Young people supposedly enjoy the luxury of time, but perhaps not if they’re particle physicists. For decades, physicists have peered into the universe’s inner workings by smashing subatomic particles together at ever higher energies. But the next highest energy collider may not be built for 50 years. And Tova Holmes, 34 and a particle physicist at the University of Tennessee, Knoxville, worries her career could slip away before she ever sees such a machine. “I will be definitely not still working, possibly not alive,” Holmes says.

That’s one reason she and dozens of her contemporaries are pushing to develop an exotic new collider. The current highest energy atom smasher, the 27-kilometer-long Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, fires protons into protons. In 2012 it discovered a long-sought particle called the Higgs boson in the field’s greatest triumph. CERN is now hoping to build a much bigger, higher energy proton collider, nearly 100 kilometers around—but not until 2070 or 2080. So Holmes and others want to explore an alternative: a collider that would smash energetic muons, heavier cousins of electrons, into equally energetic antimuons.

A muon collider could be much smaller and cheaper than a functionally equivalent proton collider, advocates say. It could fit on the 2750-hectare campus of the United States’s dedicated particle physics lab, Fermi National Accelerator Laboratory (Fermilab), enabling the U.S. to reclaim the lead in the continuing competition for the highest energy collider. Most important, younger physicists say, it might be built sooner than a more conventional competitor, perhaps in as few as 25 years. “If you want you can add 10 years to that, that’s still a lot better than when I’m dead,” Holmes says.

There’s just one catch: Nobody knows whether a muon collider can actually be built. That’s because unlike the proton or the electron, the muon isn’t eternal, but decays in just a fraction of a second. “The challenge, if you want to capture it in one word, is that the muon is unstable,” says Sergo Jindariani, a particle physicist at Fermilab. “So every stage of acceleration has to be incredibly fast.” From generating the muons, to gathering them into compact beams, to detecting the particles produced in their collisions, the machine presents novel technological challenges.

Nevertheless, some physicists, especially in the U.S., are eager to tackle the task. In December 2023, a federal advisory subcommittee called the Particle Physics Project Prioritization Panel (P5) laid out a road map for the next decade of research in the U.S. The P5 report calls for R&D on a muon collider, stating, “This is our muon shot.” But what exactly are physicists shooting for and what obstacles must they overcome to realize their dream machine?

As Albert Einstein observed, energy equals mass. So, by smashing subatomic particles together at high energies, physicists can blast out new particles—fleeting, massive entities not seen since the big bang. By creating them and watching them decay, they have pieced together a theory of fundamental particles and forces called the standard model.

The standard model describes how these particles and their antiparticles interact through three forces: electromagnetism, the strong nuclear force that binds quarks, and the weak nuclear force that enables quarks and other particles to decay and change type. (The theory ignores gravity.) The forces emerge as the matter particles exchange other particles. The electromagnetic force is conveyed by the photon, the strong force by the gluon, and the weak force by particles called the W boson and Z boson. The Higgs boson completes the theory by helping give the other particles mass.

Physicists worked out much of this with colliders, typically circular accelerators that send beams of electrically charged particles racing in opposite directions at near–light-speed to smash them together. One type fires electrons into their antimatter counterparts, positrons, but these e⁺e^– colliders struggle to reach high energies. That’s because charged particles radiate x-rays as they circulate, and less massive particles radiate more than heavier ones. With a mass just 0.05% as big as that of a proton, electrons radiate strongly enough to limit their energy. At some point, a circular e⁺e^– collider becomes like a leaky bucket, losing energy as fast as it is pumped in.

To reach higher energies, physicists usually build bigger, more powerful hadron colliders that smash protons into either protons or antiprotons. But hadron colliders have limitations, too. A proton is a bundle of quarks and gluons, so when two protons strike each other, just a single constituent in each is likely to collide directly, converting less than 10% of the protons’ energy into new particles and reducing the machine’s energy advantage. The remaining fragments of the protons also generate sprays of extraneous particles. In contrast, an electron-positron collision consumes the colliding particles’ full energy and produces no extraneous sprays, so physicists often use an e⁺e^– collider to scrutinize the new particles discovered at a hadron collider.

For example, in 1983 CERN physicists used a proton-antiproton collider with an energy of 540 gigaelectron volts (GeV) to discover the W and Z bosons. CERN then bored the 27-kilometer tunnel on the border of Switzerland and France to build the Large Electron-Positron collider, which enabled researchers to study the W and Z in detail and deduce that there are most likely just three generations of matter particles. Then, in the same tunnel, CERN built the LHC, designed to smash protons at 14 teraelectron volts (TeV). It found the Higgs.

That discovery neatly completed the standard model but left physicists vexed. The theory has obvious shortcomings—for example, it doesn’t include dark matter, the mysterious stuff thought to make up 85% of the universe’s matter. Researchers hoped the LHC would cough up other new particles that would lead to a deeper understanding. But so far, it has turned up nothing more.

So CERN plans a similar progression of bigger machines after the LHC shuts down in 2041. For 15 billion Swiss francs—roughly $17 billion—the lab envisions digging a 91-kilometer-long tunnel and installing an e⁺e^– ring called the Future Circular Collider-ee (FCC-ee) that would run at 350 GeV, the likely limit for such a machine. To be built by the mid-2040s, it would examine the Higgs as the LHC cannot. Thirty years later, CERN would replace it with a 100-TeV hadron collider called the FCC-hh that would seek new particles and could cost $50 billion.

However, a muon collider might combine the strengths of hadron and e⁺e^– colliders— and be faster and cheaper to build. With a mass 207 times an electron’s, a muon radiates far less energy as it goes in circles, so a muon collider could reach much higher energy than a circular e⁺e^– machine. At the same time, muons are fundamental particles that put all their energy into a collision, enabling a muon collider to compete with a hadron collider running at 10 times the energy. So the muon collider could be much smaller and, hence, cheaper. A 10-TeV muon collider could be had for $18 billion, advocates estimate.

The concept of a muon collider dates back decades, and in 2010 Fermilab launched a program to develop it. However, 6 years later, the Department of Energy, which funds most U.S. particle physics research, stopped the program to direct resources to another experiment: a $3.3 billion project, currently under construction, to shoot a beam of muon neutrinos from Fermilab in Illinois to a gigantic subterranean detector in South Dakota. The aborted muon collider effort was more popular with accelerator experts than particle physicists, says Diktys Stratakis, an accelerator physicist at Fermilab who took part in it. “We had a very nice product, but we didn’t have any customers,” he says.

That has changed, Stratakis says. “Now we have a lot of customers coming to us and saying, ‘Hey, can you build a collider with these parameters?’”

One reason is the realization that, technologically, building the FCC-hh will be “a lot more difficult than people thought,” says Hitoshi Murayama, a theorist at the University of California (UC), Berkeley who chaired the recent P5 team. For example, the machine would need steering magnets with fields of 16 tesla—33% higher than the current state of the art and likely unobtainable for 20 years, Murayama says.

At the same time, physicists also feel a new urgency to reach higher energies. That’s because of all that the LHC has not found—at least, so far. No particles of dark matter, the unseen stuff whose gravity holds galaxies together. No mini–black holes. None of particles predicted by supersymmetry, a concept that posits a more massive “superpartner” exists for every particle in the standard model. “If many new things had been discovered at the LHC, we would really want to study them with high precision and maybe not worry too much about going to higher energy,” says Rachel Yohay, an experimentalist at Florida State University who works on an LHC experiment. “Since we haven’t, there is a lot of interest to explore higher energies.”

Perhaps most important, some physicists say a muon collider would be the best tool to address the pressing questions raised by the discovery of the Higgs. The particle is part of a concept called the Higgs mechanism that was added to the standard model in the 1970s to explain an abstruse but important puzzle. Mathematical symmetries within the standard model suggest the weak and electromagnetic forces are different aspects of a single “electroweak” force. Yet the electromagnetic force can, in principle, reach across the universe, whereas the weak force doesn’t even reach across a nucleus.

Physicists had a standard explanation for the disparity. The electromagnetic force is long range because the photon has no mass, and the weak force is short range because the particles that convey it, the W and Z, are massive. But there was a catch. In the standard model, simply plugging in the W and Z masses ruins the mathematical symmetry that produces the weak force in the first place. So those masses have to emerge in some less direct way.

Enter the Higgs mechanism. It assumes the vacuum of empty space contains a Higgs field, like an electric field that never switches off. The otherwise massless W and Z interact with it to acquire energy and, hence, mass. The field consists of Higgs bosons lurking “virtually” in the vacuum.

But why does the Higgs field persist? The standard model assumes the field interacts with itself in a particular way that ensures its energy is lowest when, instead of vanishing, it has some nonzero strength. But nobody knows whether that assumption is right, says Nathaniel Craig, a theorist at UC Santa Barbara. “We know the Higgs field has this background value everywhere in space, but we don’t really know why.”

To probe how the Higgs field interacts with itself, scientists need collisions that produce multiple Higgs bosons. The LHC already detects the occasional Higgs pair and should see more after it is upgraded in 2026–29 to quintuple its collision rate. CERN’s planned FCC-ee would see still more. But researchers would need to spot at least a few events with three or four Higgs particles, which would require a higher energy machine. For that task a 10-TeV muon collider might have an edge over a 100-TeV proton collider.

That’s because colliding protons almost always shred each other through a strong-force interaction, which will rarely produce a Higgs. Muons interact through just the electromagnetic and weak forces, so their collisions are more likely to produce the prized events. A muon collider’s ability to more clearly probe Higgs physics sets it apart, says Donatella Lucchesi, an experimentalist at the University of Padua and Italy’s National Institute of Nuclear Physics. “This is an opportunity that we should not miss, it is super important.”

A muon collider’s sensitivity to weak interactions could also help it stalk other quarry, such as dark matter. Theorists have long thought the stuff might consist of heavy particles that interact only through the weak force, but experimenters have yet to detect such weakly interacting massive particles (WIMPs). A muon collider could produce WIMPs too massive and too weakly interacting to be seen in current searches at the LHC or in sensitive detectors underground, Craig says.

The machine might even put supersymmetry to the acid test. The decades-old concept could explain many things, such as where WIMPs come from, but its main job is to solve a more abstruse problem. Quantum mechanics predicts that known kinds of particles should fleetingly emerge from the vacuum around the Higgs and interact with it to make it hugely massive. That doesn’t happen and physicists don’t know why. Supersymmetry provides an answer. Its “central prediction” is that the Higgs has weakly interacting superpartners of modest mass that counteract that effect, Craig says, and a muon collider would be ideally suited for hunting them.

But can one actually be built? A muon collider would consist of familiar accelerator parts: so-called RF cavities resonating with radio waves to accelerate the particles, and magnets to steer and focus the beams. But it would have to work incredibly quickly because the muon is so short lived. Sitting still, a muon decays into an electron, a neutrino, and an antineutrino in 2.2 microseconds. High-energy muons traveling at near–light-speed endure longer, for milliseconds, because of the time dilation predicted by Einstein’s theory of relativity. But that’s still just a blink of an eye.

The clock would start when physicists generate the muons. They would fire a proton beam into a target to produce particles called charged pions, which, swirling in a magnetic field, would decay into muons. The same technique already generates neutrinos for other experiments. But a muon collider would require a target that could handle a proton beam with several megawatts of power.

The muons would emerge in a cloud centimeters wide. To herd them into a beam a few micrometers across, they would pass through a low-density material such as lithium hydride or liquid hydrogen. Still swirling in a horizontal magnetic field, the muons would ionize atoms in the material, dissipating energy and quelling their buzzing about. An RF cavity would accelerate the muons into the next cooling cell. That could be tricky, because RF cavities typically don’t work well in magnetic fields.

Next, two or more circular accelerators known as synchrotrons would boost the beams of muons and antimuons to their final energy. As the particles in a synchrotron gain energy, the fields generated by the steering magnets need to ramp up in synchrony to keep the particles on a circular path of a fixed radius. At the LHC that process takes 20 minutes. In contrast, to keep replenishing its beams, a muon collider would require synchrotrons that could cycle a blinding 400 times per second.

Finally, the beams would pass into a smaller accelerator called a storage ring, which could be as little as 10 kilometers long—petite compared with the LHC. Within it, one bunch of muons and one bunch of antimuons would circulate at fixed energy in opposite directions, colliding in the hearts of two detectors on opposite sides of the ring. A smaller ring provides an obvious benefit, says Stephen Gourlay, an accelerator physicist at Fermilab. “You get more turns before the muons disappear.”

The machine’s various components would push the frontiers of magnet technology, Gourlay says. “This machine is a magnet builder’s dream—or nightmare.” For example, the magnets in the rapid cycling synchrotrons must change fields by several tesla almost instantly. That process would not only exert enormous mechanical stresses on the magnets, it would also require safely shunting tens of gigawatts of power in and out of them with exquisite efficiency, Gourlay says.

Physicists will have to shield the machinery from the electrons and positrons gushing from the beams as the muons decay. Doing so is especially complicated in the detectors, as physicists must take care not to block the particles they want to observe—those coming from the muon collisions. To deal with the extraneous decay particles, a cone of tungsten would surround the beam pipe on either side of the collision point in each detector. Electrons and positrons striking the cone would convert mostly to low energy photons and neutrons, which are easier to distinguish from the desired signals.

Decaying muons also radiate energetic neutrinos, creating a novel radiation safety challenge. Shooting horizontally from the collider ring, these elusive particles would zip through the earth and emerge dozens of kilometers away. A few would strike atomic nuclei in the soil and change back into muons, which could emerge from the ground as potentially dangerous radiation. The neutrinos can’t be blocked, but digging the collider’s tunnel deeper would allow them to spread more and lower the intensity of muon radiation. Building the collider on movable mounts so the orientations of its sections could be gradually changed would also help, Stratakis says, by limiting the radiation dose at any one place.

Developers’ biggest challenge may be proving ionization cooling works. “The things that have never been done before get the highest priority because there’s the greatest chance of a surprise,” says Mark Palmer, an accelerator physicist at Brookhaven National Laboratory who headed Fermilab’s previous muon collider effort. In 2020, the International Muon Ionization Cooling Experiment at the United Kingdom’s Rutherford Appleton Laboratory showed individual muons spiraled through a cooling cell as predicted. But it did not show that the process could actually cool a muon beam, Palmer says.

Facing so many unknowns, U.S. physicists are not clamoring to start planning such a project now, as some press reports have suggested. Instead, they simply want support for basic R&D, says Patrick Meade, a theorist at Stony Brook University. “In the U.S., the zeroth-order thing is getting permission to do any research in this direction.”

Physicists envision 7 years of research at an annual cost of a few million dollars to determine what kind of demonstration project—perhaps a prototype cooling channel—would best prove a muon collider is feasible, Stratakis says. They would then spend a decade and $100 million or more building and running a demonstrator, with the aim of deciding by 2040 whether to go on to build a collider, he says.

Others are musing about muons, as well. In 2021, CERN started the International Muon Collider Collaboration (IMCC), which U.S. researchers hope to join. As for who might host a muon collider, “the goal is to hold off on that decision until the moment when funding agencies put money on the table,” says Daniel Schulte, a CERN accelerator physicist and leader of the 200-member IMCC.

Will supporters get that far? “In Europe the muon collider is something like a plan B,” Schulte says. And CERN could soon opt for the first step in plan A: the FCC-ee. In an online press conference in February, CERN Director-General Fabiola Gianotti said the lab hoped to decide by 2028 whether to build that machine, noting a muon collider is “not on the same timescale.”

Nevertheless, the idea of a muon collider continues to fascinate some physicists, especially younger researchers. Lucchesi says she can more readily find graduate students to work on the concept than to join ongoing experiments at the LHC. What’s compelling isn’t “something that you do for the fourth or fifth time just to improve the errors,” she says. “What is attractive is something new.”

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