Future colliders for particle physics—“Big and small”

Discoveries at high-energy particle colliders have established the standard model of particle physics. Technological innovation has helped to increase the collider energy at a much faster pace than the corresponding costs. New concepts will allow reaching ever higher luminosities and energies throughout the coming century. Cost-effective strategies for the collider implementation include staging. For example, a future circular collider could first provide electron–positron collisions, then hadron collisions (proton–proton and heavy-ion), and finally the collision of muons. Cooling-free muon colliders, realizable in a number of ways, promise an attractive and energy-efficient path towards lepton collisions at tens of TeV. While plasma accelerators and dielectric accelerators offer unprecedented gradients, the construction of a high-energy collider based on these new technologies still calls for significant improvements in cost and performance. Pushing the accelerating gradients or bending fields ever further, the breakdown of the QED vacuum may set an ultimate limit to electromagnetic acceleration. Finally, some ideas are sketched for reaching, or exceeding, the Planck energy.