The best-laid plans of MICE and muons did not go awry: Physicists at the International Muon Ionization Cooling Experiment, or MICE, collaboration have achieved their yearslong goal of quickly sapping energy from muons. Muons are fundamental particles that, like electrons, possess a negative charge, but they are more than 200 times as heavy. The results, reported in Nature on February 5, are the first demonstration of ionization cooling, a technique which could allow researchers to control muons for future collider applications. The achievement “is epochal,” says Vladimir Shiltsev, a physicist at Fermi National Accelerator Laboratory in Batavia, Ill., who was not involved with the new work. “It basically opens a new venue for research.”
Currently, particle physicists are at a crossroads. Existing particle colliders such as the Large Hadron Collider (LHC) at CERN near Geneva, have not generated the leads to new physics that they were expected to produce. Although the 2012 discovery of the Higgs boson at the LHC confirmed decades-old predictions about how fundamental particles get their mass, it left physicists the victims of their own success. What comes next? To reach higher, unexplored energies in which new phenomena could manifest, future particle colliders using conventional technology will have to get bigger and bigger—and much more expensive. Last year a proposal for CERN’s Future Circular Collider—a 100-kilometer ring that would be nearly four times bigger than the LHC—was criticized for its estimated cost of more than $20 billion.
“If something is prohibitively expensive, so that you could never hope to build it, then that is not how we’re going to get to where we want to be with exploring the high-energy frontier. It’s got to be affordable,” says Robert Ryne, a physicist at Lawrence Berkeley National Laboratory and author of a commentary accompanying the research paper reporting the MICE results.
Muon colliders, which promise better performance and have a smaller footprint than proton colliders, are particularly alluring. Unlike protons—which are composed of quarks and gluons and thus create messy, inefficient collisions—muons are fundamental particles, meaning they could theoretically produce clean, high-energy collisions, which could be used to study the Higgs boson or neutrinos.
Who Ordered That?
When they were discovered in 1936, muons were an unexpected addition to the then small set of known particles. Physicist Isidor Isaac Rabi is famously said to have queried of the muon, “Who ordered that?” Had he known the plight his peers would find themselves in today, Rabi might have answered, “It was 21st-century particle physicists.”
The technique for making muons in the lab has remained roughly the same for more than half a century: smash some protons into a material and wait for the resulting particles to decay into muons. This beam of muons is anything but dense, however—even after magnets funnel it into a small area. And density is what efficient experiments demand. Colliding diffuse beams would be like trying to smash two clouds into each other, Shiltsev says. To focus a muon beam, physicists need to remove the muons’ jittery up-and-down and side-to-side energy by cooling them—that is, by slowing them down.
“I liken it to the spray of particles from a shotgun,” Ryne says. “Somehow that spray needs to be turned into something that more resembles a laser beam.”
Removing energy from particles is usually very straightforward, except that muons are short-lived—on average, they decay into other particles after only two millionths of a second. None of physicists’ preexisting cooling techniques had worked fast enough.
When muons are fired through a material, they dislodge electrons, ionizing atoms. This ionization process drains kinetic energy from muons, rendering them slightly less frenetic. In their proof-of-concept experiment, MICE researchers sent muons through layers of two materials, liquid hydrogen and lithium hydride, which stripped the frenzied particles of about 10 percent of their energy.
“What we wanted to show was that we’d increased the density of the beam,” says Chris Rogers, head of the MICE collaboration and a physicist at the Science and Technology Facilities Council’s Rutherford Appleton Laboratory (RAL) in England. The technique worked: reducing the muons’ energies let the team concentrate them into a smaller area, creating a denser beam.
Ionization cooling remains in its nascent stages. To achieve the density and focus of collider-quality beams, physicists would have to sap 10,000 times more energy from muons than what MICE has demonstrated. Furthermore, MICE did not take the necessary next step of accelerating the muons. (In theory, once those particles have lost most of their energy in other directions, physicists can accelerate them forward into a nice, straight beam.)
Still, MICE represents a shift for muon physics. Critically, experimental results of ionization cooling closely matched theoretical simulations. This finding suggests that researchers are on the right track and that they understand the physics well enough to scale up the process in future experiments.
In the aftermath of completing its successful run, MICE has been decommissioned. Workers have already dismantled the school bus–sized metal apparatus at RAL, leaving an empty space in the hall.
“The next step, really, is to build a new experiment,” Rogers says. Over the next few months and years, the European, Japanese and American communities of particle physicists will meet to prioritize what research to fund and which, if any, new colliders to build. Though Scottish poet Robert Burns wrote that “foresight may be vain,” Rogers and his colleagues at MICE are cautiously optimistic that the new results will help along their best-laid plans for a muon collider.