In 1936 Carl D. Anderson and Seth Neddermeyer discovered the muon, a building block of today’s Standard Model of particle physics. While studying cosmic radiation, they detected muons, which are debris from collisions of cosmic rays with atoms in the Earth’s atmosphere. When passing through a magnetic field, muons curve less sharply than electrons due to their higher mass.

Despite being 207 times heavier than an electron, this elementary particle with spin ½ is considered light. Muons, together with their lighter cousins the electrons, their heavier cousins the tauons and their respective antiparticles, form the class of light elementary particles known as leptons, which interact with all forces except the strong force.

Thanks to their mass, muons lose less energy than electrons as they emit less synchrotron radiation during acceleration. The much smaller loss of energy makes them thus ideal candidates to accelerate and collide in circular accelerators. A muon collider, if proven feasible, would outperform any electron-positron collider by far and would combine the advantages of electron-positron and proton-proton colliders: the muon collider would produce high-energy particles with a small background. In addition, muons are suited for applications outside particle physics laboratories because their high mass allows muons to penetrate materials better than X-rays or gamma rays, for example, imaging cargo containers or searching for hidden chambers in pyramids.

Unlike electrons or protons, muons decay after just 2.2µs. Although this lifetime may seem rather short, it is longer than that of other particles, such as the tau lepton. Thanks to a phenomenon of special relativity called time dilation, the lifetime of muons increases. For the travelling muon, time moves slower the faster they are. If they travelled at the speed of light, they would be considered stable. At 7 TeV, for example, they will decay after about 150ms.

The short lifetime is the real challenge for physicists as they need to build magnets that are fast enough to increase their field to accelerate the muons before they decay. Many physicists, among them those in the International Muon Collider Collaboration, have taken up this challenge and hope to prove that it is possible to build a muon collider.