The six quarks: similar but different

The plot thickens.

There are six quarks, but physicists usually refer to them in terms of three pairs: up/down,charm/strange, and top/bottom. (In addition, for each of these quarks, there is a corresponding antiquark.) The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years.

Quarks have the unusual feature of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge. In addition to the differences noted in electric charge, all of the quarks differ in mass.

As the figure illustrates, there are six quarks, named whimsically, up, down, charm, strange, top, and bottom. [The last two were once referred to even more whimsically as truth and beauty.] Their charge, as noted in the figure, is either 2/3 or -1/3. Through a combination of these two charges, we can get our proton charge of +1. Of course, the idea of fractional charge was distasteful to most physicists of the era, possibly because of Occam’s Razor. In fact, Murray Gell-Mann wasn’t able to publish his quark theory in the usual venues. It ended up in Physics Letters. However, in this case, it turned out that Occam’s Razor favored the quarks because their existence simplified our ideas about hadrons.


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