The
W and Z⁰ bosons are the elementary particles that mediate the weak force. Their discovery has been heralded as a major success for the
Standard ModelThe Standard Model of particle physics is a theory of three of the four known fundamental interactions and the elementary particles that take part in these interactions. These particles make up all visible matter in the universe...
of
particle physicsParticle physics is a branch of physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called high energy physics, because many elementary particles do not occur under normal circumstances in nature, but can be created and detected...
.
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 chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
.
Basic properties
Two kinds of W
bosonIn particle physics, bosons are particles which obey Bose–Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein. In contrast to fermions, which obey Fermi-Dirac statistics, several bosons can occupy the same quantum state. Thus, bosons with the same energy can occupy the...
s exist with +1 and −1 elementary units of
electric chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
; the is the
antiparticleCorresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
of the . The Z boson (or Z) is electrically
neutralIn physics, a neutral particle is a particle with no electric charge.- Stable or long-lived neutral particles :Long-lived neutral particles provide a challenge in the construction of particle detectors, because they do not interact electromagnetically, except possibly through their magnetic moments...
and is its own antiparticle. All three particles are very short-lived with a
mean lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
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
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
—heavier than entire
atomThe atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
. The
massIn physics, mass commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass...
es of these bosons are significant because they act as force carriers; their masses thus limit the range of the weak interaction. The
electromagnetic forceElectromagnetism is the physics of the electromagnetic field, a field that exerts a force on particles with the property of electric charge and is reciprocally affected by the presence and motion of such particles....
, by contrast, has an infinite range because its force carrier (the
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
) is massless.
All three types have a
spinIn particle physics and quantum mechanics, spin is a fundamental characteristic property of elementary particles including the force carriers , composite particles , and atomic nuclei....
of 1. The emission of a or boson either raises or lowers the electric charge of the emitting particle by 1 unit, and alters the spin by 1 unit. At the same time a W boson can change the generation of the particle, for example changing a
strange quarkThe strange quark is a second-generation quark with a charge of − e and a strangeness of −1. It is the third-lightest quark after the up and down quarks, with a mass of somewhere between 80 and...
to an
up quarkThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
. 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 decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus...
of
cobalt-60Cobalt-60 is a radioactive isotope of cobalt. Due to its short half life of 5.27 years 60Co is not found in nature. It is produced artificially by neutron activation of 59Co. 60Co decays by negative beta decay to the stable isotope nickel-60...
, an important process in
supernovaA supernova is a stellar explosion. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months. During this short interval, a supernova can radiate as much energy as the Sun could emit over...
explosions.
- → + +
This reaction does not involve the whole cobalt-60
nucleusThe nucleus is the very dense region consisting of nucleons at the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus, with a very small contribution from the orbiting electrons....
, but affects only one of its 33
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s. The neutron is converted into a
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
while also emitting an
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
(called a
beta particleBeta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
in this context) and an electron antineutrino:
- → + +
Again, the neutron is not an elementary particle but a composite of an
up quarkThe up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, form the neutrons and protons of atomic nuclei...
and two
down quarkThe down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
s (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
flavourIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
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
chargesIn physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
are zero. The exchange of a Z boson between particles, called a
neutral currentWeak neutral current interactions are one of the ways in which subatomic particles can interact by means of the weak force. These interactions are mediated by the boson, and the interaction is called 'neutral' because the has no electric charge...
interaction, therefore leaves the interacting particles unaffected, except for a transfer of
momentumIn classical mechanics, momentum is the product of the mass and velocity of an object . For more accurate measures of momentum, see the section "modern definitions of momentum" on this page...
. Unlike beta decay, the observation of neutral current interactions requires huge investments in
particle acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
s 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 electrodynamicsQuantum electrodynamics is a relativistic quantum field theory of electrodynamics. QED was developed by a number of physicists, beginning in the late 1920s. It basically describes how light and matter interact. More specifically it deals with the interactions between electrons, positrons and photons...
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 WeinbergSteven Weinberg is an American physicist and Nobel laureate in Physics for his contributions with Abdus Salam and Sheldon Glashow to the unification of the weak force and electromagnetic interaction between elementary particles.
...
, and
Abdus SalamAbdus Salam was a Pakistani theoretical physicist, astrophysicist and Nobel laureate in Physics for his work in Electro-Weak Theory. Salam, Sheldon Glashow and Steven Weinberg shared the prize for this discovery...
, 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 theoryGauge invariance is the property of a field theory in which different configurations of the underlying fundamental but unobservable fields result in identical observable quantities. A theory with such a property is called a gauge theory...
, but the bosons in a gauge theory must be massless. As a case in point, the
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
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 mechanismIn the standard model of particle physics, the Higgs mechanism is a theoretical framework which explains how the masses of the W and Z bosons arise as a result of electroweak symmetry breaking....
, was forwarded by
Peter HiggsPeter Ware Higgs, FRS, FRSE, FKC , is an English theoretical physicist and an emeritus professor at the University of Edinburgh[....]
in the late 1960s. It predicts the existence of yet another new particle, the
Higgs bosonThe Higgs boson is a massive scalar elementary particle predicted to exist by the Standard Model in particle physics. At present there are no known fundamental scalar particles in nature....
.
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
CERNThe European Organization for Nuclear Research , known as CERN , , is the world's largest particle physics laboratory, situated in the northwest suburbs of Geneva on the Franco–Swiss border, established in 1954...
and
FermilabFermi National Accelerator Laboratory , located in Batavia near Chicago, Illinois, is a U.S. Department of Energy national laboratory specializing in high-energy particle physics. As of January 1, 2007, Fermilab is operated by the Fermi Research Alliance, a joint venture of the University of...
, its existence remains the main prediction of the Standard Model not to be confirmed experimentally.
Discovery
The discovery of the W and Z particles was considered a major success for CERN. First, in 1973, came the observation of neutral current interactions as predicted by electroweak theory. The huge
GargamelleGargamelle was a giant bubble chamber detector at CERN, designed mostly for the detection of neutrinos. Built in France, with a diameter of nearly 2 meters and 4.8 meters in length, Gargamelle held nearly 12 cubic meters of freon . It operated from 1970 to 1978 with a neutrino beam from the CERN...
bubble chamberA bubble chamber is a vessel filled with a superheated transparent liquid used to detect electrically charged particles moving through it. It was invented in 1952 by Donald A. Glaser, for which he was awarded the 1960 Nobel Prize in Physics...
photographed the tracks of a few electrons suddenly starting to move, seemingly of their own accord. This is interpreted as a
neutrinoNeutrinos are elementary particles that often travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass...
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 acceleratorA particle accelerator is a device that uses electric fields to propel ions or charged subatomic particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator...
powerful enough to produce them. The first such machine that became available was the
Super Proton SynchrotronThe Super Proton Synchrotron is a 6.9 km long particle accelerator at CERN. The SPS was designed by a team led by John Adams, director-general of what was then known as Laboratory II. Originally specified as a accelerator, the SPS was actually built to be capable of 400 GeV, an...
, where unambiguous signals of W particles were seen in January 1983 during a series of experiments conducted by
Carlo RubbiaCarlo Rubbia is an Italian particle physicist who won the Nobel Prize in Physics in 1984 for work leading to the discovery of the W and Z particles at CERN.-Biography:...
and
Simon van der MeerSimon van der Meer is a Dutch accelerator physicist who invented the concept of stochastic cooling in colliders, making possible the discovery of the W particle and the Z particle at the CERN 500 Gev proton-antiproton collider by the UA-1 experimental collaboration led by Carlo Rubbia...
. The actual experiments were called
UA1The UA1 high energy physics experiment ran at CERN from 1981 until 1993 on the SPS collider. The discovery of the W and Z bosons by this experiment and UA2 in 1982 led to the Nobel Prize for physics being awarded to Carlo Rubbia and Simon van der Meer in 1984....
(led by Rubbia) and
UA2The UA2 high energy physics experiment was one of the two major experiments and collaborations at the CERN proton-antiproton collider, and codiscovered the W and Z bosons in 1983.-External links:*****...
(led by Peter Jenni)), and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end (
stochastic coolingStochastic cooling is a form of particle beam cooling. It is used in some particle accelerators and storage rings to control the emittance of the particle beams in the machine. This process uses the electrical signals that the individual charged particles generate in a feedback loop to reduce the...
). 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 FoundationThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
.
The , , and bosons, together with the
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
, build up the four
gauge bosonIn particle physics, gauge bosons are bosonic particles that act as carriers of the fundamental forces of nature. More specifically, elementary particles whose interactions are described by gauge theory exert forces on each other by the exchange of gauge bosons, usually as virtual particles.-...
s of the
electroweak interactionIn particle physics, the electroweak interaction is the unified description of two of the four fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of...
.
Decay
The W and Z
bosonIn particle physics, bosons are particles which obey Bose–Einstein statistics; they are named after Satyendra Nath Bose and Albert Einstein. In contrast to fermions, which obey Fermi-Dirac statistics, several bosons can occupy the same quantum state. Thus, bosons with the same energy can occupy the...
s decay to
fermionIn particle physics, fermions are particles which obey Fermi-Dirac statistics; they are named after Enrico Fermi. In contrast to bosons, which have Bose-Einstein statistics, only one fermion can occupy a quantum state at a given time; this is the Pauli Exclusion Principle. Thus, if more than one...
-antifermion pairs. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the
coupling constantIn physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian of a system can be separated into a kinetic part and an interaction part...
s.
W bosons can decay to a
leptonLeptons are a family of elementary particles, alongside quarks and gauge bosons . Like quarks, leptons are fermions and are subject to the electromagnetic force, the gravitational force, and weak interaction, but unlike quarks, leptons do not participate in the strong interaction.There are six...
and
neutrinoNeutrinos are elementary particles that often travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass...
or to an up-type quark and a down-type quark. The W cannot decay to the higher-mass
top quarkThe top quark or t quark is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions...
. 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, N
C = 3. The decay widths for the W boson are then proportional to:
| Leptons |
Up quarks |
Charm quarks |
|
| 1 |
| >Vud|2 |
| >Vcd|2 |
|
| 1 |
| >Vus|2 |
| >Vcs|2 |
|
| 1 |
| >Vub|2 |
| >Vcb|2 |
Here, e
+, μ
+, τ
+ denoted the three
flavourIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s of
leptonLeptons are a family of elementary particles, alongside quarks and gauge bosons . Like quarks, leptons are fermions and are subject to the electromagnetic force, the gravitational force, and weak interaction, but unlike quarks, leptons do not participate in the strong interaction.There are six...
s (more exactly, the positive charged anti leptons). ν
e, ν
μ, ν
τ denote the three
flavourIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics flavour is a global symmetry...
s of neutrinos. The other particles starting with u and
d all denote
quarkA quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never found in...
s and anti-quarks (factor N
C is applied). V is the CKM matrix with its coefficients.
UnitarityIn mathematics, a unitary matrix is an n by n complex matrix satisfying the condition where is the identity matrix in n dimensions and is the conjugate transpose of...
of the CKM matrix implies that
|V
ud|
2 + |V
us|
2 + |V
ub|
2 =
|V
cd|
2 + |V
cs|
2 + |V
cb|
2 = 1. Therefore the leptonic
branching ratioIn particle physics and nuclear physics, the branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay. It is equal to the ratio of the partial decay constant to the overall decay constant...
s of the W boson are approximately B(e
+ν
e) = B(μ
+ν
μ) = B(τ
+ν
τ) = (~11.11%). The hadronic branching ratio is dominated by the CKM favored u and c final states, and the sum of the
hadronIn particle physics, a hadron is a particle made of quarks held together by the strong force . Hadrons are either mesons or baryons...
ic branching ratios is roughly (~66.67%). The branching ratios have been measured experimentally: B(l
+ν
l) = 10.80 ± 0.09% and B(hadrons) = 67.60 ± 0.27%.
Z bosons decay into a fermion and its antiparticle. The decay width of a Z boson to a fermion-antifermion pair is proportional to the square of the weak charge T
z − Q·x, where T
z is the third component of the
weak isospinThe weak isospin in particle physics is a quantum number relating to the weak interaction, and parallels the idea of isospin under the strong interaction. Weak isospin is usually given the symbol T or I with the third component written as Tz, T3, Iz or I3...
of the fermion, Q is the
chargeElectric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields...
of the fermion (in units of the
elementary chargeThe elementary charge, usually denoted e, is the electric charge carried by a single proton, or equivalently, the negative of the electric charge carried by a single electron. This is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called the "elementary positive...
), and x = sin
2θ
W, where θ
W is the
weak mixing angleThe Weinberg angle or weak mixing angle is a parameter in the Weinberg-Salam theory of the electroweak force. It gives a relationship between the masses of the W and Z bosons , as well as the ratio of Z boson mediated interaction which behaves like a photon, i.e. its mixing...
. Because the weak isospin is different for fermions of different
chiralityA phenomenon is said to be chiral if it is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...
, 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 N
C. The weak charge of the fermions is:
| (νe, νμ, ντ)L |
|
| (e, μ, τ)L |
|
| (e, μ, τ)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
| (νe, νμ, ντ) |
2 |
| (e, μ, τ) |
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
- Standard model (mathematical formulation)
- List of particles
- X and Y bosons
In particle physics, the X and Y bosons are hypothetical elementary particles analogous to the W and Z bosons, but corresponding to a new type of force predicted by the Georgi–Glashow model, a grand unified theory.-Details:The X and Y bosons couple quarks to leptons, allowing violation of the...
: analogous pair of bosons predicted by GUTGrand Unification, grand unified theory, or GUT refers to any of several very similar unified field theories or models in physics that predicts that at extremely high energies , the electromagnetic, weak nuclear, and strong nuclear forces are fused into a single unified field.Thus far, physicists...
- W' and Z' bosons
In particle physics, W' and Z' bosons refer to hypothetical new gauge bosons that couple to Standard Model fermions via their isospin. They are named in analogy with the Standard Model W and Z bosons.-Types of W' bosons:...
External links