the kinematic angle between the highest energy jets in a three-jet event. The angle is not measured in the lab frame, but in a frame boosted along the energy of the highest energy jet so that the second and third jets are back-to-back. By measuring the distribution of the Ellis–Karliner angle at the PETRA electron–positron storage ring at DESY, physicists determined that the gluon has spin one rather than spin zero or spin two. Subsequent experiments at the LEP storage ring at CERN confirmed this result. References Further reading Gluons Quantum chromodynamics