The real reason to build a bigger supercollider
Arguably, the 27 km Large Hadron Collider (LHC) at CERN in Switzerland has exceeded expectations. The reason why is because it was built primarily to “complete” the Standard Model of particle physics. It did so over 10 years ago by confirming the existence of the Higgs boson which confers mass to gauge bosons that mediate the strong and weak forces. By extension it is believed the Higgs boson is responsible for all forms of particle rest mass.
That is a pretty big deal and, despite the multi-billion dollar price tag, well worth it for knowledge that deep about the universe.
Nowadays CERN has been busy looking for new, physics breaking particles and we have been hearing about the potential new collider proposed that would be far larger. With a $20B price tag and a 25 year plan till operation, is it worth it?
Let’s look at its predecessor.
The LHC was first conceived in 1984 at a meeting in Switzerland. This was one year after the W and Z bosons were discovered at CERN. Its construction was approved 10 years later in 1994. It was then inaugurated in 2008 and reached 7 TeV in 2010 and 8 TeV the next year. The Higgs was discovered the following year.
When LHC was first proposed I was 3 years old. The accelerator went online when I was 28. So it took about 25 years for anyone to see any use from its operation.
That doesn’t meant that other particle physics wasn’t taking place. Physicists weren’t doing nothing that whole time, but they weren’t finding new elementary particles at CERN either. Fermilab was able to discover the top quark and the tau neutrino in the meantime because they had the more powerful collider: the 6.2 km Tevatron.
That goes to show that whoever had the most powerful collider is the most likely to discover new elementary particles. And it is a good case for building one more powerful than the LHC.
On the other hand, it seems that if it takes 25 years from initial proposal in 2020 to get from where we are now to the 100 km Future Circular Collider (FCC), we won’t see the first collisions till 2045 with construction potentially being given the go-ahead in 2028. The LHC will enter its final, High Luminosity, phase in 2029 and compete that mission by 2040.
The FCC will, it is hoped, be capabile of collisions in the 100 TeV region.
There has been some criticism of spending 20 billion dollars on a new collider, mostly misplaced. It is misplaced because people mistakenly think it is being built to find overhyped theoretical particles such as predicted by supersymmetry and string thoery.
Unlike the LHC, FCC isn’t necessarily being built to find new particles, and the particles it will be looking for have nothing to do with supersymmetry or string theory.
First, there’s a lot we don’t know about the particles we have discovered. The FCC is being conceived of as a particle factory that can help us understand the SM better and just might discover something new.
One phase of the FCC will be as an electron-positron collider, the FCC-ee. These matter-anti matter explosions will become a factory for W and Z bosons, Higgs, and top quarks. All of these particles have been “discovered” but we do not as yet have good statistics on them. In particular, we still don’t understand aspects of the electroweak force (where electromagnetism and the weak force combine) as well as fermion masses. We need to know if our predictions hold up to experiment and we can only do that with larger colliders.
The high energy hadron collider, FCC-hh, will collide protons with each other (in the same tunnel as FCC-ee) and enable the creation of particles with masses up to 15-30 TeV. This is huge. This will help discover or rule out several potential candidates for dark matter such as Supersymmetric and Kaluza-Klein Weakly Interacting Massive Particles (WIMPs).
The FCC-hh is being called a “portal” to the dark sector, which is just the area of physics that we haven’t been able to explore with current colliders. The idea is that the incompleteness of the SM will become more apparent. Besides dark matter, the SM also doesn’t explain neutrino mass or baryon asymmetry. I have my own theories about baryon asymmetry, which is just why there is so much matter and so little anti matter in the universe. (I don’t think it is a real problem.) I certainly would be interested to see if some of the dark matter candidates have any evidence at all.
Why neutrinos have mass is probably the biggest mystery we need to understand. We know they do. The 2015 Nobel was given out for that discovery. The SM, however, doesn’t account for it so that is clear evidence of a breakdown of the SM. In order to understand why, it is hoped that the FCC will be able to produce heavy neutrinos. These neutrinos may be able to resolve quite a few problems if they exist including baryon asymmetry and dark matter. In a minimal standard model with these neutrinos added, one neutrino would have a very long life (longer than the age of the universe) and be in the KeV range, and there would be a couple of degnerate heavier neutrinos. We could see these last two from the decay of Z and W bosons which the FCC is being set up to produce in copious amounts.
There are a host of other interesting areas of particle physics that we could also discover such as the coupling of the Higgs to Z0 bosons at 240 GeV which only “lepton” colliders such as FCC-ee can accomplish. These would compliment measurements of the Higgs production in the FCC-hh. There is also a planned FCC-eh which would collide electrons with protons. New physics could peak in while studying any of these but by themselves these are valuable.
It is unfortunate that the FCC will take so long to build. By the time it is operational, I will be in my 60s. Yet such things are worth it for knowledge about the building blocks of our universe that we are unlikely to be able to find anywhere else. In the meantime, we have the LHC’s final, High Luminosity phase to look forward to which on its own may discover new physics.
Boyarsky, Alexey, et al. "Exploring the potential of FCC-hh to search for particles from B mesons." Journal of High Energy Physics 2023.1 (2023): 1-18.
Benedikt, Michael, et al. "Future circular colliders succeeding the LHC." Nature Physics 16.4 (2020): 402-407.