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Cosmic Radiation and Elemenm~ Panicles 297

TABLE 10.3. Forces of nature and their exchange particles

Force Relative Range Exchange Spin Rest
strength particle mass
1. The nuclear force 1 10 -la m gluon 1 0
(the strong force)
2. The electromagnetic 10 -2 long photon 1 0
force
3. The weak force 10 -13 very Z ~ 1 91.2 GeV
short {W+,W - 1 79.9 GeV

4. Gravitation 10-38 very graviton 2 0
long

cm. The quarks can not appear free but must appear together in groups of two or three.
The second family in Table 10.2 contains the "heavy electron', the rouen and the muon
neutrino, and the charm and the strange quarks. The third family contains the tau particle,
the tau electron, and the two quarks referred to as top (or truth) and bottom (or beauty).
These quarks can only be produced in high energy particle reactions.
By combination of quarks and leptons, the true elementary particles of nature, it is
possible to systematize all known particles. The success of this theory, founded on good
experimental evidence, has been so great that its name, the Standard Model of matter, is
justified.
The force between two particles arises from the exchange of a "mediator" that carries the
force at a f'mite speed: one of the particles emits the mediator, the other absorbs it. The
mediator propagates through space and, briefly, is not lodged with either particle. These
mediators have the same properties as the elementary particles - mass, electric charge, spin
so physicists often call them particles as well, even though their role in nature is quite
-
different from that of the elementary particles. The mediators are the mortar that binds the
particle building blocks together.
Three kinds of mediators - or exchange forces, as we have called them - are known in
the nuclear world: photons which are involved in the electromagnetic force, gluons which
are the mediators of the strong nuclear force, and the weak force mediators, which
underlies the radioactive decay. We know now a great deal about the photons and the
gluons, but little about the weak force. The weak force mediators are the W +, W- and Z 0
"particles". A high energy electron is supposed to be able to emit a Z 0, and then a positron
can absorb the electron: the particles annihilate each other, leaving the Z 0 momentarily free.
Afterwards the Z 0 must decay back into a pair of elementary particles, such as an electron
and positron, or a quark and an antiquark. Z ~ particles are produced in high energy
proton-antiproton colliders; recently researchers at the CERN and SLAC laboratories were
able to determine the Z 0 mass to 91.2 GeV. This is the heaviest known unit of matter.
We conclude this chapter by Table 10.3, which summarizes the forces of nature and the
corresponding exchange particles.