Experiments in which protons and neutrons were bombarded with high-energy electrons have given indications that protons and neutrons are not amorphous masses but composed of distinct subparticles. The subparticles have been named partons, and whether or not they correspond to the hypothetical quarks remains a moot question.
The so-called partons seen in experiments at the Stanford Linear Accelerator Center were indeed quarks — a discovery that won three researchers the 1990 Nobel Prize in physics (SN: 10/27/90, p. 263). Predicted by physicists Murray Gell-Mann and George Zweig in 1964, quarks are the building blocks of most of the universe’s ordinary matter. Quarks were originally thought to come in three varieties: up, down and strange. But particle collider experiments have revealed three additional types: charm, bottom and top (SN: 4/30/94, p. 276). Quarks usually come in pairs or trios. Recently, physicists have glimpsed more elaborate tetraquarks and pentaquarks (SN: 8/1/20, p. 14).