A once-fantastical collider could answer physics’ biggest mysteries

When it comes to particle physics, Tova Holmes has been there, done that and got the T-shirt – in fact, she designed the T-shirt herself. It all started back in 2022, when she and a few colleagues arrived at a meeting of particle physicists determined to make the case for developing an entirely new kind of particle-smashing machine.

They did so by sporting tops emblazoned with a motif representing a circular particle accelerator and a single word: BUILD. “We wanted to find a way for people to visibly show how excited they were about a muon collider,” says Holmes, who is based at the University of Tennessee, Knoxville.

To its advocates, this newfangled collider would be exactly the shot in the arm that particle physics so desperately needs. The famous Large Hadron Collider (LHC) at the CERN particle physics laboratory near Geneva, Switzerland, wonderful as it is, simply hasn’t delivered any truly new discoveries in years. The answer, say Holmes and her ilk, isn’t to build ever-more powerful successors to the LHC, as some would like, but to change the game entirely. They want to collide together a strange type of particle known as the muon.

To many, though, the proposal has long seemed fanciful at best. After all, muons live for only a fraction of a second. But technological developments are now starting to make the idea more feasible – and funding organisations are eyeing it with serious interest. All of which makes it worth asking: what would it take to build this magnificent muon machine and, if we did, what secrets of reality might it reveal?

In 2012, the LHC confirmed the existence of the Higgs boson, a particle proposed nearly half a century earlier to explain how the fundamental forces of nature first split in the early universe. The boson is produced by an excitation in the Higgs field, which endows certain particles with mass – including the W and Z bosons that carry the weak force – while leaving others, such as the photon, untouched.

It was a spectacular vindication of physicists’ theories about the world of particles. But it was also unsettling. The Higgs boson’s own mass is puzzlingly small. Quantum field theory suggests it should be far larger, yet it perches, unnaturally balanced, at precisely the level required to keep the vacuum of space-time stable. Why so perfectly poised? “People talk about the Higgs discovery as the completion of particle physics,” says Patrick Meade at Stony Brook University in New York state. “But it was really the most confusing answer. It was the start.”

The next big discovery machine

But if it was indeed the start, then the engine seems to have stalled, because today experimental particle physics is at an impasse. Answering the profound questions raised by the Higgs will require a new machine, one capable of probing deeper into nature’s foundations through different or more powerful particle collisions.

The most straightforward idea is the brute-force approach: build a bigger version of the LHC. That’s the thinking behind the Future Circular Collider, a proposal being developed at CERN for a next-generation proton supercollider with a ring three to four times the circumference of the LHC. It could smash protons at over seven times the energy of its predecessor simply by stretching over a greater distance. This would allow physicists to discover particles or phenomena that emerge only at higher energies, while also probing ever-shorter distances and revealing more fundamental structures of matter.

But protons aren’t fundamental particles; they are bundles of quarks and gluons. When two protons meet head-on, it is their constituents that actually collide, producing messy sprays of secondary particles that physicists must painstakingly analyse. Plus, making a machine like the LHC any bigger would also come with an eye-watering price tag.

At the other extreme are electron-positron colliders, like the Compact Linear Collider, another proposal from CERN researchers. Electrons and positrons are fundamental, point-like particles with opposite charges, so their collisions are far cleaner and easier to interpret. The difficulty is that pushing them around a circular track at high energies causes them to shed energy copiously in the form of radiation. Linear colliders attempt to sidestep this limitation by accelerating particles along a straight track. But particles can’t be reused, unlike in a ring, which recycles them in multiple passes.

But there is also a dark horse in the running, in the form of the muon collider. Muons are essentially the heavier cousins of electrons, about 200 times more massive but with the same negative charge. You wouldn’t be able to see them in the atoms that make up everyday matter, but they are produced fleetingly when high-energy cosmic rays strike molecules in Earth’s upper atmosphere.

Their extra heft means they radiate far less energy when bent around a ring in a collider, allowing them to reach much higher energies without requiring a vastly larger tunnel. Yet, like electrons, they are fundamental particles, so their collisions would be comparatively clean. In principle, a muon collider could push beyond our current energy frontier of 13.6 teraelectronvolts (TeV) by a factor of four while fitting inside a ring not much bigger than the LHC’s, according to design studies by the US Muon Collider Collaboration.

The idea isn’t new. Physicists were already sketching proposals for muon colliders in the 1960s, but there was a catch: muons, unlike protons or electrons, have to be produced. Scientists can’t gently pluck them from atoms before accelerating them to near-light speeds. Instead, they make them by smashing protons into a target, like a solid block of graphite, and producing showers of other particles called pions, which then decay into muons. The result is less a beam and more a spray – particles fanning out in all directions, with a wide range of energies and trajectories. Turning that chaos into a tightly focused, well-behaved beam is the central technical challenge.

There is a further complication: muons are unstable. At rest, they survive for just 2.2 microseconds before decaying into other particles. By contrast, bringing protons in the LHC’s main ring up to full speed takes around 20 minutes – roughly 550 million times longer than a muon’s natural lifetime.

A muon collider is, therefore, a race against time. Physicists must capture a chaotic cloud of newborn particles, then compress and accelerate it before it is too late. “You’re starting with a beam of muons that’s like the size of a beach ball, and you want to turn it into something the thickness of a human hair,” says Meade. “And you’ve got to do it super, super fast.” Then, two of these ultrathin beams must be steered towards so they collide directly, producing high-energy Higgs bosons in the splatter.

For decades, that combination of speed and precision kept the idea on the sidelines. Muon colliders resurfaced during the 2013 Snowmass process, the once-a-decade strategy exercise in which US particle physicists map out the field’s future priorities. Even then, they tabled the muon collider for being infeasible.

Holmes was still early in her career at that time, working towards a master’s degree. But over the following decade, a series of technical breakthroughs began to transform the muon collider into a serious contender for the discovery machine of her generation.

Reviving the muon collider

One dramatic change has come about thanks to the gradual progress of the technology. Early designs of the muon collider imagined modest collision energies compared with what researchers think we can achieve today. Recent plans push up to the 30 TeV range, 100 times more energetic than initial proposals in the 1960s. At those energies, muons travel so close to the speed of light that Albert Einstein’s theory of special relativity becomes an ally. To an outside observer, time slows down for fast-moving particles. The faster the muons go, the longer they appear to live.

The effect is dramatic. In even a modest 10-TeV muon collider, muons could survive for up to a tenth of a second, roughly 45,000 times longer than their ordinary lifetime. Paradoxically, making the muons go faster buys precious extra microseconds in which to control the beam.

And researchers have learned to use that borrowed time. In 2020, the Muon Ionization Cooling Experiment, led by Kenneth Long at Imperial College London, demonstrated a technique known as ionisation cooling. Muons were passed through materials such as liquid hydrogen or lithium hydride, which reduced their momentum in all directions. The researchers then accelerated them forward using rapidly oscillating electric fields, transforming a diffuse spray into a tight, fast-moving bunch.

“It sounds completely crazy because the back of the envelope just tells you that it’s not possible,” says Jesse Thaler at the Massachusetts Institute of Technology, who was sceptical at the thought of a muon collider a decade ago. “But actually, going beyond the back of the envelope, with more scientific study, it starts to look more and more plausible.”

Researchers also, over time, gained practical experience with handling muons. Starting in 2017 at Fermilab in Illinois, the Muon g-2 experiment measured the minute wobble in muons circulating inside a magnetic field – a quantity theorists had predicted with remarkable precision. Earlier measurements hinted that the value might deviate from the standard model of particle physics, our best understanding of how three of the four fundamental forces and elementary particles work, thus raising hopes of new physics. But improved calculations eventually brought the result back in line. Even so, the experiment provided hard-won expertise in producing, storing and controlling muons at scale.

By 2022, when Holmes and her colleagues attended the next Snowmass meeting with her self-designed T-shirts, the muon collider had emerged as one of the leading candidates for the field’s next major machine. In Europe, CERN-backed International Muon Collider Collaboration (IMCC) has begun parallel studies. In the US, many physicists would like to see a future muon collider built at Fermilab, while their European counterparts are exploring whether it could one day be hosted at CERN.

“The muon collider is quite an old concept,” says Steinar Stapnes at the University of Oslo in Norway, a member of the IMCC. “Now, everybody thinks it is very interesting — scientifically and technically.”

We are at a point where it is anyone’s game. Each collider proposal we’ve mentioned must first complete technical studies and pilot demonstrations before governments decide which will secure billions in funding. In the meantime, rival camps of advocates will argue that their machine should define the next era of particle physics.

“A machine like this would be around the middle of the century,” says Holmes. “That’s if we get given a whole lot of funding.”

Sergo Jindariani, who heads the US Muon Collider Collaboration, is leading early feasibility studies for the proposed machine. “We’ve been doing things the same way for many decades,” he says. “At some point, we need a new approach, and colliding muons may be that.”

Window into the Higgs

So what would a muon collider tell us if it were built? Researchers say its central aim would be to probe the Higgs boson more deeply than any machine before it. Though it was discovered over a decade ago, the Higgs itself remains deeply baffling. “In the standard model, there are over a dozen particles, but none of them has properties like the Higgs. It’s very unique,” says Jindariani.

Physicists suspect the Higgs field shaped the early universe. As the cosmos cooled after the big bang, the field switched on during a transition that split the unified electroweak force into the separate electromagnetic and weak forces we see today. How violent that transition was could help explain one of physics’ deepest mysteries: why matter survived while antimatter vanished.

Even today, the Higgs field may not be entirely stable. Some calculations even hint that our universe sits in a precarious state, with the Higgs field not at its lowest possible energy. In that case, a quantum fluctuation could one day tip it into a deeper energy state, a process known as vacuum decay. If this happened, everything about our universe would change instantly.

“All fundamental particles that have mass would get heavier, and presumably completely reorder our elements and cause total chaos,” says Holmes.

“Essentially, it’s like somebody turning the lights on or off in the universe. If they’re off, none of us exists. If they’re on, we can live,” says Meade.

Physicists already suspect that something is amiss. Quantum theory predicts that interactions with heavy particles should drive the Higgs boson’s mass to enormous values. Instead, it sits at a relatively modest 125 gigaelectronvolts. Making the numbers work requires an extraordinary degree of fine-tuning.

For decades, physicists have proposed ways to resolve this tension. One idea is that there is not one, but multiple Higgs bosons. If every known particle in the standard model, including the Higgs boson, has a heavier partner, it would cancel the effects that scientists currently think should inflate the Higgs’s mass. Another idea is that the Higgs isn’t fundamental at all, but composite – built from smaller constituents bound together, much like protons are made of quarks.

Each of these possibilities would leave experimental fingerprints that a muon collider could detect by measuring how the Higgs couples with other particles and itself at high energies, says Holmes. It is this feature that advantages the muon collider over dedicated so-called Higgs factories – usually electron-positron colliders designed to produce vast numbers of Higgs bosons, but at lower energies than a muon machine could reach.

Before a full-scale muon collider can be built, researchers must show that its key technologies work in practice. The next step is a demonstrator facility to test whether muon beams can be prepared and controlled well enough to collide. The IMCC is developing plans for such a machine at CERN, while the US Muon Collider Collaboration, working with the IMCC, is exploring a similar demonstrator at Fermilab. The goal is to produce detailed technical designs by around 2030. If approved and funded by governments, a demonstrator could begin operating in the early 2030s, providing the proof of principle needed for a full collider.

But scientists like Holmes are in it for the long haul. She has faith that the muon collider will emerge victorious as the world’s next great project. And physicists seem to be rallying around her. She and her colleagues are no longer the only ones wearing the muon collider T-shirts: “I’m delighted to see how often I show up at another department and see them already there.”