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W and Z bosons
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The W and Z bosons are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for the Standard Model of particle physics.
The W particle is named after the weak nuclear force. The Z particle was semi-humorously given its name because it was said to be the last particle to need discovery. Another explanation is that the Z particle derives its name from having zero electric charge.
kinds of W bosons exist with +1 and -1 elementary units of electric charge; the is the antiparticle of the .
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Encyclopedia
The W and Z bosons are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for the Standard Model of particle physics.
The W particle is named after the weak nuclear force. The Z particle was semi-humorously given its name because it was said to be the last particle to need discovery. Another explanation is that the Z particle derives its name from having zero electric charge.
Basic properties
Two kinds of W bosons exist with +1 and -1 elementary units of electric charge; the is the antiparticle of the . The Z boson (or Z) is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about .
These bosons are heavyweights among the elementary particles. With a mass of and , respectively, the W and Z particles are almost 100 times as massive as the proton—heavier than entire atoms of iron. The masses of these bosons are significant because they act as force carriers; their masses thus limit the range of the weak interaction. The electromagnetic force, by contrast, has an infinite range because its force carrier (the photon) is massless.
All three types have a spin of 1. The emission of a or boson can either raise or lower electric charge of the emitting particle by 1 unit, and alter the spin by 1 unit. At the same time a W boson can change the generation of the particle, for example changing a strange quark to an up quark. The boson cannot change either electric charge nor any other charges (like strangeness, charm, etc.), only spin and momentum, so it never changes the generation or flavour of the particle emitting it (see weak neutral current).
Weak nuclear force
The W and Z bosons are carrier particles that mediate the weak nuclear force, much like the photon is the carrier particle for the electromagnetic force. The W bosons are best known for its role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernova explosions.
- → + +
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 antineutrino:
- → + +
Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks (udd). It is in fact one of the down quarks that interacts in beta decay, turning into an up quark to form a proton (uud). At the most fundamental level, then, the weak force changes the flavor of a single quark:
- → +
which is immediately followed by decay of the itself:
- → +
Being its own antiparticle, the Z boson has all its flavour quantum numbers, and all its chargess are zero. The exchange of a Z boson between particles, called a neutral current interaction, therefore leaves the interacting particles unaffected, except for a transfer of momentum. Unlike beta decay, the observation of neutral current interactions requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world.
Predicting the W and Z
Following the spectacular 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 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 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 W and Z in the process. One explanation, the Higgs mechanism, was forwarded by Peter Higgs in the late 1960s. It predicts the existence of yet another new particle, the 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. These days it is widely accepted as one of the pillars of the Standard Model of particle physics. , despite intensive search for the Higgs boson carried out at CERN and Fermilab, its existence remains the main prediction of the Standard Model not to be confirmed experimentally.
Discovery
The discovery of the W and Z particles is a major CERN success story. 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 Z 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 W and Z particles 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 W particles were seen in January 1983 during a series of experiments conducted by Carlo Rubbia and Simon van der Meer. T he actual experiments were called UA1 (led by Rubbia) and UA2 (led by 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 Z 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 , build up the four gauge bosons of the electroweak interaction.
Decay
The W and Z bosons decay to fermion-antifermion pairs. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the coupling constants.
W bosons can decay to a lepton and neutrino or to an up-type quark and a down-type quark. The W cannot decay to the higher-mass top quark. 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 colors, NC = 3. The decay widths for the W boson are then proportional to:
| Leptons | Up quarks | Charm quarks
|
|---|
| 1 | | >Vud|2 | | >Vuc|2 | | 1 | | >Vus|2 | | >Vuc|2 | | 1 | | >Vub|2 | | >Vcb|2 | |
Here, e+, µ+, t+ denoted the three flavours of leptons (more exactly, the positive charged anti leptons). ?e, ?µ, ?t denote the three flavours of neutrinos. The other particles starting with u and d all denote quarks and anti-quarks (factor NC is applied). V is the CKM matrix with its coefficients.
Unitarity of the CKM matrix implies that
|Vud|2 + |Vus|2 + |Vub|2 =
|Vcd|2 + |Vcs|2 + |Vcb|2 = 1. Therefore the leptonic branching ratios of the W boson are approximately B(e+?e) = B(µ+?µ) = B(t+?t) = (~11.11%). The hadronic branching ratio is dominated by the CKM favored u and c final states, and the sum of the hadronic branching ratios is roughly (~66.67%). The branching ratios have been measured experimentally: B(l+?l) = 10.80 ± 0.09% and B(hadrons) = 67.60 ± 0.27%.
Z bosons can decay to a fermion and its antiparticle, with the exception of the top quark, which is too massive. The decay width of a Z boson to a fermion-antifermion pair is proportional to the square of the weak charge Tz - Q·x, where Tz is the third component of the weak isospin of the fermion, Q is the charge of the fermion (in units of the elementary charge), and x = sin2?W, where ?W 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 decay width of the Z boson for quarks is also proportional to NC. The weak charge of the fermions is:
| (?e, ?µ, ?t)L | | | (e, µ, t)L | | | (e, µ, t)R | x | | (u, c, t)L | | | (d, s, b)L | | | (u, c, t)R | | | (d, s, b)R | |
Here, L and R denote the chirality of the fermions, i. e. left-handed and right-handed, respectively. The right-handed neutrinos do not exist in the standard model. (However, in some extensions of the standard model they do).
The decay widths of the Z boson are then proportional to
| (?e, ?µ, ?t) | 2 | | (e, µ, t) | 2 + x2 | | (u, c, t) | 2 + 3(-x)2 | | (d, s, b) | 2 + 3(x)2 |
For x = 0.23, the branching ratios of the Z boson are predicted to be:
- B = 20.5%,
- B = 3.4%,
- B(uu) = B(cc) = 11.8%,
- B(dd) = B(ss) = B(bb) = 15.2%, and
- B(hadrons) = 69.2%.
The branching ratios have been measured experimentally:
- B = 20.00 ± 0.06%,
- B = 3.3658 ± 0023%,
- B(uu + cc) = 11.6 ± 0.6%,
- B(dd + ss + bb) = 15.6 ± 0.4%, and
- B(hadrons) = 69.91 ± 0.06%.
See also
External links
- , the ultimate source of information on particle properties.
- page from CERN
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