The 21-kilometer ring near Protvino was once planned to accelerate protons. Today, the same tunnel could host a very different machine — a muon collider. The muon is a heavy cousin of the electron, and that's its strength. On a curve, an electron radiates like a searchlight, losing precious energy. A muon radiates a billion times less. So the ring stays modest while the collision energy is enormous.

The review authors bring together the key technologies: how to produce muons, how to cool their beam (the MICE experiment proved this is possible in the transverse plane), how to accelerate them quickly and hold them with superconducting magnets. A particular headache is the neutrino beam, which at these energies becomes dangerous. There is a solution, and it is documented. All this engineering promises physics that is otherwise out of reach: precise measurements of the Higgs boson's properties, a hunt for dark matter particles in the 'thermal window,' and nearly closing that window for WIMPs — hypothetical massive particles.

The ring near Protvino is not the only scenario. The same facility would produce intense muon beams for applied tasks: from muon-catalyzed nuclear fusion to scanning pyramids and volcanoes (muography) and isotope analysis. Russian institutes have a long tradition in these methods.