Twenty years of "seeing" gluons
1 September 1999
By the end of August 1979, physicists working on experiments on the PETRA collider at the DESY laboratory in Hamburg had gathered their first evidence for the observation of gluons - carriers of the strong force that binds together the elementary particles known as quarks to form more familiar particles such as protons and neutrons. At the time, PETRA was colliding electron and positron beams at an energy of 30 GeV (30 giga electronvolts), three times higher than the energy of any similar machine. In the "event" shown here, the collision of an electron with a positron (an anti-electron) at the centre of the TASSO detector has created three ‘jets’ of particles which leave the tracks shown. The electron and positron have annihilated to produce a quark and an antiquark, and one of these has radiated a gluon. None of the three original particles escapes on its own into the detector. Instead all three swiftly generate more quarks and antiquarks which cluster together to form the particles in the three jets. (Compare this with the "2-jet" image produced when no gluon is radiated.)
Credit: DESY / Physics Photographic Unit, University of Oxford
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