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Quarks 2008

Page history last edited by Sally Hair 15 years ago

Quarks

 

Quarks are currently considered the smallest constituents of non-lepton matter. Different combinations of quarks are known as hadrons, and include protons and neutrons along with a potentially infinite number of other possibilities. Quarks have charges of either +2/3 or -1/3, and a spin of +1/2. The most common types, or "flavors," of quark are the up and down quarks, which form the matter we see on a regular basis and part of the first "family" of elementary particles. These families increase in mass and energy, with the first being the lightest and least energetic. The second family contains the strange and charm quarks, while the third contains the top and bottom (truth and beauty) quarks:

 

The three families, or "generations," of matter, including leptons,

force carriers, and the six types of quarks: up and down, charm and strange,

 

 

Physicist Murray Gell-Mann and George Zweig first postulated the existence of quarks in 1964. Research had shown that there were many more hadrons than leptons (electrons and neutrinos), suggesting that leptons were more elementary than hadrons; in other words, that there must be smaller particles making up protons, neutrons, etc. Isospin invariance, the similarity in reaction to the strong force of both protons and neutrons, and observations of mesons ("strongly interacting particles," according to Fritzsch), led them to suggest the possibility that quarks and antiquarks combine in quark-antiquark pairs (in the case of mesons) or groups of three quarks (hadrons). The Stanford Linear Accellerator Center (SLAC) in California tested this theory in 1969 by firing high-energy electrons at protons and measuring the angles at which they were deflected. Since Gell-Mann and Zweig had already proposed their theory, the researchers knew that they were searching for angles that indicated two types of quarks, one with charge +2/3 and one with charge -1/3. The experimental data matched the theory, confirming its veracity.

 

Since then, physicists have discovered the strange quark, a more massive version of the down quark, and then the charm quark based on the assumption that there could not be four types of leptons, as currently believed, and only three types of quarks. In 1977, Fermilab discovered an additional quark, the bottom quark, by using proton collisions; in 1995, it discovered the top quark through proton-antiproton collisions.

 

In addition, a discrepancy in the symmetry of quarks' spin led W. Greenberg to propose the concept of quark "color" in 1965. This color, now considered a fundamental part of a quark's characteristics, has nothing to do with what we normally consider color. Rather, it is a way of labeling three types of quarks. All of the flavors of quarks mentioned above also come in these three colors.

 

Where Does the Name "Quark" Come From?

 

Murray Gell-Mann found the name "quark" in James Joyce's Finnegans Wake. The passage in which it appears begins:

 

Three quarks for Muster Mark!

Sure he hasn't got much of a bark

And sure any he has it's all beside the mark.

 

 

The three quarks refer to Mr. Mark's three children. Since Gell-Mann knew that the proton was composed of three of these new particles, he decided "quark" was an apt name for them.

 

Taylor H.

 

Sources and Links

 

Fritzsch, Harald. Quarks. New York: Basic Books, Inc., 1983.

History of the Discovery of Quarks. University of Florida. March 19, 2008.

Quarks. Hyperphysics. March 19, 2008.

 

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