Z boson
Classification: boson
Fundamental: yes
Mass: 91000 MeV
Interactions: weak
Spin: 1
Lifetime: 3e-25s
When the electromagnetic and weak forces were unified into electroweak theory, two force carriers were predicted. These were called the W and Z bosons, and they were discovered at the Super Proton Synchotron at CERN in 1983, thus establishing the standard model on strong experimental basis.
The Z boson interacts with particle, anti-particle pairs. It can decay into any particle that weighs less than half of its own weight. Since the Z is quite heavy, it can decay into everything except pairs of top quarks. In fact, the Z boson has been hugely useful in experimental particle physics. Since pairs of electrons or muons from Z boson decays are fairly easy to find in collisions, physicists use them to test properties of the detectors themselves. The most common method is called a tag and probe, when one lepton is tagged, and the quality of the second lepton is examined to see how good a job we did at identifying the lepton.
The Z boson also has helped put limits on the number of particle generations. The Z boson decays into pairs of neutrinos, which we can't detect. But we can establish how many Z bosons should be produced, and measure the rest of its decays, and use that to measure what fraction of decays we're missing and estimate the number of types of neutrinos. If there was a fourth family of particles with a light neutrino, the Z would decay into it and we would notice.
Actually, from Z decays we measure that there are just under three types of neutrinos. The "just under" accounts for neutrino mixing. So, if there is a fourth family of particles, its neutrino is heavy enough that the Z can't decay into it, and that would make a fourth family heavy indeed.
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