w and z bosons


  • 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.

  • The exchange of a Z boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of spin and/or

  • [12] 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
    80433±9 MeV, which is seven standard deviations above that predicted by the Standard Model, meaning that if the model is correct[13] there should only be a one-trillionth chance that such a large mass would arise by non-systematic observational

  • All three of these particles are very short-lived, with a half-life of about 3×10−25 s. Their experimental discovery was pivotal in establishing what is now called the Standard
    Model of particle physics.The W bosons are named after the weak force.

  • [9][c] Their electroweak theory postulated not only the W bosons necessary to explain beta decay, but also a new Z boson that had never been observed.

  • The neutral Z boson cannot change the electric charge of any particle, nor can it change any other of the so-called “charges” (such as strangeness, baryon number, charm, etc.).

  • At the most fundamental level, then, the weak force changes the flavour of a single quark: which is immediately followed by decay of the W− itself: Z bosons [edit] The Z0
    boson is its own antiparticle.

  • 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.

  • Discovery Unlike beta decay, the observation of neutral current interactions that involve particles other than neutrinos 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).

  • 2022 unexpected measurement of W boson mass[edit] See also: Physics beyond the Standard Model § Experimental results not explained Before 2022, measurements of the W boson
    mass appeared to be consistent with the Standard Model.

  • 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.

  • [b] Z 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 Z boson exchange) as inelastically (via W boson exchange).

  • [8] Predictions of the W+, W− and Z0 bosons Following the success of quantum electrodynamics in the 1950s, attempts were undertaken to formulate a similar theory of the weak
    nuclear force.

  • 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.

  • At the same time, the emission or absorption of a W± boson can change the type of the particle – for example changing a strange quark into an up quark.

  • Today it is widely accepted as one of the pillars of the Standard Model of particle physics, particularly given the 2012 discovery of the Higgs boson by the CMS and ATLAS

  • 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.

  • As the Z0 boson is a mixture of the pre-symmetry-breaking W0 and B0 bosons (see weak mixing angle), each vertex factor includes a factor where is the third component of the
    weak isospin of the fermion (the “charge” for the weak force), is the electric charge of the fermion (in units of the elementary charge), and is the weak mixing angle.

  • Of the four components of a Goldstone 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 relative strengths of each coupling can be estimated by considering that the decay rates include the square of these factors, and all possible diagrams (e.g.

  • [18] In 2023, the ATLAS experiment released an improved measurement for the mass of the W boson, 80360±16 MeV, which aligned with predictions from the Standard Model.

  • The fact that the W and Z bosons have mass while photons are massless was a major obstacle in developing electroweak theory.

  • [c] Weak neutral currents via Z boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in the Gargamelle bubble chamber at CERN.

  • With masses of and, respectively, the W and Z bosons are almost 80 times as massive as the proton – heavier, even, than entire iron atoms.

  • Some mechanism is required to break the SU(2) symmetry, giving mass to the W and Z in the process.

  • [16] Kotwal described it as the ‘largest crack in this beautiful theory’, speculating that it might be the ‘first clear evidence’ of other forces or particles not accounted
    for by the Standard Model, and which might be accounted for by theories such as supersymmetry.

  • The physicist Steven 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.

  • For example, in 2021, experimental measurements of the W boson mass were assessed to converge around 80379±12 MeV.

  • [14] According to Ashutosh Kotwal of Duke University 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 and a neutrino.

  • Thus, all of its flavour quantum numbers and charges are zero.

  • Relations to the weak nuclear force 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

  • Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the coupling constants.


Works Cited

[‘1. In 2018, the CMS collaboration observed the first exclusive decay of the
boson to a ψ meson and a lepton–antilepton pair.[23] Because neutrinos are neither affected by the strong force nor the electromagnetic force, and because the gravitational
force between subatomic particles is negligible, by 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
2. ^ However, see flavor-changing neutral current for a conjecture that a rare
exchange might cause flavor change.
3. ^ Jump up to:a
b The first prediction of
bosons was made by Brazilian physicist José 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:

→ charged lepton + antineutrino

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



3, for every neutrino mass eigenstate, with decay widths proportional to (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. ^ Jump up to:a b 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 isospin T3 = 0 and electric charge Q = 0, and with color charge also zero. The all-zero charges make them “sterile”, i.e. unable to interact by either the weak or electric forces no strong-force
interactions either.
7. ^ The mass of the
quark plus a
is greater than the mass of the
boson, so it does not have sufficient energy to decay into a

quark pair.
8. Tanabashi, 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.
9. ^ R. L. Workman et al. (Particle Data Group), “Mass and Width of the W Boson”, Prog. Theor. Exp. Phys. 2022, 083C01
10. ^ Jump up to:a b M. Tanabashi 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.
11. ^ Weinberg, S. (1967). “A Model of
Leptons” (PDF). Physical Review Letters. 19 (21): 1264–1266. Bibcode:1967PhRvL..19.1264W. doi:10.1103/physrevlett.19.1264.[permanent dead link] — The electroweak unification paper.
12. ^ 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.
13. ^ 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.
14. ^ “The Nobel Prize in Physics”. Nobel Foundation. 1979. Archived from the original on 3 August 2004. Retrieved 10 September
15. ^ “The discovery of the weak neutral currents”. CERN Courier. 3 October 2004. Archived from the original on 2017-03-07. Retrieved 2017-03-06.
16. ^ “Nobel Prize in Physics”. Nobel Foundation. 1979. Archived from the original on 2004-08-03.
Retrieved 2004-02-20. (see also Nobel Prize in Physics on Wikipedia)
17. ^ “The UA2 Collaboration collection”. Archived from the original on 2013-06-04. Retrieved 2009-06-22.
18. ^ “Nobel Prize in physics” (Press release). Nobel Foundation. 1984.
Archived from the original on 2004-08-03. Retrieved 2004-02-20.
19. ^ 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
20. ^ Borenstein,
Seth, Key particle weighs in a bit heavy, confounding physicists, Associated Press (AP), April 7, 2022
21. ^ Jump up to:a b Weule, Genelle (8 April 2022). “Standard Model of physics challenged by most precise measurement of W boson particle yet”.
Australian Broadcasting Corporation. Retrieved 9 April 2022.
22. ^ Wogan, Tim (8 April 2022). “W boson mass measurement surprises physicists”. Physics World. Retrieved 9 April 2022.
23. ^ Jump up to:a b c Wood, Charlie (7 April 2022). “Newly Measured
Particle Seems Heavy Enough to Break Known Physics”. Quanta Magazine. Retrieved 9 April 2022.
24. ^ 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. Retrieved 8 April 2022.
25. ^ 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. Retrieved 2022-04-09.
26. ^ Ouellette, Jennifer (24
March 2023). “New value for W boson mass dims 2022 hints of physics beyond Standard Model”. Ars Technica. Retrieved 26 March 2023.
27. ^ “Improved W boson Mass Measurement using $\sqrt{s}=7$ TeV Proton-Proton Collisions with the ATLAS Detector”.
ATLAS experiment. CERN. 22 March 2023. Retrieved 26 March 2023.
28. ^ 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. Retrieved 2013-10-21.
29. ^ Amsler, C.; et al. (Particle Data Group) (2010). “PL B667, 1 (2008), and 2009 partial update for the 2010 edition” (PDF). Archived (PDF)
from the original on 2011-06-05. Retrieved 2010-05-19.
30. ^ Sirunyan, A.M.; et al. (CMS Collaboration) (2018). “Observation of the
→ ψ ℓ+ ℓ− decay in

collisions at √s = 13 TeV”. Physical Review Letters. 121 (14): 141801. arXiv:1806.04213.
doi:10.1103/PhysRevLett.121.141801. PMID 30339440. S2CID 118950363.
Photo credit: https://www.flickr.com/photos/ywds/353897245/’]