In June, the group of Europe’s leading particle physicists released their vision for the next few years of particle physics experiments in the EU. A significant part of this is the Future Circular Collider, which would accelerate particles in a 100-kilometer underground ring around Geneva.
It’s going to be almost four times the size of the Large Hadron Collider, and it’s going to be capable of colliding particle beams with eight times the current LHC energy. The hope is that it will open the doors to new physics-and maybe break the current deadlock in our search for the theory of all. But do the FCC or other future circular collider experiments have a chance of success? Today, we’re going to address the overly optimistic proposals for future circular collider and honestly assess their chances.
Evolution of Collider technology
But first, to get some perspective, we need to take a step back and talk about how collider technology got to where it is today. Physicists have been building machines to accelerate and subsequently obliterate particles since the 1920s.
The first was the linear accelerator, or linac, which uses oscillating electric fields to accelerate charged particles in a straight line, while magnetic fields focus the beam.
The cyclotron quickly followed – here, the particles are still accelerated by electric fields, but now a constant magnetic field causes the beam to spiral outwards from its central source. Once accelerated, the particles were typically slammed into a motionless target – often just a metal slab. Some particles would collide with enough energy to be destroyed, and their energy would be released in the form of new particles. In those first collision experiments, all sorts of never-before-seen particles were observed, allowing physicists to map out the subatomic realm.
There’s a serious limit to the energy you can muster by colliding particles into a stationary object. To be precise, collision energy with a fixed target goes up with accelerator energy’s square root. Which means you’re wasting lots of energy. On the other hand, if you can collide two beams moving in opposite directions WITH each other, then you get the full oomph of the impact- twice the energy in the individual beams. The key to doing this is to store the particle beams in a ring to collide at your leisure.
Particle Beam collider
To that end, the particle storage ring was invented by Gerard K. O’Neill in the mid-1950s. Within a few years, an Italian group built the first particle beam collider – the ADA. It is a four-meter ring collided electrons and positrons.
The Soviets quickly followed with their VEP-1, which was smaller but collided electrons with one another to get a thousand times higher luminosity than ADA.
In collider-speak, luminosity is a measure of the number of particle collisions across an area over a time period. More collisions mean more chance of producing weird particles. The Americans soon followed up with a 12-meter electron-electron collider with a similar luminosity to VEP-1 but higher energies than even the ADA. Finally, we’d reached collision energies needed to test predictions of the still relatively new quantum electrodynamics.
This was enormous in solidifying our understanding of the quantum universe. Graduating from electrons and positrons, in1971, physicists started smashing protons together at CERN’s Intersecting Storage Rings facility. CERN does have a habit of extremely literal naming. Since then, colliders have only grown in size and energy.
Discovery of Higgs boson
In Illinois, the Tevatron at Fermilab was fora while the largest collider at 6.3km around, operating from 1983 to 2011. It generated collisions energetic enough to produce the very massive top quark and enabled the final Fermion- or matter particle discovery – in the standard model.
The whole project reached its peak with the Large Hadron Collider at CERN in Switzerland. It propels twin proton beams in opposite directions across a 26 km diameter ring underneath the Swiss and French countryside. The beams cross four positions where they collide with the energies of multiple terra-electron volts. Which, for comparison, is a lot of energy. And the LHC’s crowning achievement was the discovery of the Higgs boson in 2012.
The Higgs’ existence confirms our explanation of how the elementary particles acquire mass – which, of course, we’ve covered previously. In a way, this was the last missing component of the standard model-the only remaining particle that physicists believed Would exist.
Vision for the betterment of the standard model
LHC particle hunters expected that to be just the beginning – that their giant collider would go on to discover many new particles to take us beyond the standard model. The most highly anticipated were the particles predicted by supersymmetry – or SUSY. This is an evolution of the traditional model intended to address some of its major problems.
Most importantly, the issue of hierarchy-the the fact that there is a huge difference between the force of gravity and the other forces and a huge difference between the measured masses of known particles and what we believe their masses to be from the measurement of quantum field theory.
In particular, the Higgs particle is expected to have an enormous mass; our Standard Model understanding is the whole picture. The fact that the Higgs turns up at a “mere” 100 times the proton’s mass seemed to suggest that something outside the standard model was required. Among other things, those counterparts should help cancel out the interactions of the known particles with the elementary quantum fields on which those particles live, eliminating most of their mass in the process.
But the supersymmetric counterparts do their jobs most neatly if they have masses-slash-energies in a particular range: between 100 and 1000 GeV. The Large Hadron Collider reaches energy a few times higher than the top of that range, so it should have seen such particles by now. But after a decade of searches at the LHC, there’s still no sign of SUSY.
No clear hint for the next direction in the standard model
So we find ourselves at an impasse. The Standard Model is complete, but there is no clear hint of the next direction to take. So what next? Well, it’s clear that the Standard Model is not the whole picture. Besides the hierarchy problem, the standard model also doesn’t explain why neutrinos have mass. It doesn’t seem to give us a particle that could explain dark matter. And there are other anomalies like the muon’s magnetic moment.
In general, to get closer to a theory that unifies our understanding of the Standard Model’s motley zoo of particles and forces, we probably need to achieve higher energies – energies even closer to the instant of the Big Bang, when the forces of nature were literally unified, as we’ve talked about before.
There are other clever ways to probe these energies, such as using natural particle accelerators like the sun or supernovae or quasars or galactic magnetic fields, which continuously spray the earth with particles at higher energies than we can hope to replicate. But these ultra-high-energy cosmic rays are rare, and to accurately detect a new particle, we need to watch the outcome of billions of collisions, and very high luminosities are required. Bigger and better colliders may be our best shot.
Let’s start with better before we move to bigger. The LHC is currently in a long upgrade process, with the final result being a factor of a couple of increase in power and a factor of 5 increase in luminosity. This will be achieved by upgrading the existing components with more cutting edge versions – for example, better superconducting magnets for more precise control of more energetic beams.
We’ve been in the first off-line phase since2018, with operations set to resume in 2021, and there’ll be another 2-year shutdown before the new high-luminosity LHC comes online in 2027. The point of this upgrade isn’t primarily to access higher energies where new particles might exist, but rather to make the LHC much better at studying the current range.
IF either SUSY or other very high-mass particles exist, they may actually be a good way beyond the LHC’s energy range. And so for that, we need a bigger collider. And so we come to the Future Circular Collider. If it goes ahead, it’ll hit 100 TeV energies – 8 times the current LHC energy. Eventually, the Future Circular Collider will be smashing protons as the LHC does, but to start with, it’ll collide electrons and positrons with the express intention of making as many Higgs particles as possible.
Why Electron and positrons?
Okay, firstly, why electrons and positrons? Well, remember that the first particle colliders worked with electron-positron beams, and for a good reason: they are easier to work with compared to protons. It’s easier to achieve the energies and luminosities to produce, for example, large numbers of Higgs particles in relatively clean collisions.
But why build a Higgs factory? Haven’t we already discovered the Higgs? Discovered, yes, but there’s still a lot we don’t know. For example, we don’t know how often it interacts with the super heavy top quark at our current energies, a process that might contain hints about new physics.
The Higgs can also be used as a direct search for new particles. The FCC will eventually graduate to proton-proton collisions, which will further open up the discovery room.
The Future Circular Collider would cost 10 billion over its lifetime. So is this a good investment? It’ll certainly tell us more about the Higgs, and it may discover that elusive clue to take us beyond the standard model. However, there’s no guarantee that any new particles exist in the expanded energy range that the Future Circular Collider will probe. That said, you can’t know if you don’t look.
The Future Circular Collider is the priority defined by the European Strategy Group. But what about the US? Ever since Europe won the giant collider game with the LHC, particle physicists in the US have focused on smaller experiments. Fermi Lab’s Tevatron was the largest collider in the world for a while, but it was shut down in 2011 because it couldn’t compete with the LHC.
Since then, the Chicago facility has reinvented itself as leaders in, among other things, neutrino experiments – and when we visited Fermilab earlier this year, we saw the linear particle accelerator that is under development to become DUNE’s neutrino source. But the next big-ish US collider will most likely be the Electron-Ion Collider. It won’t be anywhere near the LHC size but will have a more focused mission. It will smack electrons into protons and other nucleons to probe the details structure and interactions between quarks.
The EIC is the priority program endorsed by the National Academy of Sciences, and while it doesn’t quite have final approval yet if it does happen, it’ll be at Brookhaven National Labs on Long Island, starting in around ten years. It’ll be an order of magnitude cheaper than the Future Circular Collider at an estimated 1.6 to 2.6 billion. But that’s still a chunk of change. So is it worth it to do this sort of fundamental science, even though the immediate returns to the nation and world are vague?
Well, putting aside the fact that our modern world is built on the technologies that came from this sort of fundamental physics research, I want to leave you with a quote by Fermilab’s first director, Robert Wilson, when questioned by the Senate regarding the value of the Tevatron to the US – and in particular whether it helped compete against the Russians: He said, “Especially from a long-range point of view, it is the advancement of technology. Otherwise, it’s got to do with: are we good painters, good sculptors, great poets? I mean all the things we truly worship and respect in our nation and are proud of. In that sense, this new understanding has much to do with honor and culture, but it has nothing specifically to do with defending our nation except to make it worth defending.”
Dr. Wilson was addressing the US Senate, so spoke of the nation – but the larger value of this sort of work is to elevate humanity. Whether it happens now or later – I, for one, am patriotic about being part of a species capable of doing something like this, Of coordinating thousands of scientists over many decades to build these crazy machines that can crack open the inner workings of spacetime.
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