In July, particle physicists in the US completed the Snowmass processâa decadal community planning exercise that forges a vision of scientific priorities and future facilities. Organized by the Division of Particles and Fields of the American Physical Society, this yearâs Snowmass meetings considered a range of plans including neutrino experiments and muon colliders. One new idea that generated buzz was the Cool Copper Collider (or C3 for short). This proposal calls for accelerating particles with conventional, or ânormal-conducting,â radio frequency (RF) cavitiesâas opposed to the superconducting RF cavities used in modern colliders. This âretroâ design could potentially achieve 500 GeV collision energies with an 8-km-long linear collider, making it significantly smaller and presumably less expensive than a comparable superconducting design.
The goal of the C3 project would be to operate as a Higgs factory, whichâin particle-physics parlanceâis a collider that smashes together electrons and their antimatter partners, called positrons, at energies above 250 GeV. Such a facility would make loads of Higgs bosons with less of the mess that comes from smashing protons and antiprotons togetherâas is done at the Large Hadron Collider (LHC) in Switzerland. A Higgs factory would give more precise measurements than the LHC of the couplings between Higgs bosons and other particles, potentially uncovering small discrepancies that could lead to new theories of particle physics. âI think the Higgs is the most interesting particle thatâs out there,â says Emilio Nanni from the SLAC National Accelerator Laboratory in California. âAnd we should absolutely build a machine thatâs dedicated to studying it with as much precision as possible.â
But an outsider might wonder why another Higgs-factory proposal is being added to the particle-physics menu. A similar factory designâthe International Linear Collider (ILC)âhas been in the works for years, but that project is presently stalled, as the Japanese government has not yet confirmed its support for building the facility in Japan. Waiting in the wings are several other large particle-physics proposals, including CERNâs Future Circular Collider and Chinaâs Circular Electron Positron Collider.