Fifty Years of Quarks... More Particles to Discover!

December 9, 2014 6:29 PM

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Fifty years ago, in 1964, physicists Murray Gell-Mann at Caltech and George Zweig at CERN came up with the idea of the quark as a response to the bewildering number of elementary particles that were being discovered at the huge "atom smasher" labs sprouting up all over the world. Basically, instead of the dozens of heavy particles like protons and neutrons, or medium-weight particles like pi mesons, you only needed three kinds of elementary quarks, called "up," "down" and "strange." Combining these in threes, you get the heavy particles. Combining them in twos, with one quark and one anti-quark, you get medium-weight mesons. This early idea was extended to include three more types of quarks, dubbed "charmed," "top" and "bottom" (or on the other side of the pond, "charmed," "truth" and "beauty"). These six quarks form three generations -- (U, D), (S, C), (T, B) -- in what physicists call the Standard Model of particle physics. Also included are the eight gluon particles that transmit the strong nuclear force between quarks and cause quarks always to be bound together and never found isolated in nature.

At first the quark model (or parton model, if you were an experimenter) only accounted for the then-known particles. A proton would consist of two up quarks and one down quark (U, U, D), and a neutron would be (D, D, U). A pi-plus meson would be (U, anti-D), and a pi-minus meson would be (D, anti-U)...

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