A quark is a fundamental particle which possesses both electric charge and 'strong' charge. They
combine in groups of two or three to form composite objects (called mesons and baryons,
respectively), held together by the strong force. Protons and neutrons are familiar examples of
such composite objects -- both are made up of three quarks.
The quarks come in six different species (physicists call them 'flavors'), each of which have a
unique mass. The two lightest, unimaginatively called 'up' and 'down' quarks, combine to form
protons and neutrons. The heavier quarks aren't found in nature and have so far only been observed
in particle accelerators.
How do we know they exist? At first many physicists felt they were no more than fictitious entities
invented to make certain particle physics calculations easier (legend has it that Murray Gell-Mann
took the name from a word in James Joyce's 'Finnegan's Wake'). However, particle physics
experiments over the last thirty years have proven otherwise. When protons and neutrons are struck
with particles that truly are fundamental (like electrons, neutrinos, photons, etc.) the protons
and neutrons reveal their structure in the way the colliding particle rebounds. This is analogous
to the way Rutherford discovered the nucleus within the atom by bombarding gold with radiation. The
results of these experiments show that the proton, for example, is composed of three fundamental
objects with just the right properties to be the postulated 'quarks' of Gell-Mann. Furthermore, the
theory that describes the interactions of quarks with each other also predicts the properties of
the composite objects they form. These predictions have been proven to be correct, allowing us to
develop a 'periodic table' of the known baryons and mesons -- another spectacular success of the
Answered by: Brent Nelson, M.A. Physics, Ph.D. Student, UC Berkeley
'I believe there is no philosophical high-road in science, with epistemological signposts. No, we are in a jungle and find our way by trial and error, building our road behind us as we proceed.'