Will particle physicists ever have a new toy that will take them to energies beyond those accessible through the Large Hadron Collider? History suggests it's unlikely. To save costs, the LHC was built in an existing tunnel that had hosted an earlier, less powerful accelerator. The US cancelled the construction of hardware that would have outperformed the LHC (the Superconducting Super Collider, or SSC) due to cost overruns, and it shut down its Tevatron once the LHC started up. Now, decisions on the linear collider that will be used to study the Higgs in detail are being made based on which country is likely to come up with the most money.

But physicists are apparently an optimistic bunch. Earlier this year, CERN announced that it was beginning to evaluate an LHC replacement that would require a tunnel so large—100km in circumference—that it would have to pass under Lake Geneva itself. Potentially in response, a team of US-based physicists have come up with an even more audacious plan: don't build the linear collider, resurrect the SSC's now abandoned tunnels, and use them to both host a Higgs factory and as a booster for a truly massive, 270km collider.

We'll cover the slightly less bonkers idea (CERN's) first. The protons that run around the LHC don't naturally follow a circular path; they have to be pulled around bends by powerful magnets. The more energy you put into the protons, the faster they move and the stronger your magnet needs to be to pull them around the bend. Unfortunately, there are limits to how strong we can make the magnets. When we run up against those limits, we either have to settle for lower energy or make the bends less sharp. Making the bends less sharp means making a bigger circle.

CERN would happily build the biggest tunnel it can, but it is limited by a different sort of physics. The valley it sits in is flanked by very large mountains and contains a rather deep lake. Drilling through the Alps could get prohibitively expensive, so the tunnels CERN is looking at encircle Geneva and spill out into France's Rhone valley—away from the Alps. Even so, part of the tunnel will have to go under the lake itself (though, presumably, a more shallow section).

Even so, extremely powerful superconducting magnets will be needed to keep the protons flying at 50 Tera-electronVolts, making for 100TeV collisions; the LHC will max out at 14TeV, so this is a substantial increase in energy. For now, however, the Future Circular Colliders project will remain in the study phase.

The US team would advise getting away from the mountains altogether. And, while you're at it, skip building the linear collider as well.

The secret is going back to Texas, where construction of the SSC was well underway when the funding rug got pulled out from underneath it. About 45 percent of its 87km circumference had already been dug by that point, along with a tunnel for a linear accelerator to feed ions into it. By finishing off the tunnel, the authors argue, an electron-positron collider could be built that would be able to generate the Higgs in abundance, allowing its detailed study. When not feeding the collider, the linear accelerator could act like the one at SLAC, using the electrons to generate X-rays for imaging molecules.

Electrons normally lose too much energy while travelling around a curved path (which is why we are likely to build a linear collider). But the authors seem confident that a collider of this size can hold electrons at the requisite energies for producing the Higgs.

But that's not the audacious part of their plan. They'd also accelerate protons to high energies in a second accelerator that goes through the same tunnels as the electron-positron collider. But the protons wouldn't collide there. Instead, they'd be brought up to speed and then transferred to an even bigger collider, one 270km in circumference. Remember, the LHC is only 27km around. The new collider would start at the SSC site south of Dallas and encircle the entire city.

Amazingly, the physicists argue that this makes economic sense. The cost of building and running a collider comes primarily from three factors: the cost of digging the tunnels; the cost of building all the superconducting magnets; and the cost of cooling the whole thing with liquid helium. The SSC site is great because almost all the tunneling goes through chalk, which is very easy to dig through. So, you could get a larger tunnel for the same price as you would by digging around Geneva. With a larger tunnel, you don't have to have magnets that are nearly as strong to keep the protons curving around the circumference to get to the 50TeV speeds CERN is thinking about. The cheaper magnets offset the cost of tunneling.

Of course, you'd probably need more liquid helium to cool that much more hardware, which would make operating it more expensive than the CERN plan. The authors, however, note this just once and never mention it again.

The big price advantage, however, comes if we ever decide we want to go above 100TeV. If we do, we'd already have all the drilling and liquid helium equipment set; all we'd have to do is replace the magnets with the more expensive ones that CERN was planning on using. At that point, the 270km loop would become a 300TeV collider.

While I admire the audacity of the idea, it's worth pointing out that, at the moment, we don't have any strong reason to expect that there are any particles out there massive enough to require any of this. Plus the trend in the US toward budget cutting has meant that the country is an extremely unreliable partner for large international scientific endeavors (like the European Space Agency and the ITER fusion reactor). Something like this would take years to build, and the US budgeting process hasn't demonstrated that sort of attention span in recent years.

The arXiv. Abstract number: 1402.5973 (About the arXiv).