Fermilab recently launched a new web site describing the idea of a muon collider. Why think about a future collider when we haven’t even started using the LHC yet? Because it takes a really long time to design and build one. The LHC was conceived a few decades ago, and the LHC project was approved in 1994, fifteen years ago. Also, Fermilab is soon going to shut down its Tevatron collider that has operated for almost 30 years, and is planning for its future.
I find the idea of a muon collider very intriguing. The idea is to collide positive and negative muons together, as opposed to colliding protons together as will be done at the LHC. A muon collider would be well suited to precise measurements, whereas the LHC is more of a discovery machine. This is because the protons colliding at the LHC are made of other particles called quarks and gluons, and it is those particles that actually collide in the LHC. So a collision of protons is really a collision of many quarks and gluons, which can be quite a messy thing to try to understand. A muon collider would collide muons only, so the collsions would be “cleaner” in the sense that there are fewer particles colliding at once. This makes understanding what happened in a collision easier to understand.
A muon collider would be a great tool for precisely studying whatever new particles are found at the LHC. Another option for a precise instrument is an electron collider, and there are proposals to build electron colliders as well. A muon collider has the advantage of being much smaller (a circle 2 km across instead of the proposed 30 or 50 km in a straight line proposed for electron colliders).
However, the technology necessary to build a muon collider is still being developed, whereas the technology needed to build an electron collider is already well advanced. But assuming we discover some interesting new particles at the LHC, building an electron or muon collider to follow up would be a great idea. So when will this happen? In the case of the muon collider, the schedule estimates I’ve seen currently put first collisions around 2028.
Hi Adam,
The main reason for the Muon collider is that you will have a higher cross-section for making the Higgs as compared to an electron-positron collider. Since the Higgs “couples” to the mass of the particle, muons win over electrons.
I suppose this argument will work for any new particle that couples to mass, but we need a theorist for this!
vivek
Vivek: You’re right, but I don’t know why one would call that the main reason. Adam’s comment on the size — which is primarily due, I think, to much reduced energy loss due to Bremsstrahlung — is also a very big deal.
To sum it up: Muons are the heaviest elementary particles(points)that can be accelerated and collided in any reasonable way. Time dilatation at 1TeV makes them live about 40 ms in the laboratory reference frame (instead of 2 us in their own). Bremsstrahlung is not an issue at several TeV, since they weight in at about 200 electron masses and the energy loss depends inversely on the fourth power of the particle mass. This is why the proposed collider is relatively small. Of course there are the issues of producing enough muons with a target that resembles a waterfall of mercury. Cooling, that is packaging them to bunches, and then accelerate them real fast to highly relativistic speeds before they disintegrate is another challenge.
Some recent references from Indico here.
Seth, that’s true, but if there was no identifiable physics pay-off at the end, then it is not clear if there is much of an advantage, especially, since the size reduction comes with a host of complications, e.g., accelerating, focussing and using the beams before the muons decay, how to produce the muons in the first place, how to get high luminosity, etc.
I’m completely naive about this, but if we (temporarily) ignore issues related to muon production and collimation, what would be the cost/feasibility comparison for a 1 TeV muon collider versus a similar energy ILC?
e.g. suppose the LHC tells us that there is new physics at 750 GeV, well above the proposed 500 GeV ILC. Would it be time to scrap the ILC program in favor of a muon collider, or would we still push forward with a higher-energy ILC?
Hi Flip,
I don’t think you can ignore the muon production/collimation, etc., since that is what feeds into the feasibility…
Will this new physics at 750 GeV be mass dependent, i.e., will it couple more strongly to muons than to electrons? If so, perhaps a physics case could be made, but I suspect that the muon collider is way, way behind the ILC when it comes to feasibility, R&D, etc.
vivek
Correct me if I’m wrong, but I don’t think the lepton to Higgs coupling is relevant to the Higgs production cross section. The dominant production mechanism is Higgstrahlung: l+l- -> Z -> ZH, right? The Z doesn’t care is the lepton is an e or a mu.
@Flip — I think in the case you mention, the HEP community is in for a painful era. The problem with the muon collider is that is won’t exist until 2030 (at the best). The LHC and its upgrades will deliver good physics for many years, but I hope we don’t have to wait 20 years for a lepton collider to follow up on the LHC.
Hi Josh,
At a lepton collider, you can basically tune the beam energy to sit at the Higgs resonance. At that place, the dominant production process is l+l- -> Higgs, which does depend on the coupling.
I guess the decision on the International Linear Collider will not be made till the LHC finds the Higgs; that way we will know what energy to set for the ILC. The case will be stronger if the LHC also finds other stuff, e.g., SUSY. My feeling is that it will be very hard to justify a few billion dollars just to study the Higgs in more detail.
In any case, I think the Muon Collider is very far away; no one even has a prototype of any scale. In contrast, the Linear Collider (based on electrons) has the basic design framework in place.
vivek
p.s. I have no personal stake in either machine. I am working on the LHC.
Will it be using higher temperature superconductors by then? So it can be cooled with liquid nitrogen instead of liquid hydrogen?