‘This is our Muon Shot’

In the spring of 2022, Karri DiPetrillo was gearing up for the final step of the Snowmass particle physics community planning process. It would be a once-a-decade meeting at which physicists in the United States would discuss the future of the field. For DiPetrillo, who earned her PhD from Harvard in 2019, such a discussion could define the direction of the majority of her professional life. She and other early-career scientists wanted to be sure their voices were heard.

DiPetrillo, now an assistant professor at the University of Chicago and a member of the ATLAS collaboration, went into Snowmass excited about the prospect of a new kind of particle accelerator, a muon collider. A muon collider would smash together not the standard electrons or protons, but heavy, short-lived particles called muons. Scientists have suggested that a muon collider could fit within the footprint of the US Department of Energy’s Fermi National Accelerator Laboratory.

Building a muon collider isn’t a new idea, but it is an ambitious one—scientists have yet to prove that it can even be done.

Still, no progress can be made without research and development, and physicists at a variety of career stages approached the Snowmass process with a strong desire to try. DiPetrillo and others saw the benefit in taking on the challenges that would come with the new project. They also saw the potential to generate a career’s worth of interesting science.

That’s why—in addition to contributing to two years of discussions, presentations and white papers on the topic leading up to the final meeting—DiPetrillo and a few colleagues independently made and distributed dozens of custom t-shirts in support of the muon collider.

A visible representation of individuals’ support for the muon collider, the shirts appeared throughout the auditorium crowd, the backs dotted with the bison who graze Fermilab’s campus and emblazoned with the words “The Collider We Need.”

“The point was to start the conversation,” DiPetrillo says. “And I think we did that.”

In December, the Particle Physics Project Prioritization Panel, called P5, released its recommendations for the future of the field, based on the input from the Snowmass process. Among the top priorities identified was research and development toward future accelerator technology, with a specific mention of the concept of building a muon collider in the United States.  

The report acknowledged that the technology would need to be proven before the US community could commit to such a project, but the members of P5 unanimously agreed it was a dream worth pursuing. “At the end of the path is an unparalleled global facility on US soil,” the report reads. “This is our Muon Shot.” 

The best of both worlds

Collider experiments crash beams of particles, pumped full of energy, into one another or into a target. In the crash, all of that energy can briefly convert into new particles, including ones that are short-lived or otherwise difficult to study. This is how scientists have discovered many previously unknown particles, including many of the components of the Standard Model, building the bedrock of modern physics.

That work happened largely at colliders that primarily accelerated protons or electrons. And the Large Hadron Collider at CERN, which usually accelerates protons, continues to reveal new facets of the science.

But such accelerators do have limits. As beams of particles are steered with magnets and electric fields around a circular collider, their energy can radiate away. Electrons are especially small, so in a large, modern accelerator, so much of the energy is lost that it’s difficult to accelerate them enough to produce interesting collisions.

Protons, which are more massive, don’t have this problem. But unlike electrons, protons are composite particles: They are made of quarks and gluons. Collisions between beams of more complex particles aren’t as clean, a fact that can mask signatures of new physics. And because the particles colliding are a proton’s constituent parts, only a fraction of the beam energy goes into the collision.  

Muons, on the other hand, are both fundamental—not made up of any less massive particles—and heavy—weighing about 200 times as much as an electron. A muon is often called the electron’s heavier “cousin”: The two particles are nearly the same, aside from their difference in mass. With fundamental muons smashing together, a 10-TeV muon collider is equivalent in terms of physics to a 100-TeV proton-proton machine.

“It’s really special,” DiPetrillo says. “It completely breaks this dichotomy we have between colliding electrons and colliding protons.”

That means a muon collider could, in theory, generate clean, high-energy collisions capable of producing particles that have never been seen, says Tulika Bose, a professor of physics at the University of Wisconsin-Madison and a P5 member. “The data from the LHC tells us that, if there is new physics—and we think there is, because we don’t know all the answers to all the questions—it will come at a higher energy scale, which a muon collider will provide.”

To cool a muon beam

Some particles are more stable than others. An electron, for example, lasts forever—it can travel across the universe and back without decaying. A muon, though, lasts only about 2.2 microseconds before it falls apart. That makes it extremely difficult to maintain beams of muons long enough to generate collisions.

What’s more, the most promising method for generating muons creates not tight, refined beams, but spread-out particle clouds the size of basketballs that can pass right through one another without interacting. To design a muon collider, physicists must find a way to condense the beams and bring them into collision, all before the muons decay.

In 2010, the US Department of Energy launched the US Muon Accelerator Program, or MAP, to study the feasibility of building a muon collider that could operate at the TeV scale. Shortly after its launch, though, physicists made the first detection of the Higgs boson. So when scientists met for the last Snowmass meeting in 2013, they were focused on machines that could tell them more about this long-sought particle. MAP pivoted to developing a low-energy muon-collider design, but the community opted to pursue an electron-positron collider instead, says Mark Palmer, a physicist at Brookhaven National Laboratory and former director of MAP. 

While a low-energy muon collider, like the one MAP envisioned, could produce more than 10,000 Higgs bosons a year, the LHC can generate millions, albeit in a messier environment. If there were signs the Higgs was a composite particle, a muon collider would be an effective way to probe it. But, Palmer says, “in 2013, we had no evidence for that.” 

With the LHC just beginning its decades of operation at CERN, and its predecessor, the Tevatron particle collider, shut down at Fermilab in 2011, US physicists pivoted their local program to specialize in neutrinos. In the 2014 P5 report, the committee recommended continuing major contributions to the LHC and constructing what is now called the Deep Underground Neutrino Experiment.

MAP was abandoned. But before it was scrapped, it had made progress toward cooling a muon beam with a process called ionization. Building on that progress, the International Muon Ionization Cooling Experiment, or MICE, at the Rutherford Appleton Laboratory in the United Kingdom demonstrated for the first time that the process worked.  

A vision for Fermilab

Kevin Pedro, an associate scientist at Fermilab, is searching for hypothetical models of dark matter made up of composite particles. So far, no such particles have shown up at the LHC.

It could be that the models are incorrect—but it could also be that the particles connecting the dark world to our own are extremely heavy, beyond the reach of even the world’s current most powerful accelerator. Pedro is hopeful that a muon collider could extend that reach. “To me it’s worth trying, even if there’s some stumbling blocks,” Pedro says. “We have to try something big.”

An added benefit for Pedro’s workplace: Because muons decay into neutrinos, developing a muon collider could also further neutrino studies. That would make building a muon collider at Fermilab a nearly perfect marriage of the lab’s past as an energy-frontier leader—through the Tevatron—and its present as a neutrino leader—through experiments like DUNE, Bose says. “If it all works out, it could be really a long-term future for Fermilab.”

Despite this, and even though the support of the P5 report is encouraging, the physics community will need to conduct substantial research to inform a path toward the siting, design and construction of a muon collider.

The MICE result was promising, but physicists still need to demonstrate cooling at a larger scale. They need to develop magnets that can kick into action much faster than current technology allows. And they need to find a way to quickly disburse the enormous amount of energy the collider will generate without overheating any of its elements. 

The muon collider team will need to show success in these areas by the end of the decade, Palmer says. If they could do that, they could complete a large-scale demonstration in the first years of the next decade. And if they could do that, they could build a muon collider in the 2040s or 2050s.

That’s a lot of ifs. But Palmer is optimistic. The team has yet to run into any showstoppers, he says. “We may still discover something that we don’t know how to get past, but we haven’t found it yet.”