W and Z bosons
The W and Z bosons are together known as the weak 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 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. 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 directly with the electron, since this behavior happens more often when the neutrino beam is present. In this process, the neutrino simply strikes the electron 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.
All three bosons have particle spin s = 1. The emission of a or boson either raises or lowers 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". The emission or absorption of a boson can only change the spin, momentum, and energy of the other particle.
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 and an electron antineutrino:
Again, the neutron is not an elementary particle but a composite of an up quark and two down quarks. It is in fact 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 as inelastically. Weak neutral currents via boson exchange were confirmed shortly thereafter, in a neutrino experiment in the Gargamelle bubble chamber at CERN.Predicting the W and Z
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 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 gauge theory. Some mechanism is required to break the SU 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 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 gauge coupling, is U 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. 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 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 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 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 W 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 and UA2, and were the collaborative effort of many people. Van der Meer was the driving force on the accelerator end. 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.
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 or to a quark and antiquark of opposing types. 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, NC = 3. The decay widths for the W+ boson are then proportional to:Here,,, denote the three flavours of leptons. ,, denote the three flavours of neutrinos. The other particles, starting with and, all denote quarks and antiquarks. The various Vi j denote the corresponding CKM matrix coefficients.
Unitarity of the CKM matrix implies that
Z bosons
bosons decay into a fermion and its antiparticle. As the boson is a mixture of the pre-symmetry-breaking and bosons, each vertex factor includes a factor , where is the third component of the weak isospin of the fermion, is the electric charge of the fermion, 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. The results tabulated below are just estimates, since they only include tree-level interaction diagrams in the Fermi theory.