70 years of antimatter - an electron-positron pair

25 February 1998

February and March 1928 saw the publication of two remarkable papers* by theorist P.A.M.Dirac, in which he combined relativity and quantum mechanics to derive an equation that provided a complete description of the electron. One surprising result was that the theory turned out to describe not only electrons but the particles we call positrons, or antielectrons. Dirac had predicted antimatter! Seventy years on, electrons and positrons play a routine role in experiments in particle physics. At the European laboratory, CERN, beams of high energy electrons and positrons are made to collide head-on in LEP, the Large Electron Positron collider. As Dirac's theory shows, when matter and antimatter meet, they annihilate into pure energy, which can then rematerialise as matter and antimatter. Here, in a collision at LEP caught at the centre of the Delphi experiment, the energy has rematerialised as another electron and positron. This pair move off back-to-back, leaving trails in the tracking chambers around the centre, before depositing all their energy (indicated by the lighter dots) in the ring beyond (the electromagnetic calorimeter). (Note that in this case, the initial electron and positron and created a Z boson - the neutral carrier of the weak force - which decays almost instantly to the new electron-positron pair.)

* (See Proceedings of the Royal Society 117 610 and 118 351 (1928))

Credit: CERN / DELPHI

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