intermediate vector boson
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Intermediate vector boson
One of the three fundamental particles that transmit the weak force. (An example of a weak interaction process is nuclear beta decay.) These elementary particles—the W+, W-, and Z0 particles—were discovered in 1983 in very high-energy proton-antiproton collisions. It is through the exchange of W and Z bosons that two particles interact weakly, just as it is through the exchange of photons that two charged particles interact electromagnetically. The intermediate vector bosons were postulated to exist in the 1960s; however, their large masses prevented their production and study at accelerators until 1983. Their discovery was a key step toward unification of the weak and electromagnetic interactions. See Electroweak interaction, Elementary particle, Fundamental interactions, Weak nuclear interactions
The W and Z particles are roughly 100 times the mass of a proton. Therefore, the experiment to search for the W and the Z demanded collisions of elementary particles at the highest available center-of-mass energy. Such very high center-of-mass energies capable of producing the massive W and Z particles were achieved with collisions of protons and antiprotons at the laboratory of the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. See Particle accelerator, Particle detector
Striking features of both the charged W and the Z0 particles are their large masses. The charged boson (W+ and W-) mass is measured to be about 80 GeV/c2, and the neutral boson (Z0) mass is measured to be about 91 GeV/c2. (For comparison, the proton has a mass of about 1 GeV/c2.) Prior to the discovery of the W and the Z, particle theorists had met with some success in the unification of the weak and electromagnetic interactions. The electroweak theory as it is understood today is due largely to the work of S. Glashow, S. Weinberg, and A. Salam. Based on low-energy neutrino scattering data, which in this theory involves the exchange of virtual W and Z particles, theorists made predictions for the W and Z masses. The actual measured values are in agreement (within errors) with predictions. The discovery of the W and the Z particles at the predicted masses is an essential confirmation of the electroweak theory.
Only a few intermediate vector bosons are produced from 109 proton-antiproton collisions at a center-of-mass energy of 540 GeV. This small production probability per pp ¯ collision is understood to be due to the fact that the bosons are produced by a single quark-antiquark annihilation. The other production characteristics of the intermediate vector bosons, such as longitudinal and transverse momentum distributions (with respect to the pp ¯ colliding beam axis), all provide support for this theoretical picture. See Quarks