The Subatomic Zoo
By the 1960s, physicists had discovered a large number of subatomic particles that, like the proton and neutron, attract one another through the nuclear force (also termed the strong force). Classification of all these particles, including pions, kaons, and others only seen after collisions of cosmic rays (powerful photons originating in outer space) with Earthly matter, produced results reminiscent of the chemists' classification of elements into the periodic table. In both cases, grouping entities by their observed properties revealed something about their fundamental structure.
American physicist Murray Gell-Mann (1929–), together with Israeli physicist Yuval Ne'eman (1925–) and, independently, American physicist George Zweig (1937–), introduced the idea of three basic building blocks for all particles that felt the nuclear force. Proving that physicists are not without a sense of humor, Gell-Mann called them "quarks," after the line in the James Joyce novel Finnegan's Wake, "Three quarks for Muster Mark!" (No one knows what Joyce meant by the made-up word, if anything.) Gell-Mann and his colleagues introduced three quarks and dubbed them "up," "down," and "strange"—these whimsical names being labels for each quark type's flavor (a property common to all quarks). They found that they were able to describe all baryons (e.g., protons and neutrons) as combinations of three quarks apiece and all mesons (e.g., pions and kaons) as combinations of two quarks apiece.
Quarks had to have a fractional electric charge (i.e., a fraction of the electron charge, the minimal charge unit previously conceived of) for this scheme to work—a somewhat radical idea. The up quark was proposed to have a charge of +(2⁄3)e (where e is the charge on the electron), the down quark −(1⁄3) e, and the strange quark −(1⁄3)e. Then the proton could be built from two up quarks and a down, (up + up + down), and the neutron by a complementary set opposite (down + down + up). All the ordinary matter that we see around us is made of up and down quarks (plus electrons). Quarks also have their associated antiparticles: the anti-up quark, the anti-down, and the anti-strange. The pion is an up + anti − down. Quarks also have the quantum-mechanical property of spin, equal to 1/2.
As a model this scheme could describe baryons and mesons. But did quarks actually exist? Using particle accelerators, Maurice Jacob and Peter Lanshoff smashed high-energy electrons into protons in 1980. They found that some electrons bounced off at large angles, a few even backwards—more than would be expected if the proton's charge was uniformly spread across its volume. Their results were consistent with the idea that the proton was in fact composed of three sub-particles. This and other experiments afterward established the physical reality of quarks.