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W and Z bosons

From Wikipedia, the free encyclopedia
(Redirected fromZ boson)
Bosons that mediate the weak interaction

W±
 and
Z0
 Bosons
CompositionElementary particle
StatisticsBosonic
FamilyGauge boson
InteractionsWeak interaction
TheorizedGlashow,Weinberg,Salam (1968)
DiscoveredUA1 andUA2 collaborations,CERN, 1983
MassW: 80.377±0.012 GeV/c2 (2022)[1][2]
Z: 91.1876±0.0021 GeV/c2[3]
Decay widthW: 2.085±0.042 GeV[1]
Z: 2.4952±0.0023 GeV[3]
Electric chargeW: ±1 e
Z: 0 e
Spin1 ħ
Weak isospinW: ±1
Z: 0
Weak hypercharge0
Standard Model ofparticle physics
Elementary particles of the Standard Model

Inparticle physics, theW and Z bosons arevector bosons that are together known as theweak bosons or more generally as theintermediate vector bosons. Theseelementary particlesmediate theweak interaction; the respective symbols are
W+
,
W−
, and
Z0
. The
W±
 bosons have either a positive or negativeelectric charge of 1elementary charge and are each other'santiparticles. The
Z0
 boson is electricallyneutral and is its own antiparticle. The three particles each have aspin of 1. The
W±
 bosons have a magnetic moment, but the
Z0
 has none. All three of these particles are very short-lived, with ahalf-life of about3×10−25 s. Their experimental discovery was pivotal in establishing what is now called theStandard Model ofparticle physics.

The
W
 bosons are named after theweak force. ThephysicistSteven Weinberg named the additional particle the "
Z
 particle",[4] and later gave the explanation that it was the last additional particle needed by the model. The
W
 bosons had already been named, and the
Z
 bosons were named for havingzero electric charge.[5]

The two
W
 bosons are verified mediators ofneutrino absorption and emission. During these processes, the
W±
 boson charge induces electron or positron emission or absorption, thus causingnuclear transmutation.

The
Z
 boson mediates the transfer of momentum, spin and energy when neutrinos scatterelastically from matter (a process which conserves charge). Such behavior is almost as common as inelastic neutrino interactions and may be observed inbubble chambers upon irradiation with neutrino beams. The
Z
 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.[a]

Basic properties

[edit]

These bosons are among the heavyweights of the elementary particles. Withmasses of80.4 GeV/c2 and91.2 GeV/c2, respectively, the
W
 and
Z
 bosons are almost 80 times as massive as theproton – heavier, even, than entireironatoms.

Their high masses limit the range of the weak interaction. By way of contrast, thephoton is theforce carrier of the electromagnetic force and has zero mass, consistent with the infinite range ofelectromagnetism; the hypotheticalgraviton is also expected to have zero mass. (Althoughgluons are also presumed to have zero mass, the range of thestrong nuclear force is limited for different reasons;seeColor confinement.)

All three bosons haveparticle spins = 1. The emission of a
W+
 or
W−
 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
W±
 boson can change the type of the particle – for example changing astrange quark into anup quark. The neutral Z boson cannot change the electric charge of any particle, nor can it change any other of the so-called "charges" (such asstrangeness,baryon number,charm, etc.). The emission or absorption of a
Z0
 boson can only change the spin, momentum, and energy of the other particle. (See alsoWeak neutral current.)

Relations to the weak nuclear force

[edit]
TheFeynman diagram for beta decay of a neutron into a proton, electron, and electron antineutrino via an intermediate
W−
 boson

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

W bosons

[edit]

The
W±
 bosons are best known for their role innuclear decay. Consider, for example, thebeta decay ofcobalt-60.

60
27Co
 →60
28Ni
+ +
e−
 +
ν
e

This reaction does not involve the whole cobalt-60nucleus, but affects only one of its 33 neutrons. The neutron is converted into a proton while also emitting an electron (often called abeta particle in this context) and an electron antineutrino:


n0
 →
p+
 +
e−
 +
ν
e

Again, the neutron is not an elementary particle but a composite of anup quark and twodown quarks (
u

d

d
). It is one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (
u

u

d
). At the most fundamental level, then, the weak force changes theflavour of a single quark:


d
 →
u
 +
W−

which is immediately followed by decay of the
W−
 itself:


W−
 →
e−
 +
ν
e

Z bosons

[edit]

The
Z0
 boson isits own antiparticle. Thus, all of itsflavour quantum numbers andcharges are zero. The exchange of a
Z
 boson between particles, called aneutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of spin and/ormomentum.[b]


Z
 boson interactions involvingneutrinos have distinct signatures: They provide the only known mechanism forelastic scattering of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via
Z
 boson exchange) as inelastically (via W boson exchange).[c] Weak neutral currents via
Z
 boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in theGargamellebubble chamber atCERN.[8]

Predictions of the W+, W− and Z0 bosons

[edit]
AFeynman diagram showing the exchange of a pair of
W
 bosons. This is one of the leading terms contributing to neutralKaon oscillation.

Following the success ofquantum 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 bySheldon Glashow,Steven Weinberg, andAbdus Salam, for which they shared the 1979Nobel Prize in Physics.[7][c] Theirelectroweak theory postulated not only the
W
 bosons necessary to explain beta decay, but also a new
Z
 boson that had never been observed.

The fact that the
W
 and
Z
 bosons have mass while photons are massless was a major obstacle in developing electroweak theory. These particles are accurately described by anSU(2)gauge theory, but the bosons in a gauge theory must be massless. As a case in point, thephoton is massless because electromagnetism is described by aU(1) gauge theory. Some mechanism is required to break the SU(2) symmetry, giving mass to the
W
 and
Z
 in the process. TheHiggs mechanism, first put forward by the1964 PRL symmetry breaking papers, fulfills this role. It requires the existence of another particle, theHiggs boson, which has since been found at theLarge Hadron Collider. Of the four components of aGoldstone boson created by the Higgs field, three are absorbed by the
W+
,
Z0
, and
W−
 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 theGlashow–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 theCMS andATLAS experiments.

The model predicts that
W±
 and
Z0
 bosons have the following masses:

mW±=12vgmZ0=12vg2+g′2{\displaystyle {\begin{aligned}m_{{\text{W}}^{\pm }}&={\tfrac {1}{2}}vg\\m_{{\text{Z}}^{0}}&={\tfrac {1}{2}}v{\sqrt {g^{2}+{g'}^{2}}}\end{aligned}}}

whereg{\displaystyle g} is the SU(2) gauge coupling,g′{\displaystyle g'} is the U(1) gauge coupling, andv{\displaystyle v} is the Higgsvacuum expectation value.

Discovery

[edit]
TheGargamellebubble chamber, now exhibited at CERN

Unlike beta decay, the observation of neutral current interactions that involve particlesother than neutrinos requires huge investments inparticle accelerators andparticle detectors, such as are available in only a fewhigh-energy physics laboratories in the world (and then only after 1983). This is because
Z
 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
Z
 boson.

The discovery of the
W
 and
Z
 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
Z
 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
W
 and
Z
 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 theSuper Proton Synchrotron, where unambiguous signals of
W
 bosons were seen in January 1983 during a series of experiments made possible byCarlo Rubbia andSimon van der Meer. The actual experiments were calledUA1 (led by Rubbia) andUA2 (led byPierre Darriulat),[9] 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
Z
 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 conservativeNobel Foundation.[10]

The
W+
,
W−
, and
Z0
 bosons, together with the photon (
γ
), comprise the fourgauge bosons of theelectroweak interaction.

Measurements of W boson mass

[edit]

In May 2024, theParticle Data Group estimated the World Average mass for the W boson to be 80369.2 ± 13.3 MeV, based on experiments to date.[11]

As of 2021, experimental measurements of the W boson mass had been similarly assessed to converge around80379±12 MeV,[12] all consistent with one another and with the Standard Model.

In April 2022, a new analysis of historical data from theFermilabTevatron collider before its closure in 2011 determined the mass of the W boson to be80433±9 MeV, which was seven standard deviations above that predicted by the Standard Model.[13] 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.[14] 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 DirectorJoseph Lykken reiterated that "... the (new) measurement needs to be confirmed by another experiment before it can be interpreted fully."[15][16]

In 2023, an improved ATLAS experiment measured the W boson mass at80360±16 MeV, aligning with predictions from the Standard Model.[17][18]

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.[19] 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."[19][20][11]

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 ofW→μν{\displaystyle \mathrm {W} \to \mu \nu } decays.[21][22][23]

Decay

[edit]

The
W
 and
Z
 bosons decay tofermion pairs but neither the
W
 nor the
Z
 bosons have sufficient energy to decay into the highest-masstop quark. Neglecting phase space effects and higher order corrections, simple estimates of theirbranching fractions can be calculated from thecoupling constants.

W bosons

[edit]


W
 bosons can decay to alepton and antilepton (one of them charged and another neutral)[d] or to aquark and antiquarkof complementary types (with opposite electric charges⁠±+1/3⁠ and⁠∓+2/3⁠). Thedecay width of the W boson to a quark–antiquark pair is proportional to the corresponding squaredCKM matrix element and the number of quarkcolours,NC = 3 . The decay widths for the W+ boson are then proportional to:

LeptonsQuarks

e+

ν
e		1
u

d
3|Vud|2{\displaystyle |V_{\text{ud}}|^{2}}
u

s
3|Vus|2{\displaystyle |V_{\text{us}}|^{2}}
u

b
3|Vub|2{\displaystyle |V_{\text{ub}}|^{2}}

μ+

ν
μ		1
c

d
3|Vcd|2{\displaystyle |V_{\text{cd}}|^{2}}
c

s
3|Vcs|2{\displaystyle |V_{\text{cs}}|^{2}}
c

b
3|Vcb|2{\displaystyle |V_{\text{cb}}|^{2}}

τ+

ν
τ		1Energy conservation forbids decay to
t
.

Here,
e+
,
μ+
,
τ+
 denote the three flavours ofleptons (more exactly, the positive chargedantileptons).
ν
e,
ν
μ,
ν
τ denote the three flavours of neutrinos. The other particles, starting with
u
 and
d
, all denotequarks and antiquarks (factorNC is applied). The variousVij{\displaystyle \,V_{ij}\,} denote the correspondingCKM matrix coefficients.[e]

Unitarity of the CKM matrix implies that |Vud|2+|Vus|2+|Vub|2 ={\displaystyle ~|V_{\text{ud}}|^{2}+|V_{\text{us}}|^{2}+|V_{\text{ub}}|^{2}~=} |Vcd|2+|Vcs|2+|Vcb|2=1 ,{\displaystyle ~|V_{\text{cd}}|^{2}+|V_{\text{cs}}|^{2}+|V_{\text{cb}}|^{2}=1~,} thus each of two quark rowssums to 3. Therefore, the leptonicbranching ratios of the
W
 boson are approximatelyB(e+νe)={\displaystyle \,B(\mathrm {e} ^{+}\mathrm {\nu } _{\mathrm {e} })=\,}B(μ+νμ)={\displaystyle \,B(\mathrm {\mu } ^{+}\mathrm {\nu } _{\mathrm {\mu } })=\,}B(τ+ντ)={\displaystyle \,B(\mathrm {\tau } ^{+}\mathrm {\nu } _{\mathrm {\tau } })=\,}⁠1/9⁠. The hadronic branching ratio is dominated by the CKM-favored
u

d
 and
c

s
 final states. The sum of thehadronic branching ratios has been measured experimentally to be67.60±0.27%, withB(ℓ+νℓ)={\displaystyle \,B(\ell ^{+}\mathrm {\nu } _{\ell })=\,}10.80±0.09%.[24]

Z0 boson

[edit]
See also:Weak charge


Z
 bosons decay into a fermion and its antiparticle. As the
Z0
 boson is a mixture of the pre-symmetry-breaking
W0
 and
B0
 bosons (seeweak mixing angle), eachvertex factor includes a factor T3−Qsin2θW ,{\displaystyle ~T_{3}-Q\sin ^{2}\,\theta _{\mathsf {W}}~,} whereT3{\displaystyle \,T_{3}\,} is the third component of theweak isospin of the fermion (the "charge" for the weak force),Q{\displaystyle \,Q\,} is theelectric charge of the fermion (in units of theelementary charge), andθw{\displaystyle \;\theta _{\mathsf {w}}\;} is theweak mixing angle. Because the weak isospin(T3){\displaystyle (\,T_{3}\,)} is different for fermions of differentchirality, eitherleft-handed or right-handed, the coupling is different as well.

Therelative strengths of each coupling can be estimated by considering that thedecay rates 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 theFermi theory.

ParticlesWeak isospin(T3){\displaystyle (\,T_{3}\,)}    Relative factor    Branching ratio
NameSymbolsLEFTRIGHTPredicted forx = 0.23Experimental measurements[25]
Neutrinos (all)
ν
e,
ν
μ,
ν
τ		⁠ 1 / 2⁠0 [f]3 (⁠ 1 / 2⁠)220.5%20.00±0.06%
Charged leptons (all)
e−
,
μ−
,
τ−
3 (−⁠ 1 / 2⁠ +x)2+ 3x210.2%10.097±0.003%
Electron
e−
−⁠ 1 / 2⁠ +xx(−⁠ 1 / 2⁠ +x)2+x23.4%3.363±0.004%
Muon
μ−
−⁠ 1 / 2⁠ +xx(−⁠ 1 / 2⁠ +x)2+x23.4%3.366±0.007%
Tau
τ−
−⁠ 1 / 2⁠ +xx(−⁠ 1 / 2⁠ +x)2+x23.4%3.367±0.008%
Hadrons69.2%69.91±0.06%
Down-type quarks
d
,
s
,
b
−⁠ 1 / 2⁠ +⁠ 1 / 3⁠x⁠1/3⁠x3 (−⁠ 1 / 2⁠ +⁠ 1 / 3⁠x)2+ 3 (⁠ 1 / 3⁠x)215.2%15.6±0.4%
Up-type quarks
(* except
t
)
u
,
c
⁠+ 1 / 2⁠ −⁠ 2 / 3⁠x−⁠ 2 / 3⁠x3 (⁠+ 1 / 2⁠ −⁠ 2 / 3⁠x)2+ 3 (−⁠ 2 / 3⁠x)211.8%11.6±0.6%
To keep the notation compact, the table uses x=sin2θW .{\displaystyle ~x=\sin ^{2}\,\theta _{\text{W}}~.}
* The impossible decay into atop quark–antiquark pair is left out of the table.[g]
SubheadingsLEFT andRIGHT denote thechirality or "handedness" of the fermions.[f]

In 2018, the CMS collaboration observed the first exclusive decay of the
Z
 boson to aψ meson and alepton–antilepton pair.[26]

See also

[edit]

Footnotes

[edit]
  1. ^Because neutrinos are neither affected by thestrong force nor theelectromagnetic force, and because thegravitational force between subatomic particles is negligible, bydeduction (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 forceboson. Thus, since no other neutrino-interacting neutral force carrier is known, the observed interaction must have occurred by exchange of a
    Z0
     boson.
  2. ^However, seeFlavor-changing neutral current for a conjecture that a rare
    Z
     exchange might cause flavor change.
  3. ^abThe first prediction of
    Z
     bosons was made by Brazilian physicistJosé Leite Lopes in 1958,[6] 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,[7] in 1973.
  4. ^Specifically:

    W−
     → charged lepton + antineutrino

    W+
     → charged antilepton + neutrino
  5. ^Every entry in the lepton column can also be written as three decays, e.g. for the first row, as
    e+

    ν
    1,
    e+

    ν
    2,
    e+

    ν
    3, for every neutrino mass eigenstate, with decay widths proportional to|Ue1|2,{\displaystyle \,|U_{\text{e1}}|^{2}\,,}|Ue2|2,{\displaystyle \,|U_{\text{e2}}|^{2}\,,}|Ue3|2{\displaystyle \,|U_{\text{e3}}|^{2}\,} (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.
  6. ^abIn 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 haveisospinT3 = 0 and electric chargeQ = 0, and withcolor charge also zero. The all-zero charges make them"sterile", i.e. unable to interact by either the weak or electric forces, and no strong-force interactions either.
  7. ^ The mass of the
    t
     quark plus a
    t
     is greater than the mass of the
    Z
     boson, so it does not have sufficient energy to decay into a
    t

    t
     quark pair.

References

[edit]
  1. ^abTanabashi, M.; et al. (Particle Data Group) (2018)."Review of Particle Physics".Physical Review D.98 (3): 030001.Bibcode:2018PhRvD..98c0001T.doi:10.1103/PhysRevD.98.030001.hdl:10044/1/68623.
  2. ^R. L. Workman et al. (Particle Data Group),"Mass and Width of the W Boson", Prog. Theor. Exp. Phys. 2022, 083C01 (2022).
  3. ^abTanabashi, M.; et al. (Particle Data Group) (2018)."Review of Particle Physics".Physical Review D.98 (3): 030001.Bibcode:2018PhRvD..98c0001T.doi:10.1103/PhysRevD.98.030001.hdl:10044/1/68623.
  4. ^Weinberg, Steven (1967)."A Model of Leptons"(PDF).Physical Review Letters.19 (21):1264–1266.Bibcode:1967PhRvL..19.1264W.doi:10.1103/physrevlett.19.1264. Archived fromthe original(PDF) on January 12, 2012. — The electroweak unification paper.
  5. ^Weinberg, Steven (1993).Dreams of a Final Theory: The search for the fundamental laws of nature. Vintage Press. p. 94.ISBN 978-0-09-922391-7.
  6. ^Lopes, J. Leite (September 1999)."Forty years of the first attempt at the electroweak unification and of the prediction of the weak neutral boson".Brazilian Journal of Physics.29 (3):574–578.Bibcode:1999BrJPh..29..574L.doi:10.1590/S0103-97331999000300024.ISSN 0103-9733.
  7. ^ab"The Nobel Prize in Physics 1979".Nobel Foundation.
  8. ^"The discovery of the weak neutral currents". CERN Courier. 3 October 2004.Archived from the original on 2017-03-07.
  9. ^"The UA2 Collaboration collection". Archived fromthe original on 2013-06-04. Retrieved2009-06-22.
  10. ^"Nobel Prize in Physics 1984" (Press release). Nobel Foundation.
  11. ^abAmoroso, Simone; Andari, Nansi; Barter, William; Bendavid, Josh; Boonekamp, Maarten; Farry, Stephen; Gruenewald, Martin; Hays, Chris; Hunter, Ross; Kretzschmar, Jan; Lupton, Oliver; Pili, Martina; Miguel Ramos Pernas; Tuchming, Boris; Vesterinen, Mika; Vicini, Alessandro; Wang, Chen; Xu, Menglin (18 Aug 2023). "Compatibility and combination of world W-boson mass measurements".European Physical Journal C.84 (5): 451.arXiv:2308.09417.Bibcode:2024EPJC...84..451L.doi:10.1140/epjc/s10052-024-12532-z.PMID 39512385.
  12. ^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
  13. ^Weule, Genelle (8 April 2022)."Standard Model of physics challenged by most precise measurement of W boson particle yet". Australian Broadcasting Corporation. Retrieved9 April 2022.
  14. ^Wood, Charlie (7 April 2022)."Newly Measured Particle Seems Heavy Enough to Break Known Physics".Quanta Magazine. Retrieved9 April 2022.
  15. ^Marc, Tracy (7 April 2022)."CDF collaboration at Fermilab announces most precise ever measurement of W boson mass to be in tension with the Standard Model".Fermilab. Retrieved8 April 2022.
  16. ^Schott, Matthias (2022-04-07)."Do we have finally found new physics with the latest W boson mass measurement?".Physics, Life and all the Rest. Retrieved2022-04-09.
  17. ^Ouellette, Jennifer (24 March 2023)."New value for W boson mass dims 2022 hints of physics beyond Standard Model".Ars Technica. Retrieved26 March 2023.
  18. ^"Improved W boson Mass Measurement using $\sqrt{s}=7$ TeV Proton-Proton Collisions with the ATLAS Detector".ATLAS experiment. CERN. 22 March 2023. Retrieved26 March 2023.
  19. ^abS. Navas et al.(Particle Data Group), Phys. Rev. D110, 030001 (2024)
  20. ^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
  21. ^CMS collaboration (17 September 2024)."Measurement of the W boson mass in proton-proton collisions at √s=13 TeV".CMS Document Server.
  22. ^"CMS delivers the best-precision measurement of the W boson mass at the LHC | CMS Experiment".cms.cern. Retrieved2024-09-20.
  23. ^"New results from the CMS experiment put W boson mass mystery to rest | symmetry magazine".www.symmetrymagazine.org. 2024-09-17. Retrieved2024-09-20.
  24. ^Beringer, J.; et al. (Particle Data Group) (2012)."Gauge and Higgs bosons"(PDF).Physical Review D. 2012 Review of Particle Physics.86 (1): 1.Bibcode:2012PhRvD..86a0001B.doi:10.1103/PhysRevD.86.010001.Archived(PDF) from the original on 2017-02-20. Retrieved2013-10-21.
  25. ^Amsler, C.; et al. (Particle Data Group) (2010).PL B667, 1 (2008), and 2009 partial update(PDF) (Report) (2010 ed.). Berkeley, CA:Lawrence Berkeley National Laboratory.Archived(PDF) from the original on 2011-06-05. Retrieved2010-05-19.
  26. ^Sirunyan, A.M.; et al. (CMS Collaboration) (2018)."Observation of the
    Z
     → ψ ℓ+ ℓ−     decay in
    p

    p
     collisions at√s = 13 TeV"    .Physical Review Letters.121 (14): 141801.arXiv:1806.04213.doi:10.1103/PhysRevLett.121.141801.PMID 30339440.S2CID 118950363.

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