The CERN particle accelerator that will breathe new life into physics

A new breed of collider, called plasma wakefield accelerators, can study fundamental physics in new ways by doing something the Large Hadron Collider cannot do: colliding electrons.

TO GET to Edda Gschwendtner’s experiment, you enter a small, brutalist building at CERN, Europe’s particle physics laboratory on the outskirts of Geneva, Switzerland. You head into the lift and descend 50 metres into a vast underground chamber. After a series of yellow security doors, you must traverse a kilometre along a downward-sloping tunnel – which is why Gschwendtner typically uses one of the small white bikes parked inside the doors.

Testing the plasma tube used in CERN’s AWAKE experiment
Ordan, Julien Marius/CERN

She is developing a promising kind of particle accelerator that might help us find new physics. Since the discovery of the Higgs boson in 2012, particle physics hasn’t made much progress to speak of. So, thoughts are turning to machines that can help us probe reality in different ways.

The experiment Gschwendtner works on, called AWAKE, creates a wave of plasma – a gas of charged particles – and sends electrons surfing along it. While most colliders are getting bigger and pricier, this underground machine and its ilk, known as plasma wakefield accelerators, are compact. Don’t be fooled by their size, though – they pack a punch. Compared with the likes of CERN’s vast Large Hadron Collider (LHC), over a set distance, plasma wakefield technology can manage much stronger accelerations. “Up to a factor of 1000 more,” says Gschwendtner.

It is proving effective. Over the past few years, AWAKE has had a string of successes in accelerating electrons over a distance of just metres. Last year, it passed a crucial test, and researchers are now gearing up to take it to higher energies. Gschwendtner and her colleagues hope AWAKE will provide the answers to some of the most intriguing mysteries in physics.

The results of proton collisions absorbed by screens at the LHC
Fichet, Jacques Herve/CERN

For a long time, particle physics was a successful business. Scientists spent decades cracking open the likes of atoms and protons to work out what their more fundamental ingredients are. These efforts delivjered the standard model, a list of the basic constituents of reality, such as quarks, neutrinos, electrons and the forces that govern them. The last entry on the list – the long-predicted Higgs boson – was inked-in in 2012, discovered in the shrapnel created by smashing protons apart in the LHC.

Not a lot has happened since. That bothers physicists because, while the standard model is neat enough, there is a nagging suspicion that there must be more to discover. It can’t explain why, for example, all the particles seem to come in three versions that are all exactly the same apart from their mass. Plus, the standard model has nothing to say about the identity of dark matter, the unidentified stuff allegedly hiding out in the cosmos, giving itself away only by its gravitational influence. Plenty of physicists think we need a new fundamental discovery to help work out what is missing.

This longed-for new physics could come in the form of a totally new particle, or it might be one of the particles we already know about behaving in an unexpected way. Both would be equally exciting.

For the former, we have “discovery machines” like the LHC, which uses magnets to accelerate beams of protons around its 27-kilometre ring before smashing them together at a hair less than the speed of light and examining the aftermath for hints of new particles. But proton collisions tend to create messy explosions of particle shrapnel that are tricky to interpret. This makes it fine for getting a glimpse of new particles, but unsuitable for spotting subtleties in the way they behave.

For that, we need a different type of machine. “Precision machines” can accelerate electrons, something that can’t be done in circular colliders because electrons lose too much energy as they travel around the ring. Since electrons have no internal structure, they generate clean collisions that produce exactly what physicists want, with no mess. The current front-runners in this sphere are linear accelerators, which use electric fields to accelerate particles. But, like circular accelerators, they must be big. “If you want to continue doing particle physics, discovering new phenomena at higher energies, well, we need longer accelerators,” says Matthew Wing at University College London. On the other hand, plasma wakefield accelerators (PWAs) work in a completely different way to traditional circular and linear machines. This means they can be much smaller.

Plasma wakefield accelerators

The idea for PWAs was born in 1979, when physicists Toshiki Tajima and the late John Dawson, both then at the University of California, Irvine, used a computer simulation to investigate what would happen if a laser were fired into a plasma. They found it would create a wave in the plasma and that electrons could be caught in the wake of this wave and accelerated with incredible efficiency. Like a surfer travelling down the side of a gnarly breaker, the particles get faster as they go.

It took a while for the idea to get off the drawing board, largely because for years lasers were too weak to generate useful plasma wakes. But in 2004, a trio of experimental results published simultaneously – known in the field as the “dream beam papers” – showed that laser wakefield acceleration really worked.

By this time, scientists had realised it didn’t have to be a laser driving the plasma waves, it could be a beam of electrons. A breakthrough came in 2007, when researchers bolted a PWA onto the end of the SLAC National Accelerator Laboratory at Stanford University in California. The accelerator’s electron beam was used to drive a plasma wave, and this wave accelerated a second beam of electrons. The result was impressive, effectively doubling the energy of the beam in a tube of plasma just 85 centimetres long.

One big problem remained with this set-up. To reach really high energies, researchers had to fit multiple tubes of plasma together, end to end, and re-energise the waves at each join by spurring them on with a fresh laser or electron beam. This approach, called staging, is incredibly finicky. Few have ever got it to work.

Several years later, physicist Allen Caldwell, now at the Max Planck Institute for Physics in Munich, Germany, had a bright idea. Instead of using lasers or electrons to drive the wake, you could use protons. Handily, CERN already provided an incredibly high-energy source of protons – they were so powerful, in fact, that you wouldn’t need to bother with staging at all. “You could have the protons travel for hundreds of metres, maybe a kilometre,” says Wing, “and they would still be very effectively driving a wake.”

In 2013, the project was approved by CERN and the Advanced Wakefield Experiment (AWAKE) was born. The researchers wanted to use a beam of speeding protons from CERN’s Super Proton Synchrotron (SPS), a kind of pre-accelerator to the LHC, to drive the waves in their plasma. That is why AWAKE is so deep underground, so it can be connected to SPS.

I went to visit Gschwendtner at CERN in November 2022. She took me down into the vault where AWAKE is held. The scale was incredible: like being in an underground cathedral made of concrete. To my left were the yellow security doors that guard the entrance to the tunnel leading to AWAKE. Sadly, we weren’t able to go down the tunnel on the day because the levels of radiation were too high. I did have a quick go on one of the bikes, though.

Had we been able to see AWAKE itself, we would mostly have been looking at a 10-metre-long tube surrounded by electronic equipment. “This is already the longest in the world for a plasma accelerator,” Gschwendtner told me. By 2018, Gschwendtner, Wing and their colleagues had demonstrated the protons could drive a wave and that electrons could be injected into the wake and accelerated. The results were a reminder of just how powerful this trick could be. The electrons were injected into the tube with an energy of about 19 million electron volts (eV) and accelerated to a whopping 2 billion eV, by the end of the tube, increasing their energy 100 times in 10 metres.

One nagging difficulty, though, was that protons from SPS come in bunches that are too long to form nice, tight waves in the plasma. Until 2018, the researchers were using a short laser pulse to break these bunches into smaller chunks suitable for creating a wave, a process called modulation. But that wouldn’t have worked in a tube many tens of metres long. They needed a way to get the proton bunch to modulate itself.

After painstaking work, they cleared this hurdle too. In 2022, the researchers published a demonstration of the proton beam breaking itself up into short micro-bunches. “These micro-bunches work in resonance to drive the wake, and that’s when we can really get very strong accelerations,” says Gschwendtner. “It is a big milestone.”

What makes this exciting, says Suzie Sheehy, who works on accelerator physics at the University of Melbourne in Australia, is it shows this self-modulation process can be controlled. “Gaining control to high precision is the name of the game when it comes to accelerators,” she says.

With that done, there are no more fundamental reasons why the technology can’t be scaled up. Elongate the plasma tube and you should be able to speed up the electrons more and more. Wing’s guess is that an increase from 10 metres to 30 metres would get us into interesting territory. “As soon as we start getting above 10 giga-electron volts [10 billion electron volts], you suddenly have an electron beam that isn’t available anywhere else and has the potential to be interesting for all sorts of particle physics experiments,” he says.

The search for new physics

One way to use this technology might be to help answer the vexing question of where the universe’s mass comes from. Only a small proportion of the mass of atoms comes from the particles inside them. Most of it emerges from the way particles called gluons – which carry the strong force, one of the fundamental forces of nature – bind to each other. “We don’t understand this,” says physicist Jon Butterworth at University College London. “How is the proton held together?”

Physicists suspect that we can work out the answers by firing very high-energy electrons at ions, charged atoms, and seeing how they are deflected by the gluons inside. This would help us map out what the gluons are doing in fine detail. A machine that could answer this and related questions, called the Electron-Ion Collider, is already planned at the Brookhaven National Laboratory in Long Island, New York. The aim is for construction to be completed in the 2030s. This particular collider won’t involve PWA technology, but similar machines could in future.

Another prospect would be to use AWAKE in the hunt for dark matter. One idea is that this is composed of a “dark sector” made up of a dark equivalent of every particle we know of, including particles, or photons, of light. Being massless, dark photons wouldn’t account for the gravitational effects we see from dark matter, but finding them would give us our first real evidence that the dark sector exists. Searches for dark photons usually work by making a beam of electrons hit a target, creating a beam of photons that could turn into dark photons – but only very rarely. AWAKE could help by boosting the power of the electron beam and the chances of seeing a dark photon.

A Higgs factory

Tim Nelson already works on searches for the dark sector at SLAC. He says PWAs could be useful for dark photon searches, but cautions that the first-generation machines may not offer significant advantages over conventional accelerators. “It certainly is a possible early application for PWAs, but it’s not obviously a game-changer,” says Nelson.

Arguably the most exciting use for PWAs, though, would be to study the Higgs boson. One promising avenue for new physics is the idea that the Higgs is more complicated than we have so far realised. It might come in more than one variety or perhaps interact with other particles in ways we haven’t yet spotted amid the chaos generated when they are produced at the LHC. A precision collider aimed at studying the Higgs boson, known as a Higgs factory, would enable physicists to find out – and possibly crack open a new understanding of the standard model. But PWAs still struggle to reproduce multiple beams of the same characteristics in quick succession. For this reason, says Butterworth, they aren’t a contender to power a Higgs factory quite yet.

Still, the future looks promising. Plasma wakefield accelerators might reach energies of tens of billions of electron volts in a few years. Although the LHC already reaches 13 trillion eV, “the gulf isn’t as big as you might think,” says Butterworth. In the LHC, it might only be one or two of the quarks that actually hit each other, which means the energy involved in a collision is straight away around 10 times less than the total energy of the proton. Even accounting for that, though, PWAs aren’t likely to get to the kind of energies that the LHC can manage any time soon.

Meanwhile, electron-driven PWAs are coming on apace. Richard D’Arcy works on the FLASHForward experiment at the German Electron Synchrotron (DESY) facility. The experiment uses high-energy electrons from the main DESY accelerator to create waves in a tube of plasma, which then accelerate a second group of electrons to high energies. “We are becoming more and more focused on how you can meaningfully apply this technology,” says D’Arcy. These electrons aren’t high-energy enough for particle physics, but D’Arcy says they could form the basis of an X-ray free electron laser, essentially the world’s most powerful kind of microscope. These are useful for looking in unprecedented detail at molecular processes, such as how molecules and proteins fit together to power metabolism.

When it comes to particle physics, the best bet is going to be AWAKE. “It’s very exciting,” says Gschwendtner. She and her colleagues are going to be working on that experiment for years to come, pedalling or scooting their way down the gently sloping tunnel at CERN. Each journey, and each new wave the electrons catch, will bring us closer to the new physics everyone is hoping for.

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