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Quantum Theory

Development of Quantum Theory

George Zweig, strange quark, charm quark, quark model, gluons

Scientists of the early 20th century believed that all matter was composed of three types of particles—protons, neutrons, and electrons—and that these particles could not be split into anything smaller. By the late 1950s, however, this simple model could no longer explain all the particles that physicists had discovered. Scientists needed a new model to make sense of their findings. In 1964 American physicists Murray Gell-Mann and George Zweig independently developed a theory of particle physics that proposed quarks as the building blocks of protons and neutrons. Gell-Mann borrowed the word quark from James Joyce’s novel Finnegans Wake (1939), which contains the phrase “three quarks for Muster Mark.” The two physicists needed only two types of quarks to describe the proton and neutron accurately: the up quark and the down quark. At about the same time, physicists also had discovered new elementary particles, including kaons, that they called strange. The explanation of these particles required a third type of quark, so physicists named it the strange quark.

Physicists continued to use the three-quark model through the 1960s. Experiments conducted at the Stanford Linear Accelerator in Stanford, California, during the early 1970s supported the model. In these experiments, high-energy electrons collided with a target of protons. These experiments were analogous to experiments performed by British physicist Ernest Rutherford during the early 1900s. Rutherford had determined the inner structure of atoms by observing how particles scattered off atoms. In the 1970s scientists looked at how electrons scattered off protons to learn about the internal structure of the proton. These experiments conclusively supported the existence of quarks and gluons inside protons.

Even though physicists didn’t need more than three quarks to describe existing particles, American physicist Sheldon Glashow, Greek physicist John Iliopoulis, and Italian physicist Luciano Maiani developed a theory in 1970 that predicted the existence of a fourth quark, called the charm quark. A particle containing the charm quark, the second-generation partner of the strange quark, was discovered at the Stanford Linear Accelerator and at the Brookhaven National Laboratory in Brookhaven, New York, in 1974. The discovery in 1975 of a third-generation lepton, another building block of matter, led scientists to predict the existence of a third generation of quarks. The bottom quark was discovered in 1977 and, after a long search by physicists from all over the world, the top quark was found in 1995. The long-awaited discovery of the top quark filled a hole in the standard model, a theory that physicists had developed to explain particles and their interactions. The standard model predicted that three generations of quarks should exist, each one containing two different quarks.

Current theory and data suggest that the six quarks scientists have discovered are all that exist and that quarks are fundamental—that is, they cannot be split into anything smaller. Physicists continue to conduct experiments, however, to discover whether new data will dispute their findings and to learn more about the properties of quarks. Physicists do not know how long single quarks exist (since they are always found in other particles), or whether quarks could combine in any other ways. Scientists suspect that particles containing one lepton and one quark, called leptoquarks, may exist, but they have yet to find any. Nor have experiments yet indicated how quarks acquire their masses. The standard model predicts the existence of a particle, called the Higgs boson, that would give quarks their mass, but scientists have yet to detect a Higgs boson in any experiment. Answers to these questions will have to wait for future experiments and theories that go beyond the principles of the standard model.

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