Quarks

J. I. Friedman, H. W. Kendall, R. E. Taylor, et. al., Observed Behavior of Highly Inelastic Electron-Proton Scattering, 1969

Ernest Rutherford investigated the structure of atoms by hitting them with various fast-moving particles. The particles came from radioactive substances, and were merely channelled toward the target by heavy shielding around the radiation source. In the years following, scientists began observing the collisions of cosmic rays (extremely fast moving particles from outer space) with Earth-based targets and using electromagnets to push and pull electrically charged particles, reliably accelerating them towards their targets at steadily increasing speeds. During the 1950s, a whole mess of new particles were discovered, some two-dozen quickly decaying fragments observed spinning away from high energy collisions.

During the 1960s, Murray Gell-Mann and others brought order to the menagerie of new particles, first with the eight-fold way, then with the quark model. This model treated the observed particles as though they were composed of two or three smaller particles. Because they were never observed, they were initially assumed to be entirely theoretical abstractions.

Particle accelerator technology was, by this time, increasing in leaps and bounds. Using the 2-mile long particle accelerator at Stanford (SLAC), the authors of this paper fired electrons at speeds in excess of 99.9999999% of the speed of light, aiming them at protons and expecting, much like Rutherford's graduate student, only to confirm that protons have no internal structure. Instead, they found that the electrons bounced off of the protons as though there were three point-like particles inside. Suddenly, quarks became much more than theoretical!

Later work went on to show how the strong force prevents quarks from ever breaking away from one another, explaining why they are never observed independently. The strong force binds quarks together into protons, neutrons, and the rest of the non-fundamental particles in much the same way that electromagnetism binds protons and electrons together into atoms. Also, in the same way that residual electric charge can hold atoms together, forming molecules, residual strong force binds protons and neutrons together to form atomic nuclei.

Yet more work has shown how the electroweak force acts on quarks, rather than protons and neutrons; how the strong force is transmitted by gluons, in the same way that the electroweak force is transmitted by the W⁺, W^-, Z, and photon; and has attempted (several times) to unify the strong and electroweak forces.