Z PARTICLE

electrically neutral carrier of the weak nuclear force that acts upon all known subatomic particles. It is the neutral partner of the electrically charged W particle (q.v.). The Z particle has a mass of 91.16 0.03 GeV, or nearly 100 times that of the proton. The W is slightly lighter, with a mass of 80.6 0.4 GeV. Both particles are very short-lived, having lifetimes of only about 10-25 second. The concept that the weak force is carried by intermediary messenger particles arose in the 1930s, following the successful description of electromagnetic interactions in terms of the emission and absorption of photons. For the next 30 years or so it appeared that only charged weak messengers were necessary to account for all observed weak interactions. However, in the 1960s attempts to produce a gauge-invariant theory of the weak forcei.e., a theory that is symmetric with respect to transformations in space and timesuggested unifying weak and electromagnetic interactions. The resulting electroweak theory required two neutral particles, one of which could be identified with the photon and the other as a new carrier for the weak force, called the Z. The first evidence for the Z particle came in 1973 in experiments at the European Organization for Nuclear Research (CERN) near Geneva. Experiments revealed the existence of neutral current interactions between neutrinos and electrons or nuclei in which no transfer of electric charge occurs. Such reactions could be explained only in terms of the exchange of a neutral Z particle. Z particles and W particles were later observed more directly in 1983 in further experiments at CERN that collided protons and antiprotons together. The team that undertook these experiments was led by Carlo Rubbia and Simon van der Meer. Since that time the large electron positron (LEP) collider at CERN has been used to produce thousands of Z particles by colliding together electrons and positrons at total energies around 92 GeV. Studies of the decay of the Z particles produced in this way reveal what is known as the width of the Z, or the intrinsic variation in its mass. This width is related to the particle's lifetime through the uncertainty principle, which states that the shorter the lifetime of a quantum state, the greater is the uncertainty in its energy or equivalently its mass. The width of the Z particle thus gives a measure of its lifetime, thereby reflecting the number of ways in which the particle can decay, since the greater the number of ways it can decay, the shorter is its life. In particular, measurements at CERN show that when the Z decays to neutrino-antineutrino pairs, it produces three and only three types of lightweight neutrino. This measurement is of fundamental importance, because it indicates that there are only three sets each of leptons and quarks, the basic building blocks of matter. Christine Sutton