Discovery of carrier particles of weak force W and Z

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 W⁺, W⁻, and Z. The W bosons have either a positive or negative electric charge of 1 elementary charge and are each other's antiparticles. The Z boson is electrically neutral and is its own antiparticle. The three particles have a spin of 1. The W bosons have a magnetic moment, but the Z has none. All three of these particles are very short-lived, with a half-life of about 3×10⁻²⁵ s. Their experimental discovery was a triumph for what is now known as the Standard Model of particle physics.

The W bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the "Z particle", and later gave the explanation that it was the last additional particle needed by the model. The W bosons had already been named, and the Z bosons have zero electric charge.

The two W bosons are verified mediators of neutrino absorption and emission. During these processes, the W boson charge induces electron or positron emission or absorption, thus causing nuclear transmutation. The Z boson is not involved in the absorption or emission of electrons and positrons.

The Z boson mediates the transfer of momentum, spin and energy when neutrinos scatter elastically from matter (a process which conserves charge). Such behavior is almost as common as inelastic neutrino interactions and may be observed in bubble chambersupon irradiation with neutrino beams. 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. Because neutrinos are neither affected by the strong force nor the electromagnetic force, and because the gravitational force between subatomic particles is negligible, such an interaction can only happen via the weak force. Since such an electron is not created from a nucleon, and is unchanged except for the new force impulse imparted by the neutrino, this weak force interaction between the neutrino and the electron must be mediated by an electromagnetically neutral, weak-force boson particle. Thus, this interaction requires a Z boson.

These bosons are among the heavyweights of the elementary particles. With masses of 80.4 GeV/c² and 91.2 GeV/c², respectively, the W and Z 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. (Although gluons are also presumed to have zero mass, the range of the color force is limited for different reasons; see color confinement.)

All three bosons have particle spin s = 1. The emission of a W⁺ or W⁻ 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 W 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" (such as strangeness, baryon number, charm, etc.). The emission or absorption of a Z boson can only change the spin, momentum, and energy of the other particle.

Weak nuclear force

The W and Z 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 W 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 (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 flavour of a single quark:

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

Z boson

The Z boson is its own antiparticle. Thus, all of its flavour quantum numbers and charges 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. Z boson interactions involving neutrinos have distinctive signatures: They provide the only known mechanism for elastic scattering of neutrinos in matter; neutrinos are almost as likely to scatter elastically (via Z boson exchange) as inelastically (via W boson exchange). The first prediction of Z bosons was made by Brazilian physicist José Leite Lopes in 1958, by devising an equation which showed the analogy of the weak nuclear interactions with electromagnetism. Steve Weinberg, Sheldon Glashow and Abdus Salam later used these results to develop the electroweak unification, in 1973. Weak neutral currents via Z boson exchange were confirmed shortly thereafter (also in 1973), in a neutrino experiment in the Gargamelle bubble chamber at CERN.

Discovery

Unlike beta decay, the observation of neutral current interactions that involve particles other than neutrinos requires huge investments in particle accelerators and detectors, such as are available in only a few high-energy physics laboratories in the world (and then only after 1983). This is because Z-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 Z boson.

The discovery of the W and Z 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 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 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 (led by Rubbia) and UA2 (led by Pierre 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 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 W⁺, W⁻, and Z⁰ bosons, together with the photon (γ), comprise the four gauge bosons of the electroweak interaction.

Physicists working with the Gargamelle bubble chamber experiment at CERN presented the first convincing evidence to support this idea in 1973. Neutrinos are particles that interact only via the weak interaction, and when the physicists shot neutrinos through the bubble chamber they were able to detect evidence of the weak neutral current, and hence indirect evidence for the Z boson.

At the end of the 1970s, CERN converted what was then its biggest accelerator, the Super Proton Synchrotron, to operate as a proton-antiproton collider, with the aim of producing W and Z bosons directly. Both types of particle were observed there for the first time in 1983. The bosons were then studied in more detail at CERN and at Fermi National Accelerator Laboratory in the US. In 1983 two experiments at the European Organization for Nuclear Research (CERN) detected characteristics closely approximating those predicted for the formation and decay of W and Z particles. Their findings constituted the first direct evidence of weak bosons and provided strong support for the electroweak theory. The two teams observed numerous clear-cut instances of weak bosons in proton-antiproton collision experiments that were carried out in a 540-gigaelectron-volt (GeV; 10⁹ eV) colliding-beam storage ring. All of the observed W particles had a mass of about 81 GeV, or approximately 80 times the mass of the proton, as had been predicted by the electroweak theory. The electrically neutral Z particles detected, with a rest mass of 93 GeV, were also consistent with prediction.

During the 1990s, the Large Electron-Positron collider at CERN and the SLAC Linear Collider in the US produced millions of Z bosons for further study.

These results culminated in the need to search for the final piece of the Standard Model – the Higgs boson. In July 2012, scientists at CERN announced that they had observed a new particle consistent with the appearance of a Higgs boson.

Although more time and analysis is needed to determine if this is the particle predicted by the Standard Model, the discovery of the elusive Z bosons set the stage for this important development.

The W and Z bosons decay to fermion–antifermion pairs but neither the W nor the Z bosons can decay into the higher-mass top quark. Neglecting phase space effects and higher order corrections, simple estimates of their branching fractions can be calculated from the coupling constants.

REFERENCES

CERN. Available in: https://home.cern/science/physics/z-boson. Access in: 18/11/2018.

CERN Courier. Available in: https://cerncourier.com/the-discovery-of-the-weak-neutral-currents/. Access in: 18/11/2018.

Wikipedia. Available in: https://en.wikipedia.org/wiki/W_and_Z_bosons. Access in: 18/11/2018.

Encyclopaedia Britannica. Available in: https://www.britannica.com/science/W-particle. Access in: 18/11/2018.

Nobel Prize. Available in: https://www.nobelprize.org/prizes/physics/1984/summary/. Access in: 18/11/2018.