Scientists design program for the future of US particle physics research

The US particle physics community has its plan for the coming decade of research, and it represents a mix of large-, mid-, and small-scale projects, hosted at home and abroad, bolstered by a healthy dose of theory and ambitious R&D.

The plan, created by the 30 members of the Particle Physics Project Prioritization Panel (P5), plus one ex-officio member, was approved by a federally appointed group of scientist advisors to the US agencies that fund research, the High Energy Physics Advisory Panel (HEPAP).

The P5 Report responds to two budget scenarios put to the panel by the US Department of Energy. In the more favorable scenario, there would be an initial bump provided by the CHIPS and Science Act, signed into law in 2022, then a 3% increase in the HEP budget. In the less favorable one, there would be no initial bump, then an increase of 2%, which—accounting for inflation—actually represents a budgetary decline.

The panel also reviewed research funded by the National Science Foundation.

Within both DOE budget scenarios, the panel recommended devoting funds to five major future projects, with budgets over $250 million. First, they recommended completing CMB-S4, an experiment led by Lawrence Berkeley National Laboratory in California that will observe the cosmic microwave backgroundlight from the earliest moments of the universevia the collective power of 12 telescopes installed in the South Pole and Chile.

Second, the panel recommended pursuing a re-envisioned set of enhancements to the US-hosted Deep Underground Neutrino Experiment (DUNE), led by Fermi National Accelerator Laboratory in Illinois and under construction at Sanford Underground Research Laboratory in South Dakota. In the more favorable budget scenario, the report recommended upgrading the Fermilab accelerator that will supply neutrinos to the experiment to a power of 2.1 megawatts and upgrading the detector located close to the neutrino beam at Fermilab. In both budget scenarios, the report recommended constructing a new far detector in South Dakota, bringing the total number of DUNE detectors to three.

Third, the report recommended the US provide R&D and contributions, on par with current US support for the Large Hadron Collider and High-Luminosity Large Hadron Collider, toward a future Higgs factory located abroad. The Higgs factory would conduct precision studies of the most recently discovered fundamental particle, the Higgs boson. US contributions could, for example, go toward the Future Circular Collider, or FCC-ee, proposed for construction at CERN in Europe, or toward the International Linear Collider, or ILC, proposed for construction in Japan.

Next, the P5 panel recommended building a third-generation dark matter direct detection experiment, preferably hosted in the United States, to search for dark matter particles, including the most popular candidates, called WIMPs.

In addition, the P5 Report recommended the construction of IceCube-Gen2, an upgrade to the NSF-funded IceCube experiment at the South Pole, which would contribute to both studies of neutrinos and indirect searches for dark matter.

Within any budget scenario, the panel recommended—at an even higher priority level than the pursuit of any new projects—the completion of current construction projects and continued operation of ongoing experiments. Major projects include the high-luminosity upgrade to the LHC; the first phase of the DUNE experiment and its associated PIP-II accelerator upgrade; and the Vera C. Rubin Observatory in Chile. Medium-scale projects include neutrino experiments NOvA, SBN and T2K; dark matter experiments DarkSide-20k, LZ, SuperCDMS and XENONnT; dark energy survey DESI; and flavor physics experiments Belle II, LHCb and Mu2e.

The major projects

CMB-S4’s top spot in the report shows the growing importance of cosmology in US particle physics. “We now have two paradigms: the standard models of particle physics and cosmology,” says Hitoshi Murayama, a theorist and chair of the P5 Panel. “That intersection is a new area of science, and that should definitely lead to new phenomena. Scientifically, everyone is very excited about it.”

CMB-S4, a fourth-generation CMB experiment funded by both DOE and NSF, will allow scientists to look back to the moments just after the Big Bang, a time when the universe was a small, concentrated area bursting with energy.

In her day job, University of Chicago cosmologist and P5 Panel member Abby Vieregg does research on the cosmic microwave background. “The reason I got into the CMB in the first place is that if you asked the question ‘What is the highest-energy process that I can experimentally probe?’” she says, “the answer is inflation in the early universe.”

The construction of telescopes at the South Pole for CMB-S4 has been delayed due to urgent infrastructure issues, including maintenance needs at Antarctic research stations that could not be addressed during the COVID-19 pandemic. The P5 Report recommended conserving the research stations, which will also benefit IceCube-Gen2. “The South Pole, a unique site that enables the world-leading science of CMB-S4 and IceCube-Gen2, must be maintained as a premier site of science to allow continued US leadership in these areas,” the report states.

The P5 Report’s next highest major-project priority, DUNE, was designed to study neutrinos, the most abundant matter particles in the universe. In 1998 neutrinos were discovered to oscillate between three different types. The discovery revealed the tiny particles have mass, something not predicted in the Standard Model of particle physics and a surprise scientists continue to investigate. “DUNE is about understanding this major discovery that neutrinos have mass and oscillate,” Murayama says. “That’s what DUNE is really good at, and that’s science we’d like to support.”

DUNE is unique, in part because it is the largest neutrino experiment ever built in the United States, and in part because it is being built with major contributions from countries outside the US. European research center CERN, for example, has played a significant role in the construction of DUNE, providing a large-scale demonstration of the DUNE detectors at their neutrino platform.

“DUNE is the first truly international US-hosted experiment, in a way that far exceeds [previous experiments] CDF and DZero,” says R. Sekhar Chivukula, chair of the APS Division of Particles and Fields and an ex-officio member of HEPAP.

During the community planning process that preceded and informed P5 discussions, the DUNE collaboration proposed a set of next steps for the experiment that included upgrades to the accelerator and near detector, along with the construction of a third and fourth far detector. But to ensure no single project—even the highest-priority physics project hosted in the United States—did not dominate so much of the budget that it crowded out other opportunities, the panel needed to consider alternatives, Murayama says. “To put together this exciting and ambitious program, we needed to be creative.”

Consulting with both neutrino experts and a subpanel convened specifically to advise P5 on major project budgets, the members of P5 came up with a plan that would cost less than the original proposal but boost the power of the experiment enough to reach the physics goals proposed for DUNE in the previous P5 report. “Boosting the beam, adding a third far detector and upgrading the near detector allows us to reach the milestone the previous P5 recommended, but in a different way,” Murayama says.

In their report, the panel members advocated for continued R&D into designs for a fourth DUNE detector, which they recommended adding in an expanded budget scenario.

The panel’s third highest priority recommendation was to support international plans for a future Higgs factory, a collider that could mass-produce Higgs bosons in clean collisions that would allow for close studies of the properties of the newly discovered particle.

“The US has to take its international commitments seriously,” says Jesse Thaler, a theorist at MIT and a member of the P5 Panel. “Particle physics is global science. No matter what, there is no country that can afford the next collider by themselves.”

Multiple plans for Higgs factories outside the United States are still in the works. In early 2024, scientists are scheduled to present a midterm report on the FCC Feasibility Study, looking into building a Higgs factory at CERN. In Japan, the Ministry of Education, Culture, Sports, Science and Technology has yet to indicate whether the Japanese government will support the local construction of the ILC.

To deal with the remaining uncertainty, the P5 Panel recommended convening an expert panel—or possibly more than one—sometime between this P5 and the next, to continue to advise HEPAP on plans for accelerators and accelerator R&D inside and outside the United States.

“And it’s important to note, we don’t want the US to be a bystander,” says P5 member Tulika Bose, a professor at the University of Wisconsin, Madison who conducts research on the CMS experiment. “We should actively engage in the decision process, including feasibility and design studies.”

The fourth major project the P5 Panel recommended funding is a third-generation dark matter detector. Since the previous P5 Report was approved in 2014, the number of well-motivated dark matter candidate particles has proliferated. The P5 Report endorses the construction of a next-generation detector that will be powerful enough to either discover or eliminate multiple of those candidates.

In the most restrictive budget scenario, the panel recommended supporting the construction of a third-generation detector outside of the United States. “This less favorable scenario will lead to a loss of US leadership in many areas, especially the science of the generation-3 dark matter experiment,” Murayama says.

Mid- and small-scale projects

The P5 Panel considered more than just large-scale projects like CMB-S4 and DUNE. During the Snowmass process and at subsequent P5 town hall meetings, panelists heard especially from early-career scientists that the physics program needs to include small- and medium-scale projects as well. These projects can start and finish at a quicker pace, ensuring the continuous operation of experiments where students can do their thesis research and early-career physicists can earn their chops. “Our field has a problem,” Murayama says. “There are many big, long-term projects. It’s difficult for young people to shine. They can’t wait 20 years to get tenure. We need to keep producing science they can claim for their own.”

As previously mentioned, in recent years there has been a burst of creativity in the area of dark-matter research, resulting in a long list of ideas for small-scale projects and opportunities to demonstrate new technology. “I don’t think we understood [in 2014] just how rich the dark sector could be,” Thaler says. “All of the ways to probe it—it was dizzying. It is a defining scientific question and will require an all-hands-on-deck approach.”

The panel recommended supporting a portfolio of smaller experiments with a variety of scientific aims—starting with searches for dark matter proposed as part of the 2018 Dark Matter New Initiatives program—organized in a portfolio titled “Advancing Science and Technology through Agile Experiments,” or ASTAE. The report recommended DOE formalize this small-project funding with a $35 million call for proposals each year. “We really wanted to make it a program with a framework and a structure,” says Karsten Heeger, chair of the Yale Physics Department and deputy chair of P5.

The P5 members also recommended funding DESI-II, an upgrade to the DESI wide-field spectroscopic survey, hosted by Berkeley Lab; an upgrade to the detector and accelerator for the Belle II B-factory experiment, hosted at KEK laboratory in Japan; and an upgrade to the LHCb experiment, located at the Large Hadron Collider at CERN. Thus far, the LHCb experiment has received US funding only through NSF.

A firmer foundation and a possible US-based accelerator

The P5 Plan is not just a list of recommended projects, Heeger says. “What this plan represents, in my mind, is more of a program,” he says. “We’re also recommending all of these other investments in things like R&D and personnel—that actually adds up to a fairly significant annual amount.”

A major goal of the last P5 report was to jump-start the construction of major new physics facilities, especially within the United States. The community achieved that goal, initiating the construction of US-based projects Muon g-2, ADMX, Mu2e, LZ, DESI, SuperCDMS, LSST, PIP-II, and DUNE, as well as providing major contributions to the CERN-based upgrade to the LHC, the HL-LHC.

But that emphasis in the 2014 report left little budget left for other areas of physics. Funding for theory, for example, declined by almost 40% after the 2014 P5 Report. To start to correct for that, the P5 Report recommended an increase in DOE HEP funding for university-based theory research by $15 million per year.

This year’s P5 Panel hoped to tip the balance back from construction toward accelerator and instrumentation R&D, theory and computing, which are also essential components of sustaining US involvement in particle physics. “All of us got a better sense of timelines and how long it takes,” Thaler says. “Things that were set in motion decades ago are still relevant. We realized the importance of building the pipeline in terms of R&D.”

Part of that foundation, P5 panelists say, should support a future collider with the potential to access a new energy regime, outside the reach of even the high-luminosity upgrade to the LHC. “If we look at data from the LHC and all the parameter space that has been ruled out, and think ‘Where should we be looking next?’” Bose says, “We really should be looking at new physics at a much higher scale. We need to go beyond the kind of energies we’ll have at the HL-LHC.”  

An enthusiastic contingent of participants in the Snowmass process—some of whom even wore custom matching t-shirts to promote their cause—hope to achieve these goals by constructing a muon collider, particularly one designed to fit on-site at Fermilab that could contribute to fixed-target experiments and neutrino studies there as well. The members of the P5 Panel were impressed by their plans.

“In particular, a muon collider presents an attractive option for both technological innovation and for bringing energy frontier colliders back to the US,” the P5 Report states. “Although we do not know if a muon collider is ultimately feasible, the road toward it leads from current Fermilab strengths and capabilities to a series of proton beam improvements and neutrino beam facilities, each producing world-class science while performing critical R&D towards a muon collider. At the end of the path is an unparalleled global facility on US oil. This is our Muon Shot.”

Other options are an upgrade to the proposed FCC-ee Higgs factory, which would instead collide hadrons and be called the FCC-hh; the so-called Cool Copper Collider proposed at SLAC National Accelerator Laboratory in California; or an accelerator using emerging plasma-wakefield technology. The P5 Panel recommended putting in the work on R&D toward developing the technology for all of these proposed future accelerators, with the goal of honing in on the most promising efforts and being ready to build major test and demonstrator facilities in the next 10 years.

The P5 Report says US particle physics should enhance research in particle physics theory; expand general accelerator R&D; invest in R&D in instrumentation and computing, along with R&D toward future projects such as the Spec-S5 spectroscopy experiment, the fourth DUNE detector; an Advanced Muon Facility and the technique of line-intensity mapping; support key cyberinfrastructure components, such as shared software tools; develop plans for improving the Fermilab accelerator complex; and prepare initiatives to improve workforce development, broaden engagement, and support ethical conduct in the field.

Even though those last recommendations fell outside of the P5 Panel’s charge of prioritizing physics projects, members felt it was important to include them in the report. At the HEPAP meeting, multiple commenters discussed these issues, bringing up concerns about promoting diversity, equity and inclusion and ensuring graduate students and postdocs receive a living wage.

“What the discussion highlighted is that these are issues that are important in the community,” Heeger says. “By including them, we picked up on something that was important in the Snowmass process.”

In its recommendations for NSF, the P5 Panel advocated for support of three major projects: a next-generation gravitational-wave observatory; IceCube-Gen2, an upgrade to the South Pole-based IceCube neutrino experiment; and the Cherenkov Telescope Array, to be built at two sites, in the northern and southern hemispheres.

The P5 Report also listed some opportunities the physics community could pursue if they got lucky and found their level of funding surpassed the more optimistic budget scenario. “DOE didn’t ask about a more favorable scenario,” Murayama said at the HEPAP meeting. “But we volunteer to say something about that anyway.”

In such a case, the P5 Panel recommended DOE consider a more comprehensive upgrade to the infrastructure of the Fermilab accelerator complex. Also in that scenario, the panel recommended possibly moving forward on three additional medium-scale projects: the Spec-S5 study of cosmic inflation and dark energy, the proposed fourth DUNE detector, and a second third-generation dark matter experiment. On the smaller scale, it recommended expanding support for ASTAE and accelerator R&D.

“There are amazing scientific opportunities that, given more funding, I would love to see happen or accelerated,” Thaler says.

And Thaler thinks pursuing those opportunities is possible. “Look at what happened with the last P5 report,” he says. “We worked within the constraints, but we as a community were able to advocate for budgets that were larger. This report can be used to advocate strongly for our community to the world.”

A community effort

The work of putting together the P5 recommendations began in late 2020, with the kick-off of the Snowmass community planning process. Organized by the American Physical Society’s Division of Particles and Fields, the Snowmass process sought input from physicists across the US and abroad via letters of interest, white papers, working groups, town hall meetings and finally, in summer 2022, a community-wide planning meeting at the University of Washington in Seattle.

Snowmass organizers passed their conclusions to the members of P5, who solicited additional input at six more town hall meetings, with an average registration level of about 460 participants, hosted by five national laboratories and two US universities. One session was exclusively devoted to listening to input from early-career scientists.

The panel also convened a working group—chaired by Jay Marx, who served for more than a decade as executive director for the Laser Interferometer Gravitational-wave Observatory at Caltech—specifically to advise them on estimating the cost of major future projects.

“I gained an enormously broader perspective than I had a year ago,” Vieregg says. “All of a sudden everything is on the table, and you see it all. You’re not soldering boards in your lab or thinking about managing your own project. You’re thinking about important fundamental science questions we have to answer. It got me excited about the next 10 years in particle physics.”

From May through December, the P5 panel deliberated in closed-door meetings and eventually reached a consensus. “To say I’m pleasantly surprised doesn’t exactly capture it,” Thaler says. “My colleagues all reacted how I expected them to react. There was a sense of collaboration. When there were disagreements, they were based on the science: How can we maximize the science given the constraints we have? We were all trying to come up with something that was balanced for the community as a whole.”

HEPAP, which advises the Department of Energy’s Office of High Energy Physics and the National Science Foundation’s Division of Physics, approved the P5 Report at the conclusion of their meeting today.

Their vote sends the plan back out to the community where it began. On Dec. 11, members of the physics community will convene, virtually and in-person at Fermilab, for a four-hour town-hall discussion of the plan and what to do next.

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The US particle physics community has its plan for the coming decade of research, and it represents a mix of large-, mid-, and small-scale projects, hosted at home and abroad, bolstered by a healthy dose of theory and ambitious R&D. The plan, created by the 30 members of the Particle Physics Project Prioritization Panel (P5),…

The US particle physics community has its plan for the coming decade of research, and it represents a mix of large-, mid-, and small-scale projects, hosted at home and abroad, bolstered by a healthy dose of theory and ambitious R&D. The plan, created by the 30 members of the Particle Physics Project Prioritization Panel (P5),…

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