In a talk at the CERN auditorium on 14 April Nobel laureate and Former DG Carlo Rubbia reviewed the case for a muon-antimuon (µ⁺µ^-) collider.

The interest in building a so-called "second-generation" Higgs factory that would allow better studying the properties of the recently discovered Higgs particle at 125 GeV has been revived, following the recent discovery at CERN/LHC.

This comes in analogy with the discovery of the W and Z in the UA1 and UA2 experiments and the subsequent study of the Z resonance in the pure s-channel state with LEP. A Higgs factory is needed, as the current Higgs measurements from the LHC detectors allow for a large width to invisible or other exotic decays. In addition, the coupling measurement precision, even that projected at the HL-LHC, may not be high enough to pin down physics beyond the Standard Model.

During his talk, Professor Rubbia gave a review of the existing proposals of Higgs factories: the International Linear Collider, an e⁺e^- collider in a ~30 km tunnel offering the advantage that it can run at higher energies and allow us to explore the regime beyond ZH production. However, it would allow for only one detector to be used at a time. The case for circular ee colliders like the CepC in China or the Future Circular Collider studies with ee were also reviewed, with the latter also running at the ZH threshold. With multiple detectors, they would have the potential to provide higher luminosity than at the ILC.

Rubbia proposed a µ+ µ- collider at the modest energy of 62.5 GeV and an adequate cooled muon intensity of about 6 x 1012 muons of each sign, a repetition rate of 15-50 p/s and L ≈ 1032 cm -2 s -1, corresponding to about 10,000 Higgs bosons detected per detector per year. With such an arrangement, a precise direct measurement of the Higgs boson is possible, in contrast to the ee machines, while it could also be upgradable in energy. The big advantage of this proposal is that it combines the ee circular machine benefits with a much smaller size.

A muon collider can be realized with a final ring of ~60 m in diameter, which means that the main ring and all the supporting accelerators can fit at the CERN site without the need of large scale civil engineering. It is also conceivable that the whole project can be realized in a shorter timescale than the ILC or circular ee and at a much lower cost. There are significant challenges for such a project, the most significant being that a muon cooled beam has not been fully demonstrated.

The next step, Rubbia argued, could be the realization of a muon-cooling demonstrator and experimentally demonstrating the cooling techniques which are critical for the muon collider project. Most of the important work is to develop a resonant cooling that may involve significant and unexpected conditions which are hard to predict without a practical test. The detectors for such a machine need a very challenging design since μ->e decays will result in continuous radiation, and extensive shielding must be used. Past R&D efforts done at CERN and elsewhere could address some of these challenges. 

In conclusion, Rubbia stressed that in his view the recent discovery of the Higgs puts us in the situation of having to investigate its properties in detail. We may not yet know how a muon collider would work, but if it does work then one can demonstrate its benefits compared to the other types of colliders for a Higgs factory. The operation of a demonstrator could initially be explored and demonstrated using existing muon beams, available at a number of accelerators. The ultimate muon-antimuon collider for a Higgs factory could be situated in the existing CERN site or elsewhere.