Will Future Colliders Break the Standard Model? (Script) (Patreon)

If you wanna make an omelet you gotta break a few eggs. And by omelet I mean a theory of everything, and by eggs I mean a billion billion subatomic particles obliterated in the next generation of giant particle colliders.

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In June, the consortium of Europe’s top particle physicists published their vision for the next several years of particle physics experiments in the EU. A big part of that is the Future Circular Collider, which, if it happens, will accelerate particles in a 100 kilometer circumference underground ring encircling Geneva. It’ll be nearly 4 times the size of the Large Hadron Collider, and it would be capable of colliding particle beams with 8 times the current LHC energy. The hope is that this will open the window to brand new physics - and perhaps break the current deadlock in our quest for a theory of everything. But do the FCC or other upcoming collider experiments really have a chance of succeeding? Today we’re going to discuss the incredibly ambitious plans for future colliders, and try to honestly evaluate their prospects.

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 the beam is focused by magnetic fields. 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 slab of metal. 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 begin 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 as the square root of accelerator energy. 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 so that they can be collided at your leisure. 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, or Anello di Accumuliazione. Its 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 means more chance of producing weird particles. The Americans 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 field of quantum electrodynamics. This was huge in solidifying our understanding of the quantum world.

Graduating from electrons and positrons, in 1971 physicists started smashing protons together at CERN’s Intersecting Storage Rings. CERN does have a habit of extremely literal naming. Since then, colliders have only grown in size and energy. The Tevatron at Fermilab in Illinois was for a 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 so enabled the discovery of the final Fermion - or matter particle - in the standard model.

This entire effort reached its pinnacle with the Large Hadron Collider at CERN in Switzerland. It propels twin proton beams in opposite di rections around a  26.? km circumference ring beneath the Swiss and French countrysides. The beams cross in four locations, where they collide with energies of several terra-electron volts. Which is, for reference, a lot of energy. And the crowning achievement of the LHC was the discovery of the Higgs boson in 2012. The existence of the Higgs confirms our explanation of how the elementary particles acquire mass - which of course we’ve covered previously.

In a sense this was the last missing piece of the standard model - the one remaining particle that physicists thought MUST exist. 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 extension to the standard model designed to solve several of its outstanding problems. Most notably the hierarchy problem - the fact that there’s a huge difference between the strength of gravity and the other forces, and a huge difference between the measured masses of the known particles and what we expect their masses to be from quantum field theory calculations. The Higgs particle in particular should have an enormous mass given if our Standard Model understanding is the whole picture. The fact that the Higgs turns up at a “mere” 100 times the mass of the proton seemed to indicate that something beyond the standard model was needed.

SUSY solves the hierarchy problem by proposing symmetric counterparts to the known particles. 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. But after a decade of searches at the LHC, there’s still no sign of SUSY.

So we find ourselves at an impasse. The Standard Model is complete, but there is no compelling 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 - for example 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 ultra-high energy cosmic rays are rare, and to reliably detect a new particle we need to watch the results of billions of billions of collisions - we need very high luminosities. 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 increase in power - but more importantly 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 since 2018, with operations resuming 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 its current range. IF either SUSY or other very high-mass particles do exist, then they may be a good way beyond the energy range of the LHC. And for that we need a bigger collider.

And so we come to the Future Circular Collider. If it goes ahead, it’ll hit energies of 100 TeV  - 8 times the current LHC energy. Eventually the FCC will be smashing protons like 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. Now remember the first particle colliders worked with electron-positron beams, and for good reason: they’re easier to work with compared to protons. So it’s easier to achieve the energies and luminosities to produce large numbers of Higgs 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, at our current energies, we don’t know how often it interacts with the superheavy top quark, 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 open up the discovery space.

The Future Circular Collider will cost 10s of billions over its life. 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 FCC 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. FermiLab’s Tevatron was for a white the largest collider in the world, 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 size of even the LHC, 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 yet have final approval, if it does happen it’ll be at Brookhaven National Labs on Long Island, starting in around 10 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:

“Only from a long-range point of view, of a developing technology. Otherwise, it has to do with: Are we good painters, good sculptors, great poets? I mean all the things that we really venerate and honor in our country and are patriotic about. In that sense, this new knowledge has all to do with honor and country but it has nothing to do directly with defending our country except to help 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.