US Particle Physicists Look to Future at ‘Snowmass’ Meeting

This article is republished with minor changes from the October 2022 issue of Physics Today.

Gordon Watts was nervous when U.S. particle physicists began the most recent grassroots exercise in formulating how to further their science. Watts is a high-energy physicist at the University of Washington and one of the organizers of the 2022 Snowmass process, which began more than two years ago. “I thought people would view the next steps for the field as already decided — for the High-Luminosity Large Hadron Collider at CERN and neutrino experiments in the US — and conclude, ‘Why should I spend my time thinking about what’s next?’”

Instead, “a glorious range of approaches were presented” over 10 days in July, when about 1,200 people met online and in person at the University of Washington, says Watts. “There were 511 white papers! The amount of work that represents is insane. I am very happy to be wrong.”

At Snowmass, which takes place roughly once a decade under the aegis of the Division of Particles and Fields of the American Physical Society, particle physicists share ideas for research directions, projects, and facilities. They hone their science goals and find synergies across their subfields.

Some projects and approaches drew clear support and enthusiasm, but ranking them was left for later. Reports from Snowmass will serve as guides for the Particle Physics Project Prioritization Panel (P5), which will rank projects for the Department of Energy and National Science Foundation.

Hitoshi Murayama, a particle physicist at the University of California, Berkeley, is chairing P5. “It’s a scary assignment,” he says. “We need to be realistic and maximize the science worldwide; the field is international. Community buy-in is key.” The P5 recommendations will be due in the second half of 2023.

Troubled neutrino flagship the center of attention

The 511 white papers spanned 10 “frontier” areas. The science frontiers were energy, neutrino physics, rare processes and precision measurements, cosmic physics, and theory. Crosscutting frontiers focused on instrumentation, computation, underground facilities, accelerators, and community engagement.

Snowmass reaffirmed support for completing the international Long-Baseline Neutrino Facility and Deep Underground Neutrino Experiment (LBNF/DUNE) project. The science goals for DUNE include studying neutrino oscillations as a function of energy to reveal the ordering of neutrino masses and whether and by how much neutrinos violate CP symmetry. Other goals are to search for proton decay, detect cosmic events such as supernovas, and seek signatures of new physics.

The LBNF/DUNE complex was a priority in the previous P5 in 2014. But cost overruns, delays, and management issues mired the project in controversy. In fiscal year 2021, DOE assigned Fermilab a “C” for program management; the grade wasn’t tied explicitly to any one project, but LBNF/DUNE is the lab’s flagship. The cost estimate for the beam, the “near detector” at Fermilab, and two of the planned four modules for the “far detector” at the Sanford Underground Research Facility in South Dakota has risen from a range of $1.3 to $1.9 billion in 2015 to a currently projected $3.1 billion. And data collection with the neutrino beam, originally planned to start in 2026, is now foreseen for 2031.

Contributing to the cost hikes were difficulties related to excavation, failure to account for the cost of installing the detectors, and a slower-than-anticipated ramp-up on funding, says U.S. project director Chris Mossey. “We need to deliver.” But, he adds, “the risks we had seven years ago are understood or behind us. What I worry about now is high inflation, the supply chain, the ability to travel, and the ability to get the workforce where we need it.”

The plan is to begin operating with two 17-kiloton liquid-argon detectors; two additional detectors would be part of a second phase. The neutrino beam will start at 1.2 megawatts, and later, for the second phase, it will be doubled in power. The near detector will also undergo upgrades. For now, DOE has committed funding for the first phase, and its roughly 30 international partners — major ones include CERN, Brazil, France, India, Italy, and the U.K. — have signed on assuming the full, two-phase project will go forward.

“It would be a disaster to stop now,” says Kate Scholberg, a physicist at Duke University and a co-convener of the Snowmass neutrino frontier. “Our ask of P5 will be for support to explore enhancements for phase two. We need phase two to achieve our long-baseline physics goals.”

Stakes are high around LBNF/DUNE

Community support for LBNF/DUNE is complicated by a worry that it could be scooped by Hyper-Kamiokande, under construction in Japan. But the two experiments have different strengths, says Scholberg. The water detector at Hyper-Kamiokande is cheaper and easier to scale up, she explains. And with DUNE’s argon “you get fine-grained tracks from neutrino interactions, and you can learn more about the neutrinos.” Hyper-Kamiokande is tuned to have good sensitivity for a particular range of neutrino oscillation parameters. “If nature is kind, it will do a very good job. But DUNE will have better sensitivity for a wider range of parameters.”

The broader particle physics community is mostly on board with the full, two-phase LBNF/DUNE project. “If DUNE isn’t completed, we are all in big trouble,” says Salvatore Rappoccio, a high-energy physicist at the University of Buffalo. The experiment’s success will “show the US’ technical prowess, engineering skills, and community support,” he says. Failure, he continues, would raise the specter of the “black eye” that the US particle physics community got in 1993 when the Superconducting Super Collider was canceled, and it would make obtaining funding from the federal government for future big projects more difficult.

“So even if the project goes over in price and takes money from other projects, we have to make it succeed,” says Rappoccio. He and others express confidence in Lia Merminga, who took the helm at Fermilab in April. “She has the management skills that are required to finalize this project,” he says.

Of course, not everyone shares unbridled enthusiasm for LBNF/DUNE. Michael Peskin, a theorist at SLAC, says he finds the project’s “ballooning” costs “very disturbing.” At Snowmass, he says, the issues with the project were “smoothed over, not smoothed out. There are big issues people didn’t discuss.” Peskin hopes phase one of DUNE will be sufficient “because the project will otherwise consume free energy in the high-energy community. It takes a large bite out of what the U.S. can do in other areas of particle physics — and I have big dreams in other directions.”

Going into Snowmass, says Scholberg, “I wasn’t sure how DUNE would mesh with other parts of the high-energy physics community.” There are more things people want to do than there will be resources, she says, “but most of our neutrino plans are nearer term, over the next couple of decades. The ambitious and exciting plans in the energy frontier are further out. I think the schedules will work out.”

Collider projects waiting in the wings

More than half of U.S. particle physicists work in the energy frontier. Topping their wish list is a Higgs factory, an electron–positron collider dedicated to studying the Higgs boson. A handful of circular and linear collider designs exist worldwide. The community wants whichever proposed machine can be realized most quickly.

The most technically ready design is the International Linear Collider (ILC), which would start at 250 gigaelectronvolts and could be extended to one teraelectronvolt. The project has been in limbo for years, with Japan as host waiting for financial commitments from international partners, and potential partners — including the U.S. — waiting for Japan to proceed with the project.

“Very few people in the U.S. think it will happen,” says Peskin, “but as long as you do nothing else, the ILC remains relevant.” Mark Palmer, an accelerator physicist at Brookhaven National Laboratory, was involved in the multi-teraelectronvolt collider discussions for Snowmass. “With funding,” he says, “the ILC could be built and running within a decade.”

CERN and China have aspirations for circular electron–positron colliders. They differ in detail, but each would be about 100 kilometers in circumference, would collide leptons at 240 gigaelectronvolts, and could later be converted to collide hadrons at 100 teraelectronvolts. Given existing commitments, 2048 is the earliest the proposed Future Circular Collider could turn on at CERN, the lab’s director-general, Fabiola Gianotti, said at Snowmass.

China could move faster with its Circular Electron Positron Collider (CEPC). “The design is maturing rapidly,” says Palmer, who knows people involved with the project. “If China decides to make it a national initiative in high-energy physics, it could be a contender for the next electron–positron machine.”

China has “flexibility, space, and probably money,” says Sergio Bertolucci, the Italy-based co-spokesperson for DUNE and a former director of research at CERN. China realizes that the CEPC requires international participation, he says, and is receptive to scientists coming to them. “But they cannot expect the international community to go there if they don’t also go outside.”

Because of the scope and cost, the community generally believes that at most one huge collider will be built. If China goes ahead with its proposal, international participation is unclear; it’s also unclear whether CERN would proceed with its similar project. The CEPC would be a “disrupter,” says Palmer.

High-energy physicists are also excited about a muon collider. The idea is seeing an enthusiastic revival after having been largely abandoned in 2015 following the last P5.

“With muons, you can not only push the energy reach, you can also do precise measurements,” says Sergo Jindariani, a senior scientist at Fermilab and one of a half-dozen coordinators of Snowmass’s muon collider forum. Muons are about 200 times as heavy as electrons; in a circular machine, they therefore radiate less and can be accelerated to higher energies in a more compact ring. The precision is possible because muons are leptons and, unlike hadrons, produce clean collisions. “We would like to get to 10 teraelectronvolt collisions with a circumference of about 10 kilometers,” he says. Such collisions would be about an order of magnitude higher energy than are possible at the Large Hadron Collider.

The catch is that muons are unstable. A muon’s rest lifetime is about two microseconds and increases as it is accelerated. “The lifetime is still short, but it’s long enough,” says Jindariani. Each step in forming an intense muon beam — producing the muons, and then cooling, accelerating, and colliding them — would have to be fast. And each step has challenges.

Muons are also appealing “because you can imagine going to larger rings and getting to much higher energies,” says Jindariani. “We know what type of R&D program is needed to be technologically ready for construction in 20 years,” he says.

A surprise at Snowmass “was the grassroots support for a collider on U.S. soil,” says Priscilla Cushman, a physics and astronomy professor at the University of Minnesota and past chair of the APS Division of Particles and Fields. That support is tied to the promise of the smaller size and price tag offered by either a muon collider or a Higgs factory using new cool-copper linear collider technology.

Among the other high-energy ideas floated at Snowmass were plasma wakefield accelerators and an underwater collider. More intense R&D on advanced colliders is necessary, says Palmer, “so we can actually bring concepts to maturity for the community to evaluate whether to build or not.”

“The real danger is that frontier-energy accelerators will end with the LHC,” says Peskin. “We urgently need to figure out how to build something affordable.”

The ‘dark sector’ beckons

The cosmic frontier is going “full steam ahead,” says Kimberly Palladino, an experimental physicist at the University of Oxford. “We have clear paths to the future.” Those paths include covering parameter space of weakly interacting massive particles as much as possible with current technologies and using new ones to explore lower masses, spin dependence, axions, and the matter–antimatter imbalance to possibly illuminate — or be illuminated by — dark matter.

Technology and theory related to dark matter are both exploding, says Palladino. That makes new searches possible, and “the field has become a lot more interesting than people expected.” In the past, she says, “we’ve been treated like a sideshow to the big colliders. But we are just as important — and more affordable.”

Some dark matter experiments need to be underground, notes Palladino, and they could temporarily park in the spots for DUNE’s phase-two detectors. “Between us and the neutrino frontier, we need the underground facilities,” she says.

Health of the field in the spotlight

For the first time, community engagement was a full-fledged frontier at Snowmass. It encompassed seven topics: applications and industry; career pipeline and development; diversity, equity, and inclusion; physics education; public education and outreach; environmental and societal impact; and public policy and government engagement.

The topics cut across all of physics and other areas of science, notes Kétévi Assamagan of Brookhaven National Laboratory, who convened the frontier with the University of Mississippi’s Breese Quinn. As examples of areas that need improvement, Quinn cites academic–industrial relationships, ties between physics and engineering departments, reduction of the field’s carbon footprint, and diversity.

But it’s a challenge to get people to devote effort to community engagement issues. “There is a tension,” says Assamagan, “between what physicists need to do to find jobs, teach, research, and get promotions, and the community engagement issues that impact us all.” Palladino, for example, says she was overwhelmed with other professional and family commitments and “felt guilty for not joining the frontier.”

Structural barriers discourage participation in community engagement, Assamagan notes. “We need to provide incentives, tangible support, and show that the field values work in these areas.” The community engagement frontier put together 140 recommendations directed variously at individuals, large research collaborations, institutions, and funding agencies. “Nobody has to do everything,” says Quinn, “but everybody has to do something.”

Most of the Snowmass recommendations are for P5. But while P5 will try to incorporate community engagement however it can, says Murayama, the panel may not be the best place for recommendations from that frontier. “We are figuring that out,” says Quinn. He points to two good signs: P5 is incorporating community engagement in its call for nominations, and Cushman has created a task force within the Division of Particles and Fields to come up with a structure that could shepherd the community engagement recommendations. Says Quinn, “A lot is needed to make our field healthy.”