Particle Physicists Dream of a Muon Collider

Jul 13, 2023

After years spent languishing in obscurity, proposals for a muon collider are regaining momentum among particle physicists

Particle physicists are unlikely evangelists, but in papers, at conferences and with T-shirts, stickers and memes, many of them are spreading the good word of a muon collider—a next-generation machine that would smash together muons, the massive cousins of electrons. In a 2021 manifesto, “The Muon Smasher’s Guide,” the particle partisans laid out their case. “We build colliders not to confirm what we already know, but to explore what we do not,” they wrote. “The muons are calling, and we must go.”

For proponents, the draw of a muon collider is its potential to combine the strengths of two existing types of colliders. These massive machines generally collide either protons or electrons in underground rings. By recording the aftermath of these smashups, physicists can gather information about the lay of the subatomic land. Each method has its pros and cons. Heavy protons—each of which is actually a teeming bundle of smaller, more fundamental particles—create messy, debris-clogged, high-energy collisions. Lightweight electrons collide cleanly but at lower energies.

Today’s premier facility, the Large Hadron Collider (LHC), smashes protons to probe the limits of the Standard Model, the theory that serves as a map of the most fundamental territory in the universe. As a map, the Standard Model has been successful to a fault. It precisely depicts the known landscape of elementary particles and the forces that connect them—so well that any deviation from the theory draws headlines. But like all maps, the Standard Model has borders: it does not include gravity and presently lacks answers to mysteries such as the identity of dark matter.

Physicists have never successfully collided muons, primarily because the particles live for a scant 2.2 microseconds before decaying. If muons could be wrangled, they’d create collisions that are both clean and high-energy—ideal for exploring beyond the Standard Model’s borders. In muons, “nature provided us a gift; we should take advantage of it,” argues Patrick Meade, a theorist at Stony Brook University.

The fate of any future collider rests with the alliteratively named Particle Physics Project Prioritization Panel (P5), a high-powered committee that convenes each decade to set research agendas and recommend funding for key projects. The P5 report is set to come out this fall, and many physicists hope it includes a strong push for a muon collider.

There are no guarantees that any future collider would find new particles, but advocates are enthusiastic about the discovery potential that muons hold. A future with a real live muon collider remains far-off. Even on the fastest, most optimistic time line, a muon collider wouldn’t turn on for at least two decades. But physicists are already dreaming about where they can explore with muons. “We have the opportunity to do something that’s unprecedented,” says Cari Cesarotti, a theorist at the Massachusetts Institute of Technology. “The roadblocks that were there 10 years ago are dissolving. Now is the time! So to me, it’s just like, why would you not want to do it?”

The trouble with muons is that they die. During their short lifetime, they need to be cooled, focused and accelerated to nearly the speed of light. The most viable approach begins with passing the muons through a medium such as liquid hydrogen, which saps their energy. Then powerful magnets can focus the muons and accelerate them into a loop where they collide before they decay. Variations on this plan have existed for decades—one design was dubbed “the Guggenheim” because of its resemblance to the museum’s spiraling concourse.

Curious about how feasible any of this was, in 2011 the Department of Energy founded the Muon Accelerator Program (MAP), a small research and development effort investigating the feasibility of colliding muons. A team of accelerator physicists got to work creating computer models of colliders to see which designs might work best. But just as the effort got off the ground, two discoveries seemingly spelled any muon collider’s demise.

When muons decay, they produce neutrinos—insubstantial particles that barely interact with matter. This process churns out neutrinos so profligately that “people were always intrigued with the possibility of using muons as a neutrino source,” says André de Gouvêa, a neutrino theorist at Northwestern University. For years it seemed like building a muon collider might be the only way to answer whether neutrinos behave differently than antineutrinos. But in 2012 results from the Daya Bay Reactor Neutrino Experiment, a China-based experiment that detected neutrinos from nuclear reactors, showed that the question wouldn’t be that hard to answer. Consequently, instead of a muon collider, neutrino physicists chose to go forward with the Deep Underground Neutrino Experiment, which is currently under construction in South Dakota.

The knockout blow for muon colliders was, ironically, the discovery of the Higgs boson, the particle responsible for giving other elementary particles mass. Seemingly at the center of myriad mysteries in the Standard Model, the Higgs compels many physicists to study it in as much detail as possible by producing the particle in bulk—and they’ve made plans to do this by building so-called Higgs factories. But for a muon collider, trying to smash muons together just by producing Higgs bosons is a worst-case scenario—like using a helicopter to get groceries. “If you look at the different energy scales of potential muon colliders, the Higgs factory is one of the hardest ones to actually build,” admits Mark Palmer, an accelerator physicist and former director of MAP.

So rather than risk trying to build a muon collider, the 2014 P5 report recommended an upgrade that would effectively turn the Large Hadron Collider into a Higgs factory. MAP, deemed inessential, was cut, and the program dissolved within a few years. “We had a great product, but we didn’t have a good customer,” says Diktys Stratakis, an accelerator physicist at the Fermi National Accelerator Laboratory (Fermilab), who was part of MAP.

The story might have ended there if it wasn’t for a group of Italian physicists who wanted to study a new approach for generating muons via positrons (antiparticles of electrons) without a tricky cooling process. But the Italians were starting from scratch. “We didn’t have any software. I was desperate,” says Donatella Lucchesi, a particle physicist at the National Institute of Physics in Italy. Lucchesi flew to Fermilab, which is just outside of Chicago, and pleaded with MAP physicists to scrounge up the old code, which was hiding on a dusty, forgotten computer. (The other half was discovered later, and Lucchesi had to recruit a friend to bring it back to Italy on a USB drive.)

Though the novel positron approach turned out not to be viable, across the Atlantic, U.S. researchers had heard about the Italian effort and begun to look into things themselves.

A decade ago many U.S.-based physicists had wholly discounted the prospect of a muon collider. “I just concluded that this was some fantasy,” says Nathaniel Craig, a theorist at the University of California, Santa Barbara. The technical challenges seemed too great, and it wasn’t clear why a muon collider’s capabilities might be needed.

But by 2020, as U.S. physicists were beginning to crowdsource ideas for the future of their field, the physics landscape had changed. Popular supersymmetric (SUSY) theories that were add-ons to the Standard Model had proposed a bevy of new particle counterparts waiting to be explored—the photon would have a “photino” doppelgänger, and so on. In principle, these counterparts could explain why the Higgs mass is low while also serving as excellent candidates for dark matter particles. The trouble is that ever since discovering the Higgs boson, the LHC has found no new SUSY-style particles in searches that have scaled up to about 1,000 giga-electron-volts (GeV).

This lack of new physics—sometimes dubbed a “crisis”—has compelled many physicists to seek other options and, in particular, to yearn for collisions at far higher energies. “What you really want is a sort of a laboratory for electroweak physics,” Craig says. At extremely high energies, the electromagnetic force, which controls the behavior of charged particles such as electrons, and the weak force, which governs processes such as fission decays, are unified into one “electroweak” force.

Observing the existence of the Higgs boson was a triumph. But as Craig and others argue, that discovery was only the “herald” of electroweak physics. At higher energies, and with precision measurements, physicists hope to ask more and deeper questions of the Higgs—how it couples to other particles, why its mass is so small and what its role in the early universe was. It’s an esoteric search with very real implications—if just one parameter of the Higgs were positive instead of negative, for instance, atoms would have never formed because massless electrons would never stay in their orbit. “The fact that a minus sign determines the fact that you and I are having this conversation is the weirdest thing in nature,” Meade says.

Refocused by SUSY’s lack of success, physicists scrutinized the competing collider candidates and found that only a muon collider would marry the energy and precision they wanted within a single machine. What’s more, it seemed like a muon collider was no longer a fantasy, thanks to the work of MAP and the Italian team. In early 2020 the first results from the long-delayed Muon Ionization Cooling Experiment proved that muon cooling could be done. “We had a chance to look at all the progress that has been made, and we concluded that, ‘oh my god, maybe it’s not as far off as we originally thought,’” says Sergo Jindariani, a detector physicist at Fermilab.

During the pandemic, Jindariani and his colleagues met over Zoom and brainstormed ways to solve remaining technical challenges, such as the dreaded problem of “beam-induced background.” At high energies, hurtling muons create a sort of messy cloud of roiling energy right before a collision, making it impossible to see anything. But with a new design using tungsten nozzles and an LHC-developed timing method, researchers now believe they’ll be able to filter out the mess to clearly see muons colliding.

Even though a muon collider is becoming more feasible, many wary physicists still prefer other collider options. Some are holding out hope for the Japan-based International Linear Collider (ILC), a Higgs factory that would collide electrons and positrons at low energies. Yet although plans for the ILC are “shovel-ready,” it remains in limbo—up to the whims of Japan’s government. Uncertainty creates anxiety, and privately, some physicists say the ILC is dead.

Scientists at CERN, the European laboratory for particle physics near Geneva, which built the LHC and is responsible for operating it, were intrigued by the prospect of a muon collider but not enough to displace other plans. Then and now, CERN’s next big thing has been the Future Circular Collider (FCC), which, if built, would be a colossal 90 kilometers in circumference. “A muon collider is a ‘Plan B,’” says Daniel Schulte, an accelerator physicist at CERN and head of the International Muon Collider Collaboration.

The intention is for the FCC to begin as a Higgs factory that will collide electrons and positrons. But the prospects of all Higgs factories have been hurt by hardware and software upgrades to the LHC that have increased its ability to study the particle. That was “some of the territory that we thought was unquestionably the grounds of a Higgs factory,” Craig says. “Progress has been made by the LHC.”

In the quest to reach higher energies, eventually CERN would like to upgrade the FCC to collide protons at 100,000 GeV—seven times higher than the LHC’s current capability. But the time lines are daunting. Construction on the FCC has yet to begin, and the facility’s debut is projected for no earlier than 2048. Proton collisions at the FCC would not come online until circa 2075.

“That scares the crap out of a lot of young people,” Meade says. “We’re basically saying these questions are just out of our horizon and that no one now alive is going to answer them.” For early-career researchers, the muon collider holds an additional appeal: in part because of its smaller size, it could come online around 2045—offering an epochal energy upgrade decades before the FCC would collide its first protons.

“I think that was the turning point for me,” explains Karri DiPetrillo, an experimental physicist at the University of Chicago. She and other young physicists have been a driving force behind the muon collider’s surging popularity by giving talks and trying to persuade more hesitant senior colleagues. For one of her talks, DiPetrillo includes a morbidly humorous time line: The year 2060 is marked with “Karri retires?” And at 2070—years before the FCC’s proton start—a mordant label reads, “Karri dies???”

If anywhere in the U.S. can be called a graveyard for particle physics, it is Waxahachie, Tex. Aside from some nondescript buildings, the arid landscape’s most notable feature is a series of unfinished tunnels that amount to a $2-billion hole in the ground. These are the ignominious remains of the Superconducting Super Collider (SSC), once seen as the shining pinnacle of the nation’s “big science” plans.

Had it been completed, the SSC’s ring would have spanned 87 km around and smashed protons at 40,000 GeV. In its explorations of energies that are inaccessible today, it would have easily found the Higgs (and who knows what else) possibly more than a decade before the LHC.

No single reason explains why the SSC was killed. Budget mismanagement, opposition from other physicists, competition from the International Space Station, The end of cold war–era carte blanche for high-energy physics and an unfortunate incident in which then president George H. W. Bush vomited on Japan’s prime minister all contributed to the SSC’s dismal fate.

For the past 30 years, the megaproject’s cancellation has been a grim reminder for particle physicists to temper their expectations. The dream to build a muon collider is a return to ambition. As noteworthy as anything else about the muon collider is the enthusiasm it inspires in its advocates, many of whom proudly sport muon-themed apparel. At a talk in Minneapolis this April, Nima Arkani-Hamed, a theorist at the Institute for Advanced Study in Princeton, N.J., summed up his case for a muon collider: “It’s just f—ing exciting!”

In spite of unknown rewards and certain risks, many particle physicists are flocking to the muonic fold. “If we don’t have a challenge,” Jindariani says, “the brightest people will go elsewhere.”

In other words: we choose to collide muons not because they are easy but because they are hard.

Daniel Garisto is a freelance science journalist covering advances in physics and other natural sciences. He is based in New York. Credit: Nick Higgins

Daniel Garisto

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