Higgs: Kev Precision and Cp Violation

The prospects for a muon collider operated as a Higgs factory are reviewed. The large muon mass means that the s-channel Higgs production mechanism is available, and simultaneously suppresses bremsstrahlung so that the beam energy spread can be kept to the MeV level required to exploit this. Thus this is the only machine which can make a direct scan over the Higgs resonance, and make an extraordinary mass measurement. Further possibilities such as a scan of the H and A of supersymmetry and CP violation are also mentioned. 1 Reminder of muon colliders The muon collider 1,2 is the only way of pushing the energy frontier beyond the region of applicability of electron colliders, while retaining the advantages of a point-like projectile. The mass is 200 times that of an electron, so storage rings behave like those of proton machines while a beam energy spread as low at 10 −5 may be possible, which means that narrow resonances can be scanned. The lack of beamstrahlung means that thresholds are clean, and the energy can be measured to 10 −6 via g-2. This measurement of the Higgs mass benefits from the coupling g hµµ , which gives a cross-section for s-channel production 40,000 times the electron equivalent. This allows a direct scan, giving the mass and width. This possibility depends upon a Higgs below the W threshold, so it is encouraging that EW fits 3 and direct observation 4 appear to favour this. Indeed one study 2 found 115 GeV as the optimal Higgs mass for a muon collider. The disadvantage of a muon collider is of course the muon lifetime of 2.2 µs, which means that muon production and cooling has to be performed on a similar timescale. Furthermore the electrons from the muon decay will constitute a serious detector background. Both these difficulties mean that we wish to maximize the luminosity per amp of muon current, which implies cooling the beams as much as possible.