subheadings '''{{sc|left}}''' and '''{{sc|right}}''' denote the [[chirality (physics)|chirality]] or "handedness" of the fermions.{{efn|name=wronghanded|In the Standard Model, right-handed neutrinos (and left-handed anti-neutrinos) do not exist; however, some extensions beyond the Standard Model allow them. If they do exist, they all have [[weak isospin|isospin]] {{mvar|T}}{{sub|3}}&nbsp;{{=}}&nbsp;0 and electric charge {{mvar|Q}}&nbsp;{{=}}&nbsp;0, and with [[color charge]] also zero. The all-zero charges make them [[sterile neutrino|"sterile"]], i.e. unable to interact by either the weak or electric forces no strong-force interactions either.}}

In 2018, the CMS collaboration observed the first exclusive decay of the {{SubatomicParticle|Z boson}}&nbsp;boson to a [[J/psi meson|ψ meson]] and a [[lepton]]–antilepton pair.<ref>
{{cite journal
 |last1=Sirunyan |first1=A.M. |display-authors=etal
 |collaboration=CMS Collaboration
 |year=2018
 |title=Observation of the {{nowrap |{{SubatomicParticle |Z boson}} → ψ ℓ+ ℓ−}} decay in {{SubatomicParticle|Proton}}{{SubatomicParticle|Proton}} collisions at {{radic|s}} {{=}} 13&nbsp;TeV
 |journal=Physical Review Letters
 |volume=121 |issue=14 |page=141801
 |arxiv=1806.04213 |pmid=30339440 |s2cid=118950363
 |doi=10.1103/PhysRevLett.121.141801
 |url=https://inspirehep.net/literature/1677496
}}
</ref>

==See also==
* {{Annotated link|Bose–Einstein statistics}}
* [[List of particles]]
* {{Annotated link|Mathematical formulation of the Standard Model}}
* [[Weak charge]]
* {{Annotated link|W′ and Z′ bosons}}
* {{Annotated link|X and Y bosons}}: analogous pair of bosons predicted by the [[Grand Unified Theory]]
* {{Annotated link|ZZ diboson}}

==Footnotes==
{{Notelist}}

==References==
{{Reflist|25em}}

==External links==
* {{Commons category-inline}}
* [https://pdg.lbl.gov/ The Review of Particle Physics], the ultimate source of information on particle properties.
* [https://cerncourier.com/a/the-w-and-z-particles-a-personal-recollection/ The W and Z particles: a personal recollection] by Pierre Darriulat
* [https://cerncourier.com/a/when-cern-saw-the-end-of-the-alphabet/ When CERN saw the end of the alphabet] by Daniel Denegri
* [http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/expar.html#c4 W and Z particles at Hyperphysics]

{{Particles}}
{{Authority control}}

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