Particle physicists have a reputation for building some of humanity’s biggest science 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 in at 1,400 tons. When completed, each of the largest detectors for the DUNE neutrino experiment in the United States will stand four stories tall and hold more than 18,000 tons of liquid argon.

Large projects like these have produced some of our greatest scientific discoveries, and they continue to play a vital role in research today. But in their latest planning activity, US physicists made it clear that there’s room for smaller—and less expensive—experiments, too.

In December of last year, the 2023 Particle Physics Project Prioritization Panel (P5) released its once-per-decade report outlining recommendations for the next 10 years of US particle physics research. Among its top recommendations was to “create an improved balance between small-, medium-, and large-scale projects,” with those categories broadly defined as projects costing less than $50 million, between $50-$250 million, and over $250 million, respectively.

The report recommended achieving this improved balance through the formation of a new program called ASTAE, for Advancing Science and Technology through Agile Experiments, which would comprise a new portfolio of small projects at the US Department of Energy.

The experts behind this recommendation say the problem isn’t that large projects get too much attention in the physics community; it’s that smaller projects don’t get enough, and those projects may be crucial to the future of particle physics research.

“The universe does not give up its secrets easily, and it forces us to build [large projects] to really push the limits of what we can measure,” says Lindley Winslow, a professor of physics at MIT and member of the P5 committee. “The issue is that in order to build big things, you need to start somewhere.”

If you want to investigate new physics theories, you can’t exactly jump from a research proposal to a $250 million construction project. “We need to have this middle ground between [the] very small R&D efforts and the really gigantic machines… In the last decades, that sort 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 investigations into the newest and most innovative physics theories. “The small-scale experiments are like a sandbox for exploring new ideas,” says Tien-Tien Yu, an associate professor of physics at the University of Oregon and another member of the P5 panel. “Financially, [they’re] not too big of an investment, so you’re willing to take those risks.”

Those risks can have big payoffs that open up 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 their eggs in a single billion-dollar basket. That’s important for efforts to answer the big questions in physics, like the ongoing search for the elusive “dark matter” that makes up over 80% of the universe’s mass.

“Dark matter is a very broad topic, and we still don’t have a definitive idea of what the nature of dark matter is,” Yu says. “What this means is you need to study it from many different axes, many different angles.”

For decades, the leading theory of 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 to other particles that are known to exist.

But the WIMP is not the only possible dark matter candidate. “The theorists have been very inventive,” says Dan McKinsey, a professor of physics at UC Berkeley. “They’ve come up with all sorts of models for what the dark matter could be.”

Today, leading WIMP alternatives include QCD axions, a hypothetical elementary particle whose existence could solve a widely studied inconsistency in the Standard Model known as the strong CP problem, and light dark matter, a hypothetical particle similar to WIMPs but much lighter in mass.

WIMPs are believed to be rather large by particle standards, and the search for them requires large dark-matter detectors built deep underground. QCD axions and light matter particles are theorized to be much smaller, so much so that they behave more like waves than particles. The search for particles at this smaller scale calls for different strategies.

“You no longer need, nor can you use, a giant detector [for these wavelike particles],” Winslow says. “When you’re in axion space and in the light dark matter space, these detectors actually have to be smaller.”

Smaller detectors are less costly to build than their WIMP-detecting counterparts, and they represent the kind of small- and medium-sized projects ASTAE would cover. One might think projects like these should be much easier to get off the ground than those involving much larger budgets, but in recent years, that hasn’t been the case.

Challenges of small- and medium-scale project management

The P5 recommendation for the ASTAE program includes a number of recommendations regarding its management. One is that the first set of ASTAE proposals should come from a collection of small-scale dark matter experiments originally launched in 2019 through the DOE’s DMNI—Dark Matter New Initiatives—planning activity. Through DMNI, scientists proposed technology R&D and concept studies to prepare for possible future dark-matter experiments. In 2019, DOE selected six proposals to support.

P5 panelists say there are strong scientific reasons for pursuing dark matter experiments first. Recent advances in cosmology and in technologies like quantum sensing have led to an explosion in new dark-matter theories and experimental proposals, and this abundance of fresh ideas was a key reason the 2019 DMNI program was created in the first place.

However, the motivations for putting the DMNI experiments at the forefront are also tactical. In general, DOE particle physics experiments are divided into three phases—design, construction project and operations. Each lasts roughly two years.

Many concept studies became stalled in 2020, largely due to the COVID-19 pandemic and decreased funding. “We’ve been doing a two-year design phase since 2019,” says Winslow, whose Dark Matter Radio experiment is one of the DMNI projects.

One project, dubbed the Coherent CAPTAIN-Mills experiment, needed only minimal support to finish fabrication and move into operation. At the May 2024 meeting of the High Energy Physics Advisory Panel, HEP Associate Director Regina Rameika announced that a proposed experiment called TESSERACT would move forward to project fabrication, starting in mid-2025. But other projects are still awaiting a verdict.

In many ways, ASTAE was conceived as a broader and improved version of the DMNI program, also open to projects in areas such as neutrino physics and offering a clear path through each of the project phases.

Panelists say the number of years of work invested in the DMNI experiments is a good reason to give them priority. “They’re ready to go. They are safe bets versus restarting the whole program,” Winslow says. “If some of them aren’t looking quite ready, and new ideas have come up, go ahead.”  

Small-scale projects and the future of particle physics

The enthusiasm around the ASTAE program highlights the importance of smaller projects in developing the particle physics workforce. These relatively short-term projects give students and early career scientists valuable experience in the different stages of building an experiment.

Physicists don’t necessarily get the same opportunity with larger projects. “[Large-scale] experiments of high-energy physics are things that are taking now decades to plan and to construct,” says Mayly Sanchez, a professor of physics at Florida State University and a member of the P5 panel. “That leaves the community without things that can be done in the [career] lifetime…of the younger generations.”

It’s not just the students who are affected, Winslow says. “And it’s not just the scientific workforce. It’s the technicians. It’s the engineers… How do you build big things? How do you see [a project] through all the stages?”

At the December 2024 meeting of the High Energy Physics Advisory Panel, Rameika announced that DOE did not plan to move forward with ASTAE in 2025. But she did not rule out DOE supporting it in the future.

P5 panelists say a renewed focus on small- and medium-scale projects will bring new enthusiasm from the students and early career scientists who will build the future of particle physics.

“I think it really will re-energize this sector of particle physics,” Winslow says. “I think you’re seeing so much [excitement] from the community [about] it, because it gives us sort of these near-term goals and the results and the training of the students.

“That’s the fun part,” Winslow says. “Building these things and getting the data.”