Quarks | Research & Encyclopedia Articles

Quarks

In 1869, Dmitri Mendeleev proposed the periodic law as a method for organizing the known chemical elements in a systematic way. The periodic law was important to chemists because it brought some order into the study of the 50-odd elements then known. But it was not until four decades later that an explanation was found--the Niels Bohr model of the atom--for Mendeleev's system of organization.

In 1961, the American physicist, Murray Gell-Mann, and the Israeli physicist, Yuval Ne'eman, independently suggested a method for organizing the known subatomic particles. The Gell-Mann-Ne'eman system became known as the eightfold way. The eightfold way was an important concept to physicists because it brought order to the study of more than a hundred subatomic particles discovered in the preceding two decades. Like the periodic law before it, however, the eightfold way was a purely theoretical concept. Neither Gell-Mann nor Ne'eman could explain why particles could be arranged according to this scheme.

In 1963, however, Robert Serber, a theoretical physicist at Columbia University, pointed out to Gell-Mann that one way to explain the eightfold way would be to assume that all subatomic particles are made of three fundamental particles. Over the next year, Gell-Mann developed just such a theory, in which he referred to the three fundamental particles as quarks. The unusual name was taken from a line in James Joyce's Finnegans Wake, "Three quarks for Master Mark." At about the same time, a Swiss physicist, George Zweig, attempting to solve a quite different problem, came up with a theory very similar to that of Gell-Mann's.

According to Gell-Mann and Zweig, the three kinds—or "flavors"—of quarks combine to form all subatomic particles. Those three types of quarks became known as up (u), down (d), and strange (s). A proton, for example, is now known to consist of two up quarks and one down quark (uud) and a neutron of one up quark and two down quarks (udd).

Evidence for the existence of quarks soon became available. As far back as the 1950s, Robert Hofstadter had been studying the internal structure of protons and neutrons. By firing high energy beams of electrons at atomic nuclei, Hofstadter had found that protons and neutrons are not dimensionless points, that is, fundamental particles of matter. Instead, they seemed to be fuzzy ball-like objects that had some sort of internal structure.

By 1967, the new two-mile long linear accelerator at the Stanford Linear Accelerator Center (SLAC) made it possible to repeat Hofstadter's research with even higher precision. Jerome Friedman, Henry Kendell, and Richard Taylor examined the way in which electron beams from the SLAC accelerator were diffracted as they passed near protons and neutrons. They found that the observed diffraction patterns were consistent with the quark model proposed by Gell-Mann and Zweig.

Within a matter of three years, it became obvious that the three-quark model was insufficient to account for all known subatomic particles. As a result, a group of Harvard physicists proposed in 1970 the existence of a fourth quark, which they named charm (c). The simultaneous discovery in 1974 at Brookhaven National Laboratory and SLAC of the J/ particle confirmed the existence of this fourth quark. The J/ was shown to consist of the charm quark and its antiparticle (cc).

The quark story was still not complete. In 1977, a group of physicists at the Fermi National Accelerator Laboratory discovered a new particle, the upsilon, which could best be explained as containing yet a fifth quark and its antiparticle. This quark was given the name bottom, or beauty, (b), and the upsilon was designated as a bb particle.

At this point, an effort was made to organize the known quarks by classifying them according to the energy levels at which they exist. The up and down quarks can be found in such particles as the proton and neutron that occur at energy levels common to everyday events with which we are all familiar. Particles composed of the strange and charm quarks exist only at much higher energy levels, such as those that occur in cosmic rays and in particle accelerators. Finally, the bottom quark is manifested only at the very highest energy levels of which we know, such as those available in the most powerful particle accelerators now available and at the creation of the universe.

In this scheme of classification, if would appear that one more quark may be possible, a companion of the bottom (beauty) quark at the highest energy level. The hypothesized sixth quark has been called the top, or truth, (t) quark. In experiments conducted at the European Center for Nuclear Research (CERN) in 1984, Carlo Rubbia and his co-workers claimed to have found evidence for the sixth quark. Considerable uncertainty surrounds that claim, however, and many physicists regard the t quark as hypothetical still.

Quarks interact with each other by means of the strong force. The mediating particles that carry the strong force are called gluons. The field of physics that deals with the interactions of quarks and gluons is known as quantum chromodynamics.