The Z particle is one of the intermediate vector bosons, a pair of particles predicted in 1979 in order to unify two of the fundamental forces: the electromagnetic and weak forces into the electroweak force. The Z particle has no electric charge, but is otherwise very similar to the W particle.

The precise reasons for the large difference in mass is unknown. They were detected at CERN in 1982 and the Z particle's mass was measured at 93GeV.

The theory of the weak nuclear force developed by Steven Weinberg and Abdus Salam predicted that apart from the W particles that mediated quark flavor changes and resulted in beta decay, there is another type of intermediate vector boson associated with the weak force, the Z particle, which causes the so-called "neutral current" processes (so named because the Z particle is electrically neutral), that generally involve the ghostly particles known as neutrinos. These phenomena were first observed at CERN in 1973, and gave much-needed experimental proof of the new Weinberg-Salam theory of the weak force. The actual Z particle which mediated these neutral current phenomena were observed a decade later by Simon van der Meer and Carlo Rubbia, also at CERN.

An example of a neutral current event what happens when a neutrino or some other similar particle is scattered on an atomic nucleus. Because the Z particle that causes this interaction is very, very heavy (91 GeV/c2, or about the mass of a zirconium atom), the distance at which the neutrino must approach the atomic nucleus for a neutral current reaction to occur must be correspondingly short, less than ħ/mZc, 1.4 × 10-17 m (smaller than the size of a proton!), i.e. the neutrino must hit the target nucleus very nearly head-on. This means that the chances of this event happening are extremely small, even with very high-energy neutrinos, but these chances are not zero. Very occasionally one of the neutrinos will actually hit an atomic nucleus head-on, emitting a Z particle which is absorbed by the nucleus, which breaks apart as a result. The neutrino loses energy, but is otherwise unchanged. One experiment at Fermilab involved using a particle detector that consisted of 700 tons of steel and detection apparatus. One in a billion neutrinos generated by the Tevatron managed to collide with the iron nuclei in the steel, breaking it apart.

The neutral current processes mediated by the Z particle also provided the final evidence of neutrino oscillations and served to close the solar neutrino problem. The Sudbury Neutrino Observatory (SNO) used heavy water and watched for neutral current reactions with solar neutrinos that broke apart the deuterium nuclei in the heavy water. Unlike earlier neutrino observatories like the Super Kamiokande, which used inverse beta decay (charged current reactions that involve the W particles) to find neutrinos, neutral current reactions in the SNO should occur regardless of the type of neutrino. The older detectors would have detected only electron neutrinos, so if neutrino oscillations were occurring that changed the types of neutrinos (transforming an electron neutrino generated by a nuclear fusion reaction in the sun into a muon neutrino for instance), they would see only a fraction of the sun's predicted neutrino flux. The observations of neutral current events at the SNO gave results that agreed closely with theories of the sun's neutrino output, providing persuasive proof of neutrino oscillations.

As previously noted, the Z particle has no electric charge, and does not possess any other distinguishing property, so it is also its own antiparticle (as with the photon and other uncharged field bosons). Like its partner the W particle, it also has spin 1, as the weak force is a vector field. It has a mass of 91 GeV/c2, and decays very rapidly into a quark and its matching antiquark (producing hadron jets or sometimes mesons), or a lepton and its corresponding anti-lepton, depending on how much energy the particle has.

Sources:

http://www2.slac.stanford.edu/vvc/theory/weakinteract.html

http://www2.slac.stanford.edu/vvc/theory/weakbosons.html

http://www.fnal.gov/pub/ferminews/Ferminews01-11-09.pdf

http://www.physics.carleton.ca/research/sno/results/rates.html

http://news.bbc.co.uk/1/hi/sci/tech/1943837.stm

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