The Large Hadron Collider, the most powerful particle collider humanity has ever built, resides in an enormous, 27-kilometer-long tunnel that extends under the Swiss and French countrysides. What’s often overlooked is that the tunnel was built for an earlier piece of hardware, the Large Electron-Positron collider, or LEP. LEP had been built specifically to provide a clean way to study the Z boson; only later was it converted to a higher-energy proton collider that enabled the discovery of the Higgs boson.
Now, Europe is officially committing to taking a similar approach: building a huge tunnel at the CERN facility that will collide particles to enable a clean study of the Higgs boson. But Europe is leaving the option of using the tunnel for a future collider that could reach energies nearly 10 times higher than the LHC.
Electrons vs. hadrons
Electrons and positrons are fundamental particles; as far as we know, they have no smaller particles that comprise them. That makes their collisions extremely clean. The protons collided by the LHC, in contrast, are composed of a collection of quarks and gluons, making their collisions a complicated collection of sub-collisions that can be challenging to interpret.
That makes electrons and positrons better candidates for the detailed characterization of particles. But they’re less good for discovery. As particles are forced to travel around the curved paths of circular colliders, they radiate away some energy, causing them to slow down. This limits the energy that collisions can reach. And that’s why CERN replaced LEP with the Large Hadron Collider; protons, thanks to their larger mass, lose less of their energy in the curves of a circular collider. As such, they can be boosted to much higher energy.
Thus, while the 27km tunnel that houses the LHC was originally built to study the Z boson using electrons and positrons, CERN replaced the hardware with a proton collider to discover the Higgs: the Large Hadron Collider.
But even a tunnel like the 27km one now occupied by the LHC sets limits on what we can do. To pull particles along a curved path, we need magnets that increase in strength as the particles’ energy gets higher. Given that we’re at about the limits of current magnet technology, that means we need a larger tunnel—meaning more gradual curves—to reach higher energies. And Europe’s physics community has now decided it wants to build a much larger tunnel.
Bigger is may be better?
The tunnel would be truly enormous, with a circumference of roughly 100km, meaning a diameter of roughly 30km. That would mean it would pass under the nearby Lake Geneva, requiring much deeper tunnels than those used for the LHC. It would be a major and expensive construction project. But it would potentially get us a two-for-one, just as the tunnels occupied by the LHC did. Initially, an electron-positron collider would be built for a detailed characterization of the Higgs boson.
Then, once that is done to the physics community’s satisfaction, it would be replaced by a proton collider that would allow collisions to reach energies over seven times those reached by the LHC. This could allow detection of much heavier particles than those studied by the LHC—assuming heavier particles exist (more on that in a bit).
The total costs of some of these ideas runs into the tens of billions of dollars, and the document is clear that Europe won’t be doing it on its own—international partners will be critical. And that’s where things get a bit strange, because the potential partners are in the process of considering other projects.
Japan and China
For example, Japan has been suggesting it might be willing to host a proposed alternative to a circular collider, the International Linear Collider, which would also collide electrons and positrons in order to study the Higgs. A linear collider avoids the energy loss associated with forcing particles around a curved path. But circular colliders have the advantage of being able to slowly accelerate particles each time they take a lap around the loop, meaning the acceleration could be much more gradual. Linear colliders have only one chance to accelerate particles as they run down the track toward a collision, so the track has to be much longer to reach the equivalent energies—about 30km for the International Linear Collider.
So, the costs are somewhat lower than a circular collider, but not a lot lower. And a linear collider can’t be repurposed into a high-energy hadron collider afterwards.
While Japan is very interested in hosting this collider, it hasn’t committed to fully funding it and is trying to arrange commitments from other countries to add the rest of the funding. But if that collider goes forward, it would obviate the at least half of the justification for building a giant tunnel at CERN. Yet the report claims that “The timely realisation of the electron-positron International Linear Collider in Japan would be compatible with this strategy.”
Elsewhere, the Chinese are considering a plan that largely mimics that of CERN’s: a giant circular tunnel that will house first an electron-positron collider and, later, a proton collider. China’s argument is that it’s not constrained to build in the complex geography of the Swiss-French border, with its lakes and mountain ranges. And construction costs are lower there to begin with, meaning that the whole project could be done substantially more cheaply in China. Again, no commitment has been made to build the hardware yet, but it could make the arguments for CERN’s project far more complicated if it continues to move forward.
There are also many active areas of study that would enable us to build hardware that provides greater accelerations in shorter distances. The report also argues for continued support of those, even though they would tip the balance heavily in favor of using a linear collider, since it could be made much more compact. There’s an additional mention of using muons, heavier (if unstable) relatives of the electron and positron, for the collisions, something that’s being actively pursued at the US’ Fermilab.
“The European particle physics community must intensify accelerator R&D and sustain it with adequate resources,” the report says, even though progress on any of them would make the big collider a less appealing approach.
A gap in the theory
But perhaps the biggest question facing the collider is whether there’s anything left for it to find. In addition to the Higgs, there were strong theoretical candidates, including potential dark matter particles, that were within the energy range reached by the LHC. Even though none of them turned up, that has ended up being informative, killing off a huge range of potential models for other particles and causing plenty of people to rethink models based on the idea of supersymmetry.
At the moment, in the energies that will be reached by this proposed successor, we have… not a whole lot. There are always ideas floating around that would involve extremely heavy particles, and it’s always possible that potential particles that were predicted to be lighter turn out not to be. But there’s no obvious candidates at these energies that have a well-developed theory behind them. As a result, we’ve got no strong reason to think that there’d be anything to discover if we built this machine.
Of course, we’d have decades of development in theory that might place something there before this actually becomes operational. And there’s always the chance that we’d find something unexpected if it was built. But those chances are not typically the sorts of things that are allowed to dominate the funding landscape for an entire field. Fortunately, the physics community has gotten used to shifting funding priorities over the years, and the construction of the new collider is far enough out that there is time to work out if and how to integrate this work with the proposed projects in Japan and China.