Worth the wait

By The Economist online

A timeline of the Standard Model of particle physics

ON JULY 4th researchers at CERN, Europe's main particle-physics lab, confirmed their discovery of something that looks very much like the Higgs boson. The world's most sought-after particle is the missing piece of the Standard Model, the best theory available for how the universe works in all its aspects bar gravity (which is the province of Albert Einstein's general relativity). The model divides elementary particles into two classes. First, there are the fermions, a group comprising quarks (like those which make up protons and neutrons in atomic nuclei) and leptons (such as electrons that orbit these nuclei, and ghostly neutrinos). Then there are the bosons: gauge bosons, which carry forces of nature that allow the fermions to interact, and the Higgs boson, whose role is to endow the others with mass. The concept of the Higgs was introduced in 1964, so it has taken physicists 48 years to go from idea to observation. None of the model's 16 other particles was as elusive. Indeed, as our chart shows, the muon and the tau were discovered before anyone had predicted their existence. Both are leptons, heavier versions of the familiar electron, the first elementary particle to be observed, by J.J. Thomson, a Briton, in 1897. Though the notion of a unit of negative charge had been around since the 1830s, a firmer prediction was made in 1881 by a German scientist, Hermann von Helmholz. The positron, the electron's antimatter twin (not included in our chart, since in terms of the Standard Model, particles and their antiversions are two sides of a single coin) popped out of an equation in 1928; it popped up in an experiment four years later.