W and Z bosons

Infobox Particle
bgcolour =
name = SubatomicParticle|W boson+- and Z Bosons


caption =
num_types =
composition = Elementary particle
family = Boson
group = Gauge boson
generation =
interaction = Weak interaction
theorized = Glashow, Weinberg, Salam (1968)
discovered = UA1 and UA2 collaborations, 1983
symbol =
mass = W: val|80.398|0.025|u=GeV/c2 [cite web|url=http://lepewwg.web.cern.ch/LEPEWWG/ |title=LEP EWWG Home Page |publisher=Lepewwg.web.cern.ch |author=Sld, Cdf |date= |accessdate=2008-09-10]
Z: val|91.1876|0.0021|u=GeV/c2 [http://pdglive.lbl.gov/Rsummary.brl?nodein=S044&fsizein=1]
decay_time =
decay_particle =
electric_charge = W: ±1 e
Z: 0 e
color_charge =
spin = 1
num_spin_states =

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 the fact that it has zero electric charge.

Basic properties

Two kinds of W bosons exist with +1 and −1 elementary units of electric charge; the SubatomicParticle|W boson+ is the antiparticle of the SubatomicParticle|W boson-. The Z boson (or SubatomicParticle|Z boson) is electrically neutral and is its own antiparticle. All three particles are very short-lived with a mean life of about val|3|e=-25|u=s.

These bosons are heavyweights among the elementary particles. With a mass of val|80.4|u=GeV/c2 and val|91.2|u=GeV/c2, respectively, the W and SubatomicParticle|Z boson 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 limit the range of the weak nuclear force. The electromagnetic force, by contrast, has an infinite range because its boson (the photon) is massless.

All three types have a spin of 1.

The emission of a SubatomicParticle|W boson+ or SubatomicParticle|W boson- 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 SubatomicParticle|Z boson 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 flavor 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 boson is best known for its role in nuclear decay. Consider, for example, the beta decay of cobalt-60, an important process in supernova explosions.

: Nuclide|Link|Cobalt|60 → Nuclide|Link|Nickel|60 + SubatomicParticle|link=yes|Electron + SubatomicParticle|link=yes|Electron antineutrino

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 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 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:

: SubatomicParticle|link=yes|Down quark → SubatomicParticle|link=yes|Up quark + SubatomicParticle|W boson-

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

: SubatomicParticle|W boson- → SubatomicParticle|link=yes|Electron + SubatomicParticle|link=yes|Electron antineutrino

Being its own antiparticle, the Z boson has all zero quantum numbers. 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 [http://www.nobel.se/physics/laureates/1979/ 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. As of 2008, 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. (The 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 [http://www.nobel.se/physics/laureates/1984/ 1984 Nobel Prize in physics] , a most unusual step for the conservative Nobel Foundation.

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, N_C = 3. The decay widths for the W boson are then proportional to:

For x = 0.23, the branching ratios of the Z boson are predicted to be:
:B(SubatomicParticle|neutrino SubatomicParticle|antineutrino) = 20.5%,:B(SubatomicParticle|leptonSubatomicParticle|antilepton) = 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(SubatomicParticle|neutrino SubatomicParticle|antineutrino) = 20.00 ± 0.06%,:B(SubatomicParticle|leptonSubatomicParticle|antilepton) = 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

* Standard model (mathematical formulation)
* List of particles
* X and Y bosons: analogous pair of bosons predicted by GUT
* W' boson
* Z' boson

References

External links

* [http://pdg.lbl.gov/ The Review of Particle Physics] , the ultimate source of information on particle properties.
* [http://intranet.cern.ch/Chronological/Announcements/CERNAnnouncements/2003/09-16WZSymposium/Courier/HeavyLight/Heavylight.html W and Z] page from CERN
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html#c4 W and Z particles at Hyperphysics]
* [http://www.everything2.com/index.pl?node=Z%20particle Z particle at Everything2]