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. 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 force--i.e., a theory that is symmetric with respect to
transformations in space and time--suggested 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.

This article was written by Christine Sutton, who is a Research
Associate in the Department of Nuclear Physics at the University of
Oxford. She is author of The Particle Connection (1984).

Copyright © 1997 Encyclopædia Britannica, Inc. All Rights
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