70 years of antimatter - making quarks and antiquarks

11 March 1998

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Continuing the series on what can happen when matter meets antimatter, in this week's image an electron and a positron (antielectron) have annihilated at the centre, and the energy has rematerialised as a quark and an antiquark. This image is from the OPAL experiment, at the European laboratory, CERN. Like recent examples in Picture of the Week, it shows the results of an annihilation in LEP, the Large Electron Positron collider. However this week we cannot see directly the particle and antiparticle that the annihilation produces, as quarks and antiqaurks cannot exist alone. Instead, as soon as they emerge they begin to make other quarks and antiquarks from the energy of the interaction, so that they can form composite particles (hadrons). These are the particles that leave the two "jets" of trails in the tracking chambers around the centre of the image - the two jets shoot off in opposite directions, just like the original quark and antiquark from which they form. The hadrons then lose all their energy in the next layers (the calorimeters) and come to a stop. ((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 initial quark-antiquark pair. You can contrast this with Z decays to other particle-antiparticle combinations detected in Opal here.)

Credit: CERN / OPAL

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