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{{Infobox Particle| bgcolour =| name = W± and Z Bosons| image =| caption =| num_types =| composition = Elementary particle| group = [Gauge boson| theorized = [Sheldon Glashow,
Steven Weinberg,
Abdus Salam (
1968)] and
UA2 collaborations,
1983/[speed of light2
Z: 91.1876±0.0021
GeV/speed of light2 | decay_time =| decay_particle =| electric_charge =
W: ±1 Elementary charge
Z: 0 e| color_charge =| spin = 1| num_spin_states =-->
In
physics, the
W and Z bosons are the elementary particles that mediate the
weak force. Their discovery at CERN in
1983 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
Boson exist with +1 and −1 elementary units of
electric charge; the W+ is the
antiparticle of the W−. The Z boson (or Z0) is electrically
neutral and is its own antiparticle. All three particles are very short-lived with a half-life of about 3 × 10−25 seconds.
These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c2, respectively, the W and Z0 particles are almost 100 times as massive as the proton—heavier than entire atoms of iron. The
mass of these bosons are significant because they limit the range of the weak nuclear force. The
electromagnetism, by contrast, has an infinite range because its boson (the photon) is massless.
All three types have a spin (physics) of 1.
The emission of a W+ or W– 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 an
strange quark to an
up quark. The Z0 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).
The weak nuclear force
for beta decay of a
neutron into a proton, electron, and
neutrino via an intermediate heavy
W bosonThe 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 the explosion of
supernova.
{}^{60}_{27}\hbox{Co}\to{}^{60}_{28}\hbox{Ni}+\hbox{e}^-+\overline{\nu}_e
This reaction does not involve the whole cobalt-60
Atomic 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:
\hbox{n}\to \hbox{p}+\hbox{e}^-+\overline{\nu}_e
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 (particle physics) of a single quark:
\hbox{d}\to\hbox{u}+\hbox{W}^- \,
which is immediately followed by decay of the W− itself:
\hbox{W}^-\to\hbox{e}^-+\overline{\nu}_e
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
showing the exchange of a pair of W bosons. This is one of the leading terms contributing to neutral Kaon oscillation.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. As of 2007, 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 of the W and Z
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, 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 Prize.
See also
External links
- The Review of Particle Physics, the ultimate source of information on particle properties.
- W and Z page from CERN
- W and Z particles at Hyperphysics
- Z particle at Everything2
{{Infobox Particle| bgcolour =| name = W± and Z Bosons| image =| caption =| num_types =| composition =
Elementary particle| group = [Gauge boson| theorized = [Sheldon Glashow, Steven Weinberg, Abdus Salam (
1968)] and UA2 collaborations,
1983/[speed of light2
Z: 91.1876±0.0021 GeV/
speed of light2 | decay_time =| decay_particle =| electric_charge =
W: ±1
Elementary charge Z: 0 e| color_charge =| spin = 1| num_spin_states =-->
In
physics, the
W and Z bosons are the
elementary particles that mediate the weak force. Their discovery at CERN in
1983 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
Boson exist with +1 and −1 elementary units of
electric charge; the W+ is the
antiparticle of the W−. The Z boson (or Z0) is electrically
neutral and is its own antiparticle. All three particles are very short-lived with a
half-life of about 3 × 10−25 seconds.
These bosons are heavyweights among the elementary particles. With a mass of 80.4 and 91.2 GeV/c2, respectively, the W and Z0 particles are almost 100 times as massive as the
proton—heavier than entire
atoms of
iron. The
mass of these bosons are significant because they limit the range of the weak nuclear force. The electromagnetism, by contrast, has an infinite range because its boson (the photon) is massless.
All three types have a spin (physics) of 1.
The emission of a W+ or W– 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 an strange quark to an
up quark. The Z0 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).
The weak nuclear force
for beta decay of a
neutron into a
proton,
electron, and
neutrino via an intermediate heavy
W bosonThe 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 the explosion of
supernova.
{}^{60}_{27}\hbox{Co}\to{}^{60}_{28}\hbox{Ni}+\hbox{e}^-+\overline{\nu}_e
This reaction does not involve the whole cobalt-60
Atomic 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:
\hbox{n}\to \hbox{p}+\hbox{e}^-+\overline{\nu}_e
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 (particle physics) of a single quark:
\hbox{d}\to\hbox{u}+\hbox{W}^- \,
which is immediately followed by decay of the W− itself:
\hbox{W}^-\to\hbox{e}^-+\overline{\nu}_e
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
showing the exchange of a pair of W bosons. This is one of the leading terms contributing to neutral
Kaon oscillation.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. As of 2007, 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 of the W and Z
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, 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 Prize.
See also
External links
- The Review of Particle Physics, the ultimate source of information on particle properties.
- W and Z page from CERN
- W and Z particles at Hyperphysics
- Z particle at Everything2
W and Z bosons - Wikipedia, the free encyclopedia
In physics, the W and Z bosons, colloquially known as Weakons, are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for ...
W and Z bosons - Hutchinson encyclopedia article about W and Z bosons
W particle. Elementary particle, one of the intermediate vector bosons responsible for transmitting the weak nuclear force. The W particle exists as both W + and W
Investigating the Production of W and Z bosons at LHCb
Histogrammed parameters: Pseudorapidity; Rapidity; Angular Distributions; Transverse mass; Invariant mass . Plotted for each boson and compared to theory.
Weak
The carrier particles of the weak interactions are the W +, W-, and the Z particles. The W's are electrically charged and the Z is neutral. The Standard Model has united ...
Electroweak
The difference between their observed strengths is due to the huge difference in mass between the W and Z particles, which are very massive, and the photon, which has no mass as ...
Theory: Weak Interaction Carrier Particles (SLAC VVC)
Weak Interaction Carrier Particles W Bosons. The mass of the W boson is about 80 GeV/c 2, that is about eighty times the mass of the proton or neutron, or roughly the ...
The W±and Z bosons
The W ± and Z bosons On the other side a electro-weak interaction is developed by Weinberg and Salam. A few years later 't Hooft shows that it is a well-posed theory.
W and Z bosons definition of W and Z bosons in the Free Online ...
W and Z particles, elementary particles elementary particles, the most basic physical constituents of the universe. Basic Constituents of Matter
W bosons - definition of W bosons by the Free Online Dictionary ...
An elementary particle that has a mass approximately 160,000 times that of the ... W and Z bosons W and Z bosons W and Z bosons W and Z class destroyer W and Z particles
Reference for W and Z bosons - Search.com
W and Z bosons ... Wikipedia. Licensed under the GNU Free Documentation License. Are you an expert in this subject?
