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particle physics

The study of the properties of elementary particles and of fundamental interactions (see forces, fundamental).

Pioneering research took place at the Cavendish laboratory, Cambridge, England. In 1895 Joseph Thomson discovered that all atoms contain identical, negatively charged particles called electrons that could easily be freed. By 1913 Ernest Rutherford had shown that electrons surround a very small, positively-charged nucleus (which in the case of a hydrogen atom consists of a single positively charged particle, a proton, identified by James Chadwick in 1932). The nuclei of all the other elements are made up of protons and uncharged particles called neutrons. 1932 also saw the discovery of a particle (predicted in 1928 by Paul Dirac) with the mass of an electron, but an equal and opposite charge, called the positron. This was the first example of an antiparticle; it is now known that almost all particles have corresponding antiparticles. In 1933 Wolfgang Pauli argued that a hitherto unsuspected particle must accompany electrons in beta-ray emission - the so-called electron-neutrino.

Particles and fundamental forces

By the mid-1930s, four fundamental kinds of force had been identified: (1) The electromagnetic force acts between all particles with electric charge, and was thought to be related to the exchange between the particles of photons, packets of electromagnetic radiation. (2) In 1935 the Japanese physicist Hideki Yukawa suggested that the strong force (holding protons and neutrons together in the nucleus) was caused by the exchange of particles with a mass of about a tenth that of a proton; these particles, called pions (originally pi-mesons), were found by the British physicist Cecil Powell in 1946. (3) Theoretical work on the weak nuclear force began with Enrico Fermi in the 1930s. The existence of the particles that carry the weak force, the W and Z particles (weakons), was confirmed in 1983 at CERN. (4) The fourth fundamental force, gravitation, is experienced by all matter, the postulated carrier of this force being the graviton.

Leptons, hadrons, and quarks

The electron, three types of neutrinos, and the positive and negative muons are the leptons - particles with half-integral spin that ‘feel’ the weak, but not the strong, force. The muon (found by US physicist Carl Anderson in cosmic radiation in 1937) produces the muon neutrino when it decays. The tauon, a surprise discovery of the 1970s, produces the tauon neutrino when it decays. Hadrons (particles that ‘feel’ the strong force) started to turn up in bewildering profusion in experiments in the 1950s and 1960s. They are classified into mesons, with whole-number or zero spins, and baryons (which include protons and neutrons), with half-integral spins. It was shown in the early 1960s that if hadrons of the same spin are represented as points on suitable charts, simple patterns are formed. This symmetry enabled a hitherto unknown particle, the omega-minus, to be predicted from a gap in one of the patterns; it then turned up in experiments. In 1964, Murray Gell-Mann suggested that all hadrons were composed of just three types or ‘flavours’ of a new particle with half-integral spin and a charge of magnitude either 1/3 or 2/3 that of an electron; Gell-Mann christened the particle ‘quark’. Mesons are quark-antiquark pairs (spins either add to one or cancel to zero), and baryons are quark triplets. To account for new mesons, such as the psi, the number of quark flavours was conjectured to be six (up, down, top, bottom, strange, and charm), and this had been confirmed by 1985.

In 2003 scientists at the High-Energy Accelerator Research Organization in Tsukuba, Japan, discovered a remarkable new particle. The unknown particle, which the researchers classified as X(3872), was detected in the decay products of mesons consisting of bottom (or beauty) quarks; it weighed as much as a single atom of helium and lasted a billionth of a trillionth of a second, an unusually long lifetime for a subatomic particle of this size.

A new class of subatomic particle was created in 2004 at the Hadron-Electron Ring Accelerator in Hamburg, Germany. It was made up from five quarks by firing beams of electrons and protons together. The existence of the new particle, a new type of ‘pentaquark’, called a ‘charmed’ pentaquark, was determined from the decay products that remained, since the particle itself survived less that 10−23 seconds.

CERN Charpak, Georges

colour (quantum physics) Cronin, James Watson

nuclear physics nucleon

Paul, Wolfgang Politzer, Hugh David

Taylor, Richard E