Particle Physics

By the early 1960s accelerators had found dozens of short-lived particles, a "zoo" with no apparent order. Murray Gell-Mann organised them with representation theory: they fell into patterns of eight and ten predicted by the symmetry group SU(3), and the missing member of one pattern, the omega-minus, was found in 1964 with the predicted mass. The same year, Gell-Mann and George Zweig proposed that the patterns arise because the particles are made of three smaller constituents, quarks, with fractional charges. No free quark has ever been seen. But in 1968 electron-scattering experiments at SLAC found hard, point-like objects inside protons, as a beam of bullets might reveal stones inside a cloud.

Why quarks are never seen alone was made plausible in 1973 by asymptotic freedom: the strong force, carried by gluons, weakens at short distances and strengthens as quarks are pulled apart, so separating them takes ever more energy. A proof of this confinement is still lacking.

In 1967 Steven Weinberg, and independently Abdus Salam, building on the work of Sheldon Glashow, combined electromagnetism and the weak force into one quantum field theory of Yang–Mills type. The weak force's carriers are heavy because of a field that fills space, the mechanism found in 1964 by François Englert and Robert Brout, by Peter Higgs, and by others. The W and Z particles were found at CERN in 1983 by the UA1 team led by Carlo Rubbia and the rival UA2 team, at the predicted masses. On 4 July 2012 the Large Hadron Collider found the Higgs boson itself, completing the Standard Model.

The Sun shines by fusing hydrogen into helium. Each completed reaction turns four protons into one helium nucleus and releases about 26.7 MeV of energy, along with exactly two neutrinos. That is enough to count the neutrinos reaching Earth using only the brightness of sunlight.

Sunlight delivers 1361 watts per square metre at Earth's distance. One reaction releases $26.7 \times 10^6 \times 1.60 \times 10^{-19} \approx 4.28 \times 10^{-12}$ joules. So the reactions needed to supply each square metre's sunlight number

and each produces two neutrinos, so about $6.4 \times 10^{14}$ neutrinos cross each square metre every second. That is $6.4 \times 10^{10}$ per square centimetre, roughly 65 billion through your thumbnail every second, day and night, since they pass straight through the Earth. (The neutrinos carry off about 2% of the energy themselves, which barely changes the estimate.)

Almost none of them interact. That is why Raymond Davis needed a tank of 600 tonnes of cleaning fluid nearly a mile underground to catch a few. From 1968 his experiment found only about a third of the rate predicted by John Bahcall's detailed model of the Sun. For thirty years physicists argued over whether the model or the experiment was wrong. Neither was. The Sun makes only electron neutrinos, and Davis could detect only those. In 1998 Super-Kamiokande in Japan, in an analysis led by Takaaki Kajita, found that neutrinos made in the atmosphere change type in flight, and in 2001 the Sudbury Neutrino Observatory in Canada, led by Arthur McDonald, counted all types and found the full predicted total. Neutrinos oscillate, so they have mass, and the Standard Model had to be extended.

The Standard Model has passed every test at accelerators. It is nonetheless incomplete. It does not include gravity, it says nothing about the dark matter that makes up most of the matter in the universe, neutrino masses had to be added, and it cannot explain why the universe is made of matter rather than equal parts matter and antimatter. Whether the next step comes from larger colliders, underground detectors or the sky is unknown.