The report by the Particle Physics Project Prioritization Panel (P5) was approved Friday, Dec. 8, by the High Energy Physics Advisory Panel (HEPAP) and will be sent to the two main funding agencies for physics in the U.S. - the Department of Energy (DOE) and the National Science Foundation (NSF) - to aid them in their decisions about which research to fund. The HEPAP, a permanent advisory committee to DOE and NSF, constitutes a prioritization panel every 10 years.

The panel, consisting of 31 members and one ex-officio member from the U.S. and abroad, considered only large- and medium-sized physics research projects - the kind that can take years or decades to plan and build, enlist contributions from thousands of scientists and cost billions of dollars.

To fit within budget constraints - likely less than $5 billion from the two agencies over 10 years for new projects - the panel had to combine or reconfigure many proposed projects and turn down perhaps two-thirds of them.

"Fiscal responsibility has been a big thing on our mind to make sure that the recommendations are actionable by agencies and can be followed up," said Murayama, the MacAdams Professor of Physics at the UC Berkeley. "We had to be really realistic about our plan."

The five recommended projects with estimated budgets exceeding a quarter of a billion dollars each are:

The Cosmic Microwave Background Stage IV experiment (CMB-S4), which will use telescopes sited in Chile and Antarctica, supported by U.S. infrastructure at the South Pole, to study the oldest light from the beginning of the universe. The polarization of the CMB can tell cosmologists about the gravitational waves generated during inflation in the early universe and help them understand what was going on when the cosmos was still microscopic.

Enhancements, including an upgrade in power and experimental capabilities, to the Deep Underground Neutrino Experiment (DUNE) in South Dakota. The DUNE is the centerpiece of a decades-long program to reveal the mysteries of elusive neutrinos. The U.S.-hosted international project will exploit a unique underground laboratory, the Sanford Underground Research Laboratory, now nearing completion, and neutrino beams produced at Fermi National Accelerator Laboratory in Illinois.

A Higgs boson factory, located in either Europe or Japan, to advance studies of a still mysterious particle that was only discovered in 2012, yet which gives mass to all other forms of matter. An accelerator that produces lots of Higgs bosons would allow precise measurements of the boson's properties and help physicists understand how the particle fits into current models of the universe and whether it is connected with dark matter.

A Generation 3 (G3) Dark Matter experiment that would combine four different international experiments - including the LZ experiment led by Lawrence Berkeley National Laboratory - into one comprehensive program to probe the enigmatic nature of dark matter, which makes up a significant portion of the universe's mass and energy and has been one of the most enduring mysteries in modern physics. The panel recommended that this experiment be built in the U.S.

Expansion at the South Pole of a neutrino observatory, which earlier this year mapped for the first time the sources of neutrinos from the Milky Way galaxy and outside our galaxy. Called IceCube-Gen2, it would be an international collaboration operated by the University of Wisconsin-Madison. The observatory now consists of detectors embedded in 1 cubic kilometer of ice; the expansion would increase the observatory's sensitivity by a factor of 10.

The panel also recommended investing in studies of a future muon collider. While most particle accelerators today rev up electrons or protons and smash them together, a muon collider would accelerate short-lived muons, which are fundamental particles like electrons (they're both leptons), but much heavier. A muon collider could explore new frontiers of physics with much less energy input than a proton collider. The panel proposed Fermilab as a good place to build a demonstration collider to test the unique technology.

"In the P5 exercise, it's really important that we take this broad look at where the field of particle physics is headed, to deliver a report that amounts to a strategic plan for the U.S. community with a 10-year budgetary timeline and a 20-year context. The panel thought about where the next big discoveries might lie and how we could maximize impact within budget to support future discoveries and the next generation of researchers and technical workers who will be needed to achieve them," said Karsten Heeger, P5 panel deputy chair and Eugene Higgins Professor and chair of physics at Yale University.

The panel also urged DOE to establish a fund, like NSF, that would support small-scale projects.

"We need to really look at the balance between big things - of course, we're all excited about them - but also small things, to really keep young people going," Murayama said. "In some cases, small projects can involve thinking really outside the box and can be high-risk, high-return, in terms of scientific results. That kind of combination we feel very strongly about."

The panel was also tasked with looking at diversity issues within the particle physics community.

"We came up with actionable recommendations for how we can improve the climate in the community, which is still very much dominated by white males. I hate to say this, but that's true," Murayama said. "One of the big discussions we had was about how to make the community more inclusive and mutually caring for each other. We have clear recommendations along those lines."

The report built on the output of a Snowmass 2021 high energy physics community planning exercise in Seattle, Washington, organized by the American Physical Society, the only independent body in the U.S. that represents particle physics community as a whole. The new knowledge and new technologies discussed there set the stage for the P5 report.

"The Higgs boson had just been discovered before the previous P5 process, and now our continued study of the particle has greatly informed what we think may lie beyond the standard model of particle physics," Murayama said. "Our thinking about what dark matter might be has also changed, forcing the community to look elsewhere - to the cosmos. And in 2015, the discovery of gravitational waves was reported. Accelerator technology is changing, too, which has shifted the discussion to the technology R and D needed to build the next-generation particle collider."

He noted that the two triangles on the cover of the report are meant to emphasize that looking at smaller and smaller things - the realm of traditional particle physics - must be combined with a look at larger structures, such as the evolution of universe, to get a complete picture of what the report describes as the "smallest constituents of our vast and complex universe."

"The P5 report will lay the foundation for a very bright future in the field," said R. Sekhar Chivukula, 2023 chair of the APS Division of Particles and Fields and a Distinguished Professor of Physics at the University of California, San Diego. "There are extraordinarily important scientific questions remaining in particle physics, which the U.S. particle physics community has both the capability and opportunity to help address, within our own facilities and as a member of the global high energy physics community."

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