physicists were keen to learn about the nuclear force which was presumed to bind protons and neutrons together to form nuclei. This led to the construction of larger and larger particle colliders throughout the decades, the most recent being the Large Hadron Collider in Switzerland. These accelerators have revealed the existence of hundreds of new particles, and almost all of them can be classed as hadrons . The discovery of these hadrons was not greeted with glee: it seemed incredulous to believe that such a large number of particles could all be fundamental. Fortunately, the initial skepticism has been validated. A lot of work in the 1950s and 1960s has shown that all hadrons (which include the protons and neutrons) are composite particles: they themselves are made up of even smaller particles, called quarks. It is currently believed that there are six flavours of quark, each of which exist in one of three ‘colours’: red, green or blue. Almost all known hadrons can be accounted for by combining a suitable combination of quarks in varying levels of excitation. The entire spectrum can be explained by allowing gluons (the quanta of the QCD gauge field) to form particles too. 

Why quarks are not seen as free entities of their own is a very deep question, and is essentially the question of colour confinement . Physicists hypothesise that the force between two quarks does not diminish as they are separated (contrast this with QED and, say, a positron and electron). Therefore it would take an infinite amount of energy to separate two quarks. Hence quarks are forever bound into colourless combinations, such as red- green-blue or red-antired. These colourless combinations are the hadrons we see in particle detectors and cosmic rays. Confinement, although widely believed, is yet to be analytically proven. 

When quarks were first discussed in theoretical papers it was not certain if they were just convenient mathematical fictions, or whether they were 

