Fitting Quarks Into An Overall View of Matter
The quark theory fit into a larger scheme of matter, the Standard Model. According to the Standard Model, matter is made up of 12 fundamental particles divided equally into two major families, quarks and leptons. Leptons, which are thought to be fundamental particles with no smaller parts, include the electron, two heavier versions of the electron, and three uncharged particles called neutrinos. Neutrinos were long thought to be massless, but recent research has indicated that they may possess a tiny amount of mass.
Only the two lightest members of each family—the up and down quarks, and the electron and electron neutrino—play any major role in the universe today. All the heavier quarks and heavier charged leptons have only a momentary existence, because they are highly unstable. These particles can be created in particle accelerators, but they existed in nature in abundance only in the first few moments after the big bang, the colossal explosion of matter and energy that astronomers believe gave birth to the universe billions of years ago. When physicists push particle accelerators to higher and higher energies, they are in effect duplicating the conditions of the big bang.