APS News | Research

The P5 Report is Here: Particle Physicists Set Sights on the Multi-Billion-Dollar Road Ahead

U.S. physicists share their vision for the future of high energy physics — and ask for the funding to achieve it.

Published Dec 08, 2023
A view of the ProtoDUNE cryostat at CERN.
A view of the ProtoDUNE cryostat at CERN.
Credit: CERN

The discovery of the electron in 1897 marked a major shift in physics: If atoms weren’t the smallest building blocks of the universe, what else could there be? A lot, it turns out. Exploration into the 20th century yielded a veritable ‘particle zoo’ — quarks, muons, pions, and beyond. By the 1970s, physicists had developed a unifying theory, the Standard Model, to explain the existence of so many particle types and their relationships to each other.

Many mysteries remain. How much mass do neutrinos have? What is dark matter made of? Why is most of the universe made of matter, not antimatter? But studying tiny particles requires some of the largest experiments ever built, such as multi-billion-dollar accelerators that smash particles together at 99.9999991% the speed of light, like CERN’s Large Hadron Collider in Switzerland — famed home of the 2012 Higgs boson discovery, which further validated the Standard Model.

Projects of this scale require an extraordinary amount of time and money, and physicists have come up with an extraordinary way to prioritize. About once a decade, a panel of physicists drafts a budget-conscious plan, known as the Particle Physics Project Prioritization Panel (P5) report, which recommends where the field should go experimentally and how to get there technologically. The report is given to the federal High Energy Physics Advisory Panel (HEPAP), which advises the U.S. Department of Energy and National Science Foundation, which in turn decide how to dole out more than $1 billion in funding to high energy physics each year.

On the morning of Dec. 8, the P5 panel released its 2023 report to the public.

“We're very fortunate that the agencies that support high energy physics [in the U.S.] … ask their advisory panel, P5, to take community input and use it as the basis for constructing an effective particle physics program,” says R. Sekhar Chivukula, 2023 chair of APS’s Division of Particles and Fields (DPF) and a theoretical particle physicist at the University of California, San Diego.

“Community input” is an understatement: The P5 report relies on the feedback of hundreds of physicists, who joined a planning process known as ‘Snowmass,’ organized by DPF. First convened in Colorado in 1982, Snowmass brings together physicists from across the United States and around the world to plan the future of the field.

Last year’s Snowmass process culminated in hundreds of white papers, fodder for the P5 panel as it began the arduous task of distilling everyone’s hopes into a realistic vision for the next 10 years and beyond. It’s scientific “democracy at work,” says Hitoshi Murayama, P5 panel chair and a theoretical particle physicist at the University of California, Berkeley.

The Snowmass process is triggered when a sufficient number of advances in the field set the stage for the next generation of scientific discovery. For example, the last P5 report was published in 2014, just two years after the Higgs boson was observed. A year later, in 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected long-theorized gravitational waves. And today, the IceCube Neutrino Observatory is revealing awe-inspiring features of our galaxy through a neutrino ‘camera.’

These discoveries have spurred a major shift in thinking within the field, says Karsten Heeger, P5 panel deputy chair and experimental neutrino physicist at Yale University. “We’ve found ourselves now in a situation where we can learn about the fundamentals of particle physics not just by looking at the smallest things, with colliders, but by also looking at the universe and the cosmos,” Heeger says.

For example, many physicists searching for dark matter’s clues aren’t running experiments at an accelerator. Instead, they’re looking at the night sky, scouring black holes for new evidence. “The way we think about what may lie beyond the Standard Model has really changed,” says Murayama.

The P5 report captures these shifts, says Heeger, while balancing the need to sustain investment in major activities already underway that the P5 panel deems most important. This includes a high-luminosity upgrade to the LHC (HL-LHC), to increase its brightness by a factor of 10; the Deep Underground Neutrino Experiment (DUNE), to answer persistent questions about neutrinos; and the Vera C. Rubin Observatory, intended to help scientists understand the drivers of cosmic evolution. The report also urges funding for ongoing smaller experiments on dark matter, like DarkSide-20k, and deeper exploration of particle decay paths through Belle II, among many other projects.

Developing the report was like “putting together a big jigsaw puzzle,” says Murayama: The goal is to fit priorities together in a way that maximizes opportunities for physicists, while balancing constraints. For example, “big projects are very exciting,” he says, “but they take a long time.” DUNE is one such project, designed to yield decades’ worth of data — but because graduate students need to complete their degrees within much shorter timeframes, funding must also support small-scale efforts. The P5 team had to consider this carefully, he says.

One of the panel’s grandest recommendations is for the United States to develop the capacity to design and someday build a 10-TeV parton center-of-momentum (pCM) collider, perhaps using muon beams, a feat that many proponents argue would solidify the country’s leadership in collider science. The project faces daunting challenges: As elementary particles, muons would be simpler and more energy-efficient to study than the protons, composed of quarks held together by gluons, that CERN’s LHC smashes together — but muons also have extremely short lifespans.

U.S. physicists call this aspirational collider their ‘muon shot,’ like ‘moonshot,’ a problem that can only be solved with a radical new way of thinking. “It’s worth spending time and money on it, to at least see if it’s going to work or not before we can talk about building it,” says Murayama. That means investing in fundamental science now, ahead of a future funding initiative.

The report also recommends supporting a broad portfolio of new projects across the field. For example, some of the new lines of support recommended by P5 would help physicists study the universe’s birth through the cosmic microwave background at the South Pole (CMB-S4) and refine the properties of the Higgs boson through a new offshore ‘Higgs factory,’ designed to churn out the elusive particles for study.

The report recommends initiatives to nurture the nation’s advanced technological workforce, too — including increases in existing federal funding, like ramping up DOE funding for research on high energy physics theory in universities by $15 million per year, and accelerator R&D programs by $10 million per year.

And the report is clear-eyed about the consequences of underfunding — what the panel dubbed a “less favorable budget scenario” that would not keep pace with inflation. In this scenario, the report warns, “the US will cede its leadership” in many areas, like off-shore dark-matter detection experiments.

This is important because many of P5’s recommended projects would be collaborative and international. “The scale of particle physics experiments these days is such that it is not possible, in most cases, to have multiple experimental programs undertaking the same work,” says Chivukula. “There needs to be a certain amount of sharing and prioritization, as there is in the case of the LHC and DUNE,” like providing in-kind support to Europe or Japan to construct the Higgs factory.

“What we recommend here, it’s not just exclusively important for particle physics,” says Heeger. “It’s important for U.S. R&D and the science landscape.”

“We need to have structural support at all levels from funding agencies, and individuals, to make all of this happen.”

Liz Boatman

Liz Boatman is a science writer based in Minnesota.

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