Hence the experimenters looked for 'three-jet events', and that is what they found.

In particle physics, a three-jet event is an event with many particles in final state that appear to be clustered in three jets.

This difference between the three-jet events and the two-jet events with a high-energy photon, which indicates that the third jet has unique properties under the strong interaction, can only be explained by the original particle in that jet being a gluon.

As shown in 1978 summer conferences the PLUTO experiments at the electron-positron collider DORIS (DESY) reported the first evidence that the hadronic decays of the very narrow resonance Y(9.46) could be interpreted as three-jet event topologies produced by three gluons.