There are several possible accelerators that could follow the Large Hadron Collider. Credit: Denis Balibouse/Reuters/Alamy
Momentum is building to build a particle accelerator in the United States that smashes muons, electrons’ heavier cousins. The collider will follow the world’s next big accelerator, which is yet to be built, and physicists hope it will discover new elementary particles. Although the short-lived nature of muons makes such a collider technically difficult to build, its main advantage is that it would be smaller and potentially cheaper than competing collider designs. The vision remains distant, 2040 at the earliest, but research and development must begin now, its advocates say.
It’s a “bold and promising vision,” says Carrie Di Petrillo, a particle physicist at Fermilab in Batavia, Illinois. Physicists around the world are considering the feasibility of such a collider, but hosting it in the United States “would be a game-changer for my generation of physicists,” she says.
Support among physicists for a muon collider emerged during Snowmass, a major planning effort by the US particle physics community that lays out its scientific vision once a decade. The exercise concluded with a ten-day workshop held in Seattle, Washington, from July 17 to 26. Organizers will now distill the views of thousands of scientists into a report that describes the major questions in the field and what is needed to solve them, which will ultimately affect US federal funding. Almost a third of the white papers physicists contributed to the “energy limit” section of the exercise were about muon colliders, and excited supporters of the meeting sold T-shirts supporting the plans.
Higgs factory
The muon machine will follow the construction of the “Higgs factory”, a major collider that a collaboration in Europe, China and Japan is already racing to build to study the elementary particle known as the Higgs boson in precise detail (see Future Colliders ). The Large Hadron Collider (LHC) at CERN, Europe’s particle physics laboratory near Geneva, Switzerland, detected the Higgs, which binds to the field that gives the particle mass, in 2012. But it did not detect the other new particles, which are very physicists expected, and some now believe it may be beyond the machine’s reach.
A Higgs factory will combine electrons with their antimatter counterparts, positrons, in collisions that are cleaner than proton-proton smashing at the LHC, enabling precise studies. In contrast, the muon collider would be a “discovery” machine, trying to find new particles through collisions of unprecedented energy and clarify the reason for the discrepancies found in the results of previous experiments.
Muons can be accelerated to higher energies than electrons because they lose less energy as synchrotron radiation. And they have a big advantage over proton collisions. These involve collisions between the particles’ constituent quarks, each of which carries only a small fraction of the total collision energy. Since muons are fundamental particles, each collision involves all of the particle’s energy. This means that a 10-trillion-electron-volt (TeV) muon collider about 10 kilometers long can produce particles that have as much energy as those produced by the 100-TeV, 90-kilometer proton machine that CERN plans to build in the second half a century.
The concept of a muon collider has been around since the 1960s. But only in recent years have viable technologies been developed that may be able to deal with the muon’s quirks, which include the fact that it decays easily, creating annoying background noise, and is unlikely to be inclined to form an intense beam. The excitement among American physicists now is because there is plenty of time to develop and build the machine to succeed the Higgs factory, and plenty of people to work on it, says Priscilla Cushman, a physicist at the University of Minnesota in Minneapolis.
Whether it will be built in the United States depends on funding and politics as well as technical feasibility, says Joel Butler, a particle physicist at Fermilab and chairman of the Snowmass steering group. CERN also organized an international collaboration to investigate the feasibility of a muon collider. For all the collider options on the table, American physicists need to do enough research and development “so that when choices have to be made, they can be made in a good way,” he says.
Enthusiasm for the muon collider has been matched by a growing focus on cost and sustainability, said Caterina Vernieri, a particle physicist at Stanford University in California who is part of a group proposing a cheaper Higgs factory design known as the Cool Copper Collider. as part of the Snowmass process.
Dark matter
Long-range colliders were only a small part of Snowmass’ agenda. Among their near-term plans, the physicists highlighted their commitment to a high-intensity upgrade of the LHC from 2026, which will produce more than ten times the data created so far. They also reiterated their desire to press ahead with the phased construction of a 1,300-kilometer US-based experiment called DUNE, which is designed to probe the nature of elusive particles called neutrinos. Some argued for a green light for CMB-S4, a next-generation survey of the cosmic microwave background.
A cross-disciplinary appeal was to ensure that a wide range of facilities exists to search for dark matter. The failure to find a theoretically predicted type of dark matter known as weakly interacting massive particles (WIMPs) over the past ten years, either in the massive detectors designed to look for them or at the LHC, means that dark matter must be even more exotic than was thought.
Physicists want to look for much lighter candidates for dark matter and reformulate their search to consider that it may exist as a whole family of particles rather than just one, says Suchita Kulkarni, a dark matter physicist at the University of Graz in Austria, who attended the meeting in Snowmass. Finding it will take a few large, sensitive experiments — like those already looking for WIMPs — and many other small, experimental ones, says Stanford University physicist Micah Bouk.
Funding Recommendations
The two-year Snowmass process, to which physicists from around the world submitted 521 papers, was “exhausting but exciting,” says Cushman, who is a member of the steering group.
The key time will come next year, when the U.S. federal particle physics prioritization panel, known as P5, will use the Snowmass findings — and budget considerations — to make investment recommendations to funders at the Department of Energy and the National Science Foundation for the next ten years.
Physicists are now working on how best to communicate with funders and the public, Kulkarni says. Over the past decade, they haven’t found what many expected — a deviation from the Standard Model, their best description of particle physics, which they know is incomplete. “The community is making an effort to create a coherent and honest narrative,” says Kulkarni. “We’re doing the best we can and we’re going to learn something from it. But breakthroughs are fickle lovers, and you never know when you’ll get them.
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