Carlo Rubbia: A Millimole of Muons for a Higgs Factory
Here I am offering you a quick-and-dirty transcript of part of Rubbia’s presentation. If any of the sentences below is garbled or does not make much sense, I believe you know who you should blame…
Rubbia started by asking the question of what we are going to do in the next years ? CMS and ATLAS have observed a narrow line at 125 GeV mass compatible with SM Higgs boson. The results of the experiments exclude Higgs bosons above and until 600 GeV. Experimental resolutions are much larger than the intrinsic width. So this raises a question: new physics must also appear at the TeV scale. This is also one of the main reasons for believing that SUSY might be nearby. It is based on the argument that the otherwise divergent self-interaction of Higgs requires a cutoff at the TeV scale.
However the argument does not hold since for 125 GeV stability may allow without novelties a legitimate cutoff up to the Planck Mass. Thus there migh indeed be only one standard model Higgs to be confirmed experimentally.
During the next 20 years CERN plans to pursue the hadronic production of Higgs, and searching for SUSY. Rubbia claimed here that the existence of additional Higgs particles is assumed as unlikely within LHC energy range – I do not know where he got this from, but I concur! He thus argues that studies should concentrate on the properties of what is discovered already. The LHC at high luminosity will be a Higgs factory. And there are plenty of opportunities to check the couplings because of the rich decay phenomenology at 125 GeV of the new-found particle.
In particular, like the decays of the Z boson, the width determination will be crucial for the nature of the particle and the underlying theory. The SM prediction for the natural width is only 4 MeV, so getting there is a formidable task. The signal shape is unaffected only if the RMS of the energy resolution (for instance, in a leptonic collider, the beam energy) is less than a few MeV.
Rubbia then asked what precision is needed to search for possible deviations from SM under the assumption that there is no other additional H state at the LHC. The predicted ultimate LHC accuracties fo exotic alternatives have been studied. With 3000 fb-1 the LHC can only go to 8%, 10%, and 15% precision respectively in resolving the hVV, htt, hbb couplings. In SUSY, the sensitivity to TeV-scale new physics for a 5-sigma discovery may need percent to sub percent accuracies in the measured rates.
The scalar sector is one of the keys to the future understanding of HEP. Once the Higgs mass is known, the SM may be entirely defined – with the notable exception of neutrino masses, nature and mixings.
After the Z discovery, the detailed studies at LEP and SLAC in clean conditions have been a necessary second phase. A similar phase may also be necessary for the Higgs. It would be necessary to produce at least 10000 events per year in a clean environment. Two future alternatives can be compared:
– An e+e- collider at L>10^34, yielding a ZH signal (associated production) of 200 fb. The circumference of such a thing is about 80 km for a LEP-like ring.
– A muon-muon collider at L>10^32, yielding a H signal of 20000 fb in s state production. The collider is way smaller, R=50m.
Various options of LEP-like rings throughout the world are being put forth. For instance, a Super Tristan in KEK area, a TLEP tunnel in Geneva area.
The luminosity has to be pushed to the beamstrahlung limit. Collisions are at angle, with fewer bunches than for a B factory, in a nano-beam scheme. Luminosity cost and power consumption (100 MW) are comparable to those of ILC. In order to reach the luminosity and power consumption the cures are: a huge ring, extremely small vertical emittance, with beam crossing size of 0.01 microns (3 microns for LEP2). The circular ring (E=250 GeV) performance is at a feasibility broderline. However, the H width cannot be directly detected in this scheme.
So one can compare a super-Tristan scheme with a ILC. They have comparable costs and power consumptions. The more conservative ring alternative appears preferable. A nuclear power plant is needed for powering this thing.
The muon collider has a great enhancement in the higgs cross section. Like in the well known case of Z, the H production offer cleanliness. A unique feature is that actual mass and narrow width may be measured with accuracy. A muon collider will evidence a resonant signal, that, if sharp enough, will dominate backgrounds.
Then Rubbia started talking about the details of how to cool the muon beam. He recalled the story of Liouvillian and non Liouvillian accelerators: Liouville’s theorem states that when there is a Hamiltionian, then the six dimensional phase space is preserved. Therefore we need some kind of dissipative non-L drag force working against it, if we want to achieve the needed parameters.
Colling is essential whenever secondary particles are produced from initial conditions and later accelerated. At high-energy, muons may be stable enough to offer a reasonable number of muon-muon collisions for a H resonance in s state or for muon decays which can be an excellent source of a high intenstiy neutrino beam, for instance to study CP violation. Ionization cooling is specific for muons. This, called “dE/dx cooling”, resembles the damping of relativistic electrons. It produces a fast cooling compared to other methods. This is a necessity for muons. Transverse betatron oscillations are cooled by a target foil.
There have been lots of developments in this technology. During the late nineties, studies were carried out in the US and in UK (MICE). Conclusions are that muon cooling could be done for a 3 TeV collider and L=7E34, or a 600 GeV collider and L=10^33, or a Higgs factory at 110 GeV and 2.2E31. Also a neutrino factory.
Rubbia then discussed the “early cooling scenarios”, which require something called a “Guggenheim channel”. This is made up by many RF cavities that restore the energy lost in low-Z absorbers.
He argued that we need an initial cooling experiment. The next step could be the practical realization of a full scale cooling demonstrator. This means realizing a cascade of very small rings of few meters radius, to achieve the longitudinal and transverse emittances with asymptotically cooled muons. Muons may be extracted from an existing accelerator at low intensity. Work is going on to explore this possibility.
Rubbia then discussed with a lot of detail the physics of possible cooling options, flipping formula-thick slides at relativistic speed. He far exceeded my typing capabilities, so anyway I believe you should check the original slides if you really want to get a headache – I don’t want to be corresponsible. So I will content myself with just reporting his conclusions:
The development of a new collider will be necessary in order to study LHC potential discoveries beyond the present SM. A high-energy muon collider is the only possible circular high energy lepton collider that can be situated with in the cern or fnal sites. However it requires two major developments, the production of a millimole of muons and the phase compression of its beams.
At the end of the talk I managed to ask Rubbia a question. Here it is:
“You recalled the history of the Z discovery and subsequent LEP studies in motivating a muonic Higgs factory. I would like to point out that at the LHC we have not measured a non-zero Hmumu coupling yet (nor the Hee for that matter, nor we likely ever will). Arguably the Hmumu coupling could be zero, or equal to that of electrons. So we are relying on the SM to hold in order to have a chance to see it breaking down. It is a lose-lose scenario, at least to some extent.”
Rubbia’s answer: “What can I say…”