Particle physicists are renowned for having carried out some of humanity’s greatest scientific experiments. The Large Hadron Collider in Europe accelerates particles around a loop 17 miles in circumference. The Belle II detector, which scientists use to study collisions at the SuperKEKB particle accelerator in Japan, weighs 1,400 tons. When completed, each of the largest detectors at the DUNE neutrino experiment in the United States will rise four stories high and contain more than 18,000 tons of liquid argon.
Major projects like these have given rise to some of our greatest scientific discoveries and continue to play a vital role in research today. But in their latest planning activity, American physicists have made it clear that it is also possible to carry out smaller, less expensive experiments.
In December of last year, the 2023 Particle Physics Project Prioritization Group (P5) released its 10-year report outlining recommendations for the next 10 years of particle physics research in the United States. One of its main recommendations was to “create a better balance between small, medium and large scale projects”, with these categories defined broadly as projects costing less than $50 million, between $50 million and $250 million, respectively. dollars and more than 250 million dollars. .
The report recommends achieving this better balance through the creation of a new program called ASTAE, for Advancing Science and Technology through Agile Experiments, which would include a new portfolio of small projects within the U.S. Department of Energy.
The experts behind the recommendation say the problem isn’t that big projects get too much attention from the physics community; it’s that small projects don’t get enough, and these projects could be crucial to the future of particle physics research.
“The universe doesn’t give up its secrets easily and it requires us to build (big projects) to really push the limits of what we can measure,” says Lindley Winslow, professor of physics at MIT and member of the P5 committee. “The problem is that to build great things you have to start somewhere. »
If you want to study new physical theories, you can’t go from a research proposal to a $250 million construction project. “We need to find a middle ground between really small R&D efforts and really gigantic machines… Over the last few decades, that kind of middle ground has been lost,” Winslow says.
The scientific importance of small- and medium-scale experiments
P5 panelists say small-scale projects are the logical choice for early research into the latest and most innovative physics theories. “Small-scale experiments are like a sandbox for exploring new ideas,” says Tien-Tien Yu, associate professor of physics at the University of Oregon and another member of the P5 panel. “Financially, it’s not too big an investment, so you’re willing to take those risks.”
These risks can have significant implications that open the way to entirely new areas of science.
Small-scale projects also allow the particle physics community to explore the same question from multiple angles, rather than putting all of its eggs in a single billion-dollar basket. This is important for efforts to answer big questions in physics, such as the ongoing search for the elusive “dark matter” that makes up more than 80 percent of the mass of the universe.
“Dark matter is a very broad topic and we still don’t have a definitive idea of its nature,” says Yu. “That means you have to study it from many different axes and angles.”
For decades, the dominant theory about dark matter has been that it is made up of weakly interacting massive particles, or WIMPs. Physicists say these extremely heavy hypothetical particles fit neatly into the mathematics of the Standard Model of particle physics, functioning as a kind of partner for other particles that are known to exist.
But WIMP is not the only possible candidate for dark matter. “Theorists have been very inventive,” says Dan McKinsey, a physics professor at UC Berkeley. “They came up with all kinds of models for what dark matter could be.”
Today, major alternatives to WIMPs include QCD axions, a hypothetical elementary particle whose existence could resolve a widely studied inconsistency in the Standard Model known as the strong CP problem, and light dark matter, a particle hypothetical similar to WIMPs but much lighter in mass.
WIMPs are thought to be rather large by particle standards, and searching for them requires large dark matter detectors built at depth. QCD axions and light matter particles are theoretically much smaller, so much so that they behave more like waves than particles. Searching for particles at this smaller scale requires different strategies.
“You no longer need or can use a giant detector (for these wave particles),” says Winslow. “When you’re in axion space and dark matter light space, these detectors actually have to be smaller.”
The smaller detectors are less expensive to build than their WIMP-detecting counterparts, and they represent the type of small- to medium-sized projects that ASTAE would cover. You would think that projects like these should be much easier to start than those involving much larger budgets, but in recent years this has not been the case.
The challenges of managing small and medium-scale projects
Recommendation P5 for the ASTAE program includes a number of recommendations regarding its management. The first is that the first set of ASTAE proposals is expected to come from a set of small-scale dark matter experiments initially launched in 2019 as part of the Dark Matter New Initiatives (DMNI) planning activity of the DOE. Through DMNI, scientists have proposed technological research and conceptual studies to prepare for possible future dark matter experiments. In 2019, DOE selected six proposals for support.
P5 panelists say there are strong scientific reasons to pursue dark matter experiments first. Recent advances in cosmology and technologies such as quantum sensing have led to an explosion of new theories and experimental proposals about dark matter, and this abundance of new ideas was one of the main reasons why the 2019 DMNI program was created.
However, the motivations for bringing DMNI experiments to the forefront are also tactical. In general, DOE particle physics experiments are divided into three phases: design, construction project, and operations. Each lasts approximately two years.
Many conceptual studies remained stalled in 2020, largely due to the COVID-19 pandemic and dwindling funding. “We’ve been doing a two-year design phase since 2019,” says Winslow, whose Dark Matter Radio experiment is one of DMNI’s projects.
One project, dubbed the Coherent CAPTAIN-Mills Experiment, needed only minimal support to complete manufacturing and move into operation. At the May 2024 meeting of the High Energy Physics Advisory Group, HEP Associate Director Regina Rameika announced that a proposed experiment called TESSERACT would move into project fabrication, starting in mid-2025. But other projects are still awaiting a verdict.
In many ways, ASTAE was designed as a broader and improved version of the DMNI program, also open to projects in areas such as neutrino physics and providing a clear path through each of the project phases.
Panelists say the number of years of work invested in DMNI experiments is a good reason to prioritize them. “They are ready to go. These are safe bets rather than restarting the entire program,” says Winslow. “If some of them don’t seem quite ready and new ideas come up, go for it.”
Small-scale projects and the future of particle physics
The enthusiasm generated by the ASTAE program highlights the importance of small projects in developing the particle physics workforce. These relatively short-term projects provide students and early-career scientists with valuable experience in the different stages of building an experiment.
Physicists don’t necessarily have the same opportunities in larger projects. “(Large-scale) high-energy physics experiments are things that now take decades to plan and build,” says Mayly Sanchez, professor of physics at Florida State University and member of the P5 panel. “It leaves the community without things that can be done over the course of the life (career)… of the younger generations.” »
It’s not just students who are affected, Winslow says. “And it’s not just about scientific staff. These are the technicians. They are the engineers… How do we build great things? How do you see (a project) through all the stages?
At the December 2024 meeting of the High Energy Physics Advisory Group, Rameika announced that DOE does not plan to move forward with ASTAE in 2025. But she did not rule out that DOE support him in the future.
P5 panelists say a renewed focus on small- and medium-scale projects will spark new enthusiasm from students and early-career scientists who will build the future of particle physics.
“I think this will really revitalize this area of particle physics,” Winslow says. “I think you see so much (enthusiasm) from the community (about it), because it kind of gives us these short-term goals and outcomes and training students.
“That’s the fun part,” Winslow says. “Build these things and get the data.”