A “muon shot” aims to study the basic forces of the cosmos. But meager federal budgets could limit its ambitions.
Friday, Dec. 8: This story was updated to include a vote that occurred after it was first published.
Scientists approved a blueprint for the next two decades of particle physics in the U.S. on Friday. It aims to restore American primacy in high energy particle physics.
The new strategy calls on physicists to begin laying the groundwork for a revolutionary particle collider that could be built on American soil. The machine would collide tiny, point-like muons, which resemble electrons but are more massive. Muons provide more bang for the buck than the protons used in the Large Hadron Collider at CERN, and would push the search for new forces and particles deeper than ever into the unknown.
The siting of such a project, perhaps at the Fermi National Accelerator Laboratory in Illinois, would restore American particle physics to a position of pre-eminence that was ceded to Europe in 1993 when Congress canceled the giant Superconducting Super Collider. But it will take at least 10 years to demonstrate that the muon collider could work and how much it would cost.
“This is our muon shot,” the committee, charged with outlining a vision for the next decade of American particle physics, said in a report titled “Exploring the Quantum Universe: Pathways to Innovation and Discovery in Particle Physics.” It was presented and discussed at a meeting in Washington, D.C., on Thursday and Friday, and will be discussed further at Fermilab next week.
The report also highlighted a need to invest in next-generation experiments probing the nature of subatomic particles called neutrinos; the cosmic microwave background, relic radiation from the Big Bang; and dark matter, the gravitational glue holding galaxies together. The panel also recommended participating in a future facility in either Europe or Japan, dedicated to studying the Higgs boson, the discovery of which in 2012 was key for understanding how other particles get their mass.
“The size of the universe we now see as 14 billion light-years across was actually smaller than the size of a nucleus” early in cosmic time, said Hitoshi Murayama, a physicist at the University of California, Berkeley, who led the committee. “So our field is actually not just looking for the fundamental constituents, but getting a bigger picture of how the universe works as whole.”
The committee, formally known as the Particle Physics Project Prioritization Panel, or P5, was tasked by the U.S. Department of Energy and the National Science Foundation to lay out a road map for the future of the field. The three-year process began by soliciting input from the particle physics community at large, and the final report will serve as a recommendation for what national agencies should prioritize over the next decade.
The last P5 report, “Building for Discovery: Strategic Plan for U.S. Particle Physics in the Global Context,” was published in 2014 on the heels of the Higgs boson discovery. That was a tremendous success for the Standard Model, a suite of quantum equations that explains everything that scientists know and test in the laboratory about the forces and particles in nature, and which has reaped numerous Nobel Prizes for its originators.
But the Standard Model has nothing to say about gravity and thus about black holes or the “dark energy” that is pushing the universe apart. Nor does it explain dark matter, the invisible matter swaddling galaxies. Nor, for that matter, does it explain the Higgs.
In the last decade, physicists have made little progress on these fronts. They have failed to identify dark matter, and some of their most popular hypotheses, particularly a notion called supersymmetry, could be on the verge of being ruled out, at least as an explanation for dark matter.
The process of building a report for the next decade included the Seattle Snowmass Summer Study in 2022, for which physicists submitted hundreds of papers proposing future initiatives in the field. A summary of the study was compiled into a 700-page book. “I characterize this really as democracy at work,” Dr. Murayama said, referring to it as a “bottom-up” process.
Sally Seidel, a physicist at the University of New Mexico and chair of the Department of Energy’s High Energy Physics Advisory Panel, described the process as “a remarkable display of curiosity” that brought together thousands of researchers. “I cannot recall a more exciting time to explore particle physics,” she wrote in an email.
Recommendations by the P5 committee took into account two budget scenarios given to them by the U.S. Department of Energy. In one “base line” case the department’s budget is expected to rise by 3 percent a year, basically keeping up with inflation. In this scenario, the committee emphasized pursuing major projects like a system of telescopes in Chile and Antarctica to study the cosmic microwave background, the realization of an offshore Higgs factory and a scaled-up version of IceCube, an observatory frozen in the ice of the South Pole that captures neutrinos from exotic sources in the universe.
With this budget scenario, there would also be room to support the vision for a particle collider based in the United States.
Particle colliders like the CERN machine get their oomph from Einstein’s revelation that energy and mass are interchangeable. The more energy released in a collision, the more massive the particles that can be made. Because protons are messy bags of smaller particles known as quarks and gluons, smashing them together releases only a fraction of the proton’s total energy. Muons, on the other hand, are elementary; with no internal constituents (as far as scientists know), their collisions harness more energetic results.
A muon collider is one of three options being considered as the successor to CERN’s Large Hadron Collider, which is currently the largest collider in the world and is expected to dominate particle physics for the next decade. China and CERN have each explored building a new collider 60 miles or so in circumference, which would reach collision energies of 100 trillion electron volts compared with the Large Hadron Collider’s 14 trillion, opening up vistas of energy and time.
Another possibility, which in principle could be performed on a table top rather than in miles of underground tunnels, is called wake field acceleration, in which the particle is propelled like a surfer on waves of highly ionized gas, a plasma.
Toyoko Orimoto, a physicist at Northeastern University, found these recommendations ambitious and exciting. “The next 10 years are going to be a very thrilling time for particle physics,” she said.
The report also considered a grimmer budget scenario consisting of only a 2 percent yearly increase in funds, which would amount to an effective cut in support. In that case, the committee said, the United States would have to abandon hopes of hosting a next-generation dark-matter experiment in a new underground laboratory in South Dakota and scratch planned upgrades to the already-expensive, ongoing construction of the Deep Underground Neutrino Experiment, or DUNE, further diminishing the nation’s leadership in those areas.
“The U.S. will have to cede leadership in certain areas in particle physics,” said Karsten Heeger, a physicist at Yale University who is the P5 deputy chair. “That would be an impact that would be felt in the field, and beyond.”
Failing all of that, the report urges the federal government to stay the course on projects to which it is already committed, including cranking up the luminosity, or collision rates, of the Large Hadron Collider for deeper studies of the Higgs and other rare phenomena; continuing construction of the Vera C. Rubin Observatory, a telescope in Chile designed to create time-lapse movies of the cosmos; and a limited version of DUNE.
“They did what they had to do,” Lisa Randall, a physicist at Harvard who was not part of the P5 committee, said, producing a hopeful vision for the future while preparing to weather what could be fraught budgets right now.
Michael Turner, a retired cosmologist from the University of Chicago, and Maria Spiropulu of the California Institute of Technology, who are leading a related study for the National Academy of Sciences, called it “a bold plan” in a joint emailed statement.
Because the lifetimes of these projects span decades, the committee emphasized support for early-career scientists who will eventually take over the projects. “They are the future,” Dr. Murayama said.
Up-and-coming physicists are excited about the endorsement. “As someone who still has the better part of her career in front of her, this is the kind of field I want to be a part of,” said Tova Holmes, a physicist at the University of Tennessee who has been working on muon collider designs. “One with big ambitions that tries new things, develops new technologies and believes in its own potential.”
The committee will pivot its focus to gaining support for the plan, both within and outside of the physics community. In particular, Dr. Murayama hoped it would grab the attention of staff members who communicate with members of Congress about how to vote on the department’s budget.
“Basic research is a tough sell,” Dr. Murayama said. “It’s not an immediate benefit to society.” But the payoff is worth it, he added: Particle physics has led to revolutions in medical applications, materials science, and even the creation of iPhones and the World Wide Web.
But according to Dr. Murayama, the benefits transcend the impact the field has on society. “Particle physics is really at the heart of what we are, who we are,” he said, adding that all of us, physicist or not, “would like to understand why we exist, where we came from and where we’re going.”
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