Murray Gell-Mann · Quarks
Can a hundred-odd particles be reduced to fewer things?
That time, that place
As accelerators grew through the 1950s, new particles poured out.
At first proton, neutron and electron sufficed. Then came the pion, the kaon, the lambda, sigma, xi — past a hundred names, most of them vanishing as soon as they were made.
Fermi is said to have remarked that if he could remember all these names he would have been a botanist. A hundred fundamental particles are not fundamental.
Why this question
Gell-Mann thought of what the periodic table had done. Mendeleev arranged elements by property, gaps were left, and the gaps were later filled.
Gell-Mann laid the particles out by their properties. With electric charge on one axis and, on the other, a property measuring how 'strangely' long a particle lived, they fell into neat octagons and triangles. Another physicist across the sea reached the same arrangement the same year. In 1961 Gell-Mann named the arrangement the Eightfold Way.
One place in a triangle stood empty. He predicted the mass, charge and strangeness of whatever belonged there.
In 1964 the omega-minus turned up at Brookhaven, close to the predicted values.
What was found
But why that shape?
Gell-Mann found that if you assume three smaller things combining, the table falls out by itself. Group them in twos and threes and the octagons and triangles reproduce exactly.
The difficulty was the charge those things had to carry: one third and two thirds of the electron's. Nobody had ever seen a fractional charge, and nobody has yet seen one pulled out on its own.
Gell-Mann took the name from a line in Joyce and called them quarks. In the first paper he left room for their being a convenient bookkeeping device rather than real particles. That same year another young physicist reached the same idea alone, and his paper was refused publication.
In 1968 at Stanford, electrons were fired hard into protons. The spread of scattering angles was the pattern you get if three hard grains sit inside — the same reasoning by which the nucleus had been found half a century earlier by firing particles at gold foil.
Quarks were not a device.
The hundred-odd particles were each taken to be fundamental in themselves
The predicted omega-minus turning up, and the 1968 scattering that showed three grains inside the proton
The Standard Model of quarks and force carriers was built, its last box filled by the Higgs in 2012