The Epic Interactive Encyclopedia 1998/Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso/P/Particle_physics/INFOTEXT

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The study of elementary particles that make
up all atoms. Atoms are made up of positively
charged protons and, except for hydrogen,
neutrons (which have no charge) in the
nucleus, surrounded by negatively charged
electrons. Nuclei do not split apart easily;
they usually need to be bombarded by
particles such as protons, raised to very
high kinetic energies by particle
accelerators. Pioneering research took place
at the Cavendish laboratory, Cambridge,
England. In 1895, Joseph Thomson discovered
that all atoms contain identical, negatively
charged particles (electrons) which could
easily be freed. By 1913, Ernest Rutherford
had shown that electrons surround a very
small, positively-charged nucleus, thus, the
nucleus of a hydrogen atom consists of a
single positively charged particle, a proton
(identified by James Chadwick in 1932). The
nuclei of other elements are made up of
protons and uncharged particles called
neutrons. 1932 also saw the discovery of a
particle, predicted by Paul Dirac, with the
mass of an electron, but an equal and
opposite charge - the positron. This was the
first example of an antiparticle; it is now
believed that almost all particles have
corresponding antiparticles. The following
year, Wolfgang Pauli argued that a hitherto
unsuspected particle must accompany electrons
in beta-emission; the so-called
electron-neutrino. Neutrino radiation is
extremely penetrating. particles and
fundamental forces By the mid-1930s, four
fundamental kinds of force had been
identified. The electromagnetic force acts
between all particles with electric charge,
and was thought to be related to the exchange
between the particles of photons, packets of
electromagnetic radiation. The strong force
(holding quarks together to form protons and
neutrons, and binding these particles
together inside the nucleus) is transmitted
by the exchange of particles called gluons
between quarks and antiquarks. Theoretical
work on the weak force (responsible for beta
radioactivity of the Sun) began with Enrico
Fermi in the 1930s; current theory suggests
the exchange during weak interactions of W
and Z particles with masses some 100 times
that of the proton. The existence of W and Z
particles was confirmed in 1983 at CERN. The
fourth fundamental force, gravity, is
experienced by all particles; the projected
go-between particles have been dubbed
gravitons. leptons and quarks The electron
and electron neutrinos are examples of
leptons - particles with half-integral spin
that `feel' the weak, but not the strong
force. There are known to be two more
electron-like leptons plus their neutrinos:
the muon (found by US physicist Carl Anderson
in cosmic radiation in 1937), and the tauon,
a surprise discovery of the 1970s. hadrons
(particles that `feel' the strong force)
started to turn up in bewildering profusion
in experiments in the 1950s and 1960s. They
are classified into mesons, with whole-number
or zero spins, and baryons, with
half-integral spins. It was shown in the
early 1960s that if hadrons of the same spin
are represented as points on suitable charts,
simple patterns are formed. This symmetry
enabled a hitherto unknown particle, the
omega-minus, to be predicted from a gap in
one of the patterns; it duly turned up in
experiments. In 1964, Murray Gell-Mann
suggested that all hadrons were built from
just three types or flavours of a new
particle with half-integral spin and charge
of magnitude either 1/3 or 2/3 that of an
electron, which Gell-Mann christened the
quark. Mesons are quark-antiquark pairs
(spins either add to one or cancel to zero)
and baryons are quark triplets. To account
for new mesons such as the psi the number of
quark flavours had risen to six by 1985.