by Brad Lambert
For many years, the nuclear physics community postulated that atoms were made of three particles that they considered "elementary". There were the proton and the neutron, which appeared to differ only in charge, that resided in the nucleus, and the negatively charged electrons orbiting in a cloud around the positive protons and neutral neutrons in the nucleus. Recently, evidence in high-energy collisions of protons and/or antiprotons and neutrons and/or antineutrons began to show that protons and neutrons may not be the smallest components of the atomic nucleus. These theoretical subunits of protons and neutrons were called "quarks".
The theory developed around the hypothesis that baryons, particles such as the proton and neutron, consisted of a combination of three quarks, and that mesons, particles with a mass of less than that of a proton, consisted of a pair of quarks. To conserve charge of the hadrons, the group of baryons and mesons combined, each quark was assigned a fractional charge of e/3, -e/3, or 2e/3, where e is the charge of the electron, 1.602 * 10-19 coulombs. Since nothing else was known to have a fractional charge, scientists set out to search for this strange phenomenon. It was reasoned that something as odd as a partial electron charge should be easy to find. Samples of matter both terrestrial and extraterrestrial were probed for these exotic pieces of charge, and little was found. Researchers did find some evidence for fractional charges in a very few of the materials, but in all of the cases, the evidence was not firm enough to solidly support the existence of quarks. (Ohanian 489)
After considering these results, scientists thought that perhaps if quarks did not exist in nature freely, it could be possible to collide two particles that are made of quarks and cause the constituent particles to fly apart from each other. This was tested in many laboratories, and researchers found that shattering protons seemed impossible. (Neutrons were not used, as magnetic fields can only accelerate charged particles.) The next line of approach was one that would attempt to "see" the inside of a proton using high-energy electrons, similar to the way Ernest Rutherford "saw" the interior of an atom using alpha particles. Using Stanford University's linear accelerator (SLAC), electrons could be accelerated to a speed such that their energy was 22 GeV (22 billion electron-volts, or [2.2 * 1010] * [1.602 * 10-19 joules (J)]. At this energy, the resolution was approximately 10-17 m. Since the radius of a proton is on the order of 10-15 m, this energy would be sufficient to explore the inner structure of the proton. At lower electron energies, the protons acted as single balls of charge, as previous nuclear theory suggested. However, at energies at or near 22 GeV, the proton's interior began to be seen as an area consisting of smaller, compact masses that resembled point charges. Seemingly, the quark's existence was confirmed. (Ohanian 489-490)
Originally, three flavors, or types, of quarks were postulated to exist, since all baryons were composed of no more than three quarks. These three flavors were named "up", "down", and "strange". Two problems arose with the three quark-three antiquark model. First, several particles were observed to be composed of only one flavor of particle. This seemed to contradict the Pauli Exclusion Principle, which states that no two particles can exist in the same quantum state simultaneously. (Ohanian 491) To solve this problem, a new property of quarks, "color", was introduced. Three colors of quarks were proposed: red, green, and blue. As with flavors and antiflavors, the same number of colors and anticolors would exist. This property was soon found to govern the "color force", the part of the strong nuclear force that acts on quarks. This force differs from the strong nuclear force that acts upon nucleons in that its strength is independent of separation distance of quarks. Since this was true, scientists could then explain why they could never find fractional charges in nature and confirm them as lone quarks. The color field, as the theory of quantum chromodynamics (QCD) called it, prevented quarks from existing outside a baryon. This color field, scientists found, was capable of interacting with itself so strongly that it could create particles inside a baryon without quarks. These uncharged particles are called gluons or glueballs. They are the reason that the mass of the baryon is so much greater than the sum of the masses of the constituent quarks. (Weingarten 117)
Later, high-energy interactions of particles showed some significant discrepancies in observations of results and hypothesized results. One by one, three new flavors of quarks were introduced to deal with such discrepancies. The first was the "charm", followed by the "bottom", then finally the recently-confirmed "top". To date, the six quark-six antiquark and three color-three anticolor model is capable of explaining most, if not all, known reactions. (Ohanian 492)
Quarks have been a subject of great fascination for many years. Many doubted their existence, and many others doubted the models proposed to explain their existence and role in particle physics. Even today, some doubt that the present model is correct. Some say that more flavors and more colors will be found soon. This may very well happen. Whether or not this occurs, the future looks bright. With all the experimental evidence found so far, the particle physics community can rest assured that only modifications to present theory lay ahead; the quark theory of matter seems quite safe.
Works Cited
Ohanian, Hans C. Modern Physics. Englewood Cliffs, NJ: Prentice Hall, Inc. 1987.
Weingarten, Donald H. "Quarks by Computer". Scientific American, February 1996,
p. 117, vol. 274, no. 2.