Gluons were detected by the jets of hadronic particles they produce in a particle detector soon after they are first created. In particular, at the PETRA collider at the DESY facility in Hamburg, Germany positrons and electrons were collided producing an intermediate photon that then produced a quark and an anti-quark as it decayed. This process results in two jets of hadronic particles as the quarks form hadrons. Sometimes one of the final-state quarks radiates a gluon just before it "hadronizes" (that is, forms into hadrons such as protons, pions, neutrons, etc.). This gluon will form a jet of its own, producing an event with three jets, instead of the usual two.
Four different detectors at PETRA in 1979 reported observing the three-jet events with just the right frequency and just the right characteristics to be consistent with gluon production. Later experiments at CERN and Fermilab confirmed these results.
One of the original papers that reported the three jet events is in Physical Review Letters, volume 43, page 830. A more detailed discussion can be found in the Cahn and Goldhaber book "Experimental Foundations of Particle Physics."
Answered by: Brent Nelson, Ph.D., Research Fellow, University of Michigan
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