W and Z bosons

W and Z bosons
In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are and . The bosons have either a positive or negative electric charge of 1 elementary charge and are each other's antiparticles. The boson is electrically neutral and is its own antiparticle. The three particles each have a spin 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 of particle physics.
The bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the " particle", 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.
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 – 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 gluons are also presumed to have zero mass, the range of the color force is limited for different reasons; see color confinement.)
All three bosons have particle spin s = 1. The emission of a or boson either lowers or raises the electric charge of the emitting particle by one unit, and also alters the spin by one unit. At the same time, the emission or absorption of a boson can change the type of the particle – for example changing a strange quark into an up quark. The 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.). The emission or absorption of a boson can only change the spin, momentum, and energy of the other particle. (See also Weak neutral current.)
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, but affects only one of its 33 neutrons. The neutron is converted into a proton while also emitting an electron (called a beta particle in this context) and an electron neutrino:
→ + +
Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks. It is 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 boson is its own antiparticle. Thus, all of its flavour quantum numbers and charges are zero. The exchange of a boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of spin and/or momentum.
boson interactions involving neutrinos have distinct signatures: They provide the only known mechanism for elastic scattering of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via boson exchange) as inelastically (via W boson exchange). Weak neutral currents via boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in the Gargamelle bubble chamber at CERN.
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. 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, 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, 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 and bosons to form their longitudinal components, and the remainder appears as the spin 0 Higgs boson.
The combination of the SU(2) gauge theory of the weak interaction, the electromagnetic interaction, and the Higgs mechanism is known as the Glashow–Weinberg–Salam model. Today it is widely accepted as one of the pillars of the Standard Model of particle physics, particularly given the 2012 discovery of the Higgs boson by the CMS and ATLAS experiments.
The model predicts that and bosons have the following masses:
where is the SU(2) gauge coupling, is the U(1) gauge coupling, and 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 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 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 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, where unambiguous signals of bosons were seen in January 1983 during a series of experiments made possible by Carlo Rubbia and Simon van der Meer. The actual experiments were called UA1 (led by Rubbia) and UA2 (led by Pierre Darriulat), and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end (stochastic cooling). UA1 and UA2 found the boson a few months later, in May 1983. Rubbia and van der Meer were promptly awarded the 1984 Nobel Prize in Physics, a most unusual step for the conservative Nobel Foundation.
The and bosons, together with the photon, comprise the four gauge bosons 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 .
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 , which is seven standard deviations above that predicted by the Standard Model, meaning that if the model is correct there should only be a one-trillionth chance that such a large mass would arise by non-systematic observational error. According to Ashutosh Kotwal of Duke University 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. The team deliberately encrypted its data and withheld any preliminary results from themselves until the analysis was complete, to prevent "confirmation bias" bending their interpretation of the data. Kotwal described it as the 'largest crack in this beautiful theory', speculating that it might be the 'first clear evidence' of other forces or particles not accounted for by the Standard Model, and which might be accounted for by theories such as supersymmetry. The Nobel-winning theoretical physicist Frank Wilczek 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 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[Collider Detector at Fermilab] and the Standard Model – we first have to understand why the CDF measurement is in strong tension with all [other measurements]".
In 2023, the ATLAS experiment released an improved measurement for the mass of the W boson which aligned with predictions from the Standard Model.
Decay
The and bosons decay to fermion 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 fractions can be calculated from the coupling constants.
W bosons
bosons can decay to a lepton and antilepton (one of them charged and another neutral) or to a quark 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, The decay widths for the W boson are then proportional to:
{| class="wikitable" style="text-align:center;"
!colspan="2" width="100"|Leptons
!colspan="6" width="100"|Quarks
|-
|
| 1
|
| 3
|
| 3
|
| 3
|-
|
| 1
|
| 3
|
| 3
|
| 3
|-
|
| 1
|colspan="6"|Energy conservation forbids decay to .
|-
|}
Here , denote the three flavours of leptons (more exactly, the positive charged antileptons). denote the three flavours of neutrinos. The other particles, starting with and , all denote quarks and antiquarks (factor is applied). The various denote the corresponding CKM matrix coefficients.
Unitarity of the CKM matrix implies that
thus each of two quark rows Therefore, the leptonic branching ratios of the boson are approximately 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
Z0 boson
bosons decay into a fermion and its antiparticle. As the boson is a mixture of the pre-symmetry-breaking and bosons (see weak mixing angle), each vertex factor includes a factor 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. Because the weak isospin is different for fermions of different chirality, 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 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 the Fermi theory.
{| class="wikitable" style="text-align:center;"
!colspan=2| Particles
!colspan=2| Weak isospin
!rowspan=2| Relative factor
!colspan=2| Branching ratio
|-
! Name
! Symbols
!
!
! Predicted for = 0.23
! Experimental measurements
|-
| align="left" | Neutrinos (all)
|
|
| 0
|
|
|
|-
| align="left" | Charged leptons (all)
|
|colspan=2|
|
|
|
|-
| align="right" | Electron
|
|
|
|
|
|
|-
| align="right" | Muon
|
| − +
|
|
|
|
|-
| align="right" | Tau
|
|
|
|
|
|
|-
| align="left" | Hadrons
|colspan=4|
|
|
|-
| align="right" | Down-type quarks
|
|
|
|
|
|
|-
| align="right" | Up-type quarks( )
| ,
| −
| −
|
|
|
|}
To keep the notation compact, the table uses
* The impossible decay into a top quark–antiquark pair is left out of the table.
Subheadings and denote the chirality or "handedness" of the fermions.
In 2018, the CMS collaboration observed the first exclusive decay of the boson to a ψ meson and a lepton–antilepton pair.
See also
List of particles
Weak charge
: 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
W and Z particles at Hyperphysics
Bosons
Elementary particles
Electroweak theory
Gauge bosons
Standard Model
Force carriers
Subatomic particles with spin 1
de:W-Boson