The Standard Model
By the mid 1930s, the understanding of the fundamental structure of matter
seemed almost complete.
Decades before, Rutherford had shown that atoms have relatively tiny but massive
nuclei. The quantum
theory had made sense of atomic spectra and electron orbitals. The discovery of
the neutron
had explained nuclear isotopes. So protons, neutrons, and electrons provided the
building blocks
of all matter. Some puzzles remained, however: What holds the protons, neutrons,
and electrons together to
form the nucleus and what are the three themselves made of?
To answer these questions, particle accelerators were built. Atoms, protons,
neutrons, and other known particles were smashed together at dazzling speeds..
An unexpected wealth
of new types of particles were discovered. Some were similar to protons and
neutrons. Others
were completely different. By the early 1960s a hundred or so more kinds of
particles had been identified, but physicists
still had no complete understanding of the fundamental forces.
Then in 1964, Murray Gell-Mann and George Zweig separately came to the idea that
protons and neutrons
were made of yet smaller objects, called quarks. Protons and neutrons
were made of three
quarks. Quarks could bond in threesomes or pairs.
The current, so-called Standard Model summarizes the conventional
discoveries of the sub-
atomic world:
Standard Model Chart
There are 6 known quarks: up, down, charm, strange, top, bottom. Up, charm, and
top all have +2/3
electric charge, while down, strange, and bottom quarks have -1/3 charge.
Quarks that group together
as threesomes are called baryons. A proton is composed of two up
quarks and one down
quark. A quark may bond with an antiquark to form
a meson. An antiquark has the same mass, but opposite charge. For
example, an anti-up
quark has -2/3 charge.
There is another set of 6 "fundamental" particles called leptons: electron,
electron-neutrino, muon,
muon-neutrino, tau, tau-neutrino. As the reader may notice, the electron, muon,
and tau have
corresponding neutrinos. Neutrinos are massless, neutral particles traveling at
the speed of light, involved
in the beta decay of neutrons.
There are four basic forces of the universe: strong, electromagnetic, weak,
gravity. Each force
has corresponding "force-carriers". Richard Feynman thought up the mechanisms
for attraction (or repellation)
of particles through such force-carriers. For example, the force-carrier for
electromagnetism is
the photon. When two electrons near each other, photons are exchanged and the
electrons are repelled.
Conversely, a photon "bond" is formed between electrons and protons that attract
the two together.
Quarks are held together by gluons, carriers of the strong force. The
weak force is carried
by W+, W-, Z0 particles and elusive gravitons are the medium of the
gravitational force.
The strong force, ie. gluons, occurs only between quarks. Any particles of
non-zero electric
charge may interact via electromagnetism. Gravity is observed between any
objects with mass.
The weak force may occur for leptons and quarks. Neutrinos, being massless,
neutral leptons, may
only form or interact through the weak force.
