252 The Poetry of Physics and The Physics of Poetry
different states of gluons. Quarks also have three different colour charges
and antiquarks have three kinds of anticolours.
The interactions of quarks and gluons are described by Quantum
Chromodynamics (QCD), which is a theory of the fundamental force of
nature, the strong interaction of baryons and mesons, i.e. hadrons, and is
patterned on Quantum Electrodynamics. It is an essential part of the
Standard Model of particle physics and describes and explains a huge
body of experimental data collected over many years.
The Weak Interaction
The weak interaction is one of the four basic forces of nature along
with the strong interaction, gravity and electromagnetism. It is the
force that accounts for the decay of the strongly interacting hadrons and
is extremely weak with a strength of 10-11 that of the electromagnetic
force and 10-13 of the strong interaction. The first example of the
weak interaction that was encountered was the beta decay of a free
neutron into a proton, an electron and an antineutrino, n → p + e + νe.
[A note on notation: the neutrino will be represented by νe and the
antineutrino by νe] The neutrino and antineutrino are uncharged
elementary particles with a minuscule, but nonzero mass that travels very
close to the speed of light and is difficult to observe because they pass
through ordinary matter for the most part without interacting. The
existence of this particle was first suggested by Wolfgang Pauli to
explain the lack of conservation of energy, momentum and angular
momentum when neutron decay into a proton and an electron was first
observed. Neutrino was first detected in 1956 when they were observed
in induced beta decay (νe + p → n + e+) in which antineutrinos colliding
with protons produced pairs of neutrons and positrons that could be
detected.
Another example of an early encounter of the weak interaction
was the decay of the muon into electrons and neutrinos and the decay of
the pions into muons and e-neutrinos, νe and μ-neutrinos, νμ. Muons
were first discovered in cosmic ray showers and were observed to
decay with a mean life of 2.2 x 10-6 seconds as follows: μ– → e– + νe + νμ
and μ+ → e+ + νe + νμ. Pions were also first discovered in cosmic ray
showers and were observed to decay with a mean life of 2.6 × 10-8
seconds as follows: π+ → μ+ + νμ and π– → μ– + νμ.