Nature 578, 53–59 (2020)

Due to the muon’s larger mass compared to the electron, synchrotron radiation losses would be much lower at muon than at electron colliders, allowing for more compact colliders. However, the quality of muon beams is not good enough yet to be used in such a machine. Part of the reason lies in the production mechanism of muons: by shooting protons onto a target, pions are created that can decay further into muons. Thus, the brightness of the muon beam is too low to collide it with its antiparticles.

The MICE collaboration demonstrated ionization cooling, where muons are passed through an absorber material, losing momentum through ionization. The trick to increasing the beam brightness is then that the lost momentum is only restored in the direction of the beam by radio-frequency cavities. These results represent a milestone for the development of muon colliders.