What is the Particle Physics Project Prioritizing Panel (P5) Process?

The Particle Physics Project Prioritization Panel (P5) is an advisory group tasked with evaluating and prioritizing projects and program initiatives in particle physics. Since 2003 P5 has been guiding the future direction of the field and funding priorities in the United States. The P5 process culminates with a Report that proposes a prioritized project and program agenda under a couple of different funding scenarios, to be pursued over 10 years, in the context of a 20-year global strategy.

• P5 is a temporary subcommittee of HEPAP, the High Energy Physics Advisory Panel, a committee jointly formed by the U.S. Department of Energy’s (DOE) Office of Science and the National Science Foundation (NSF), the principal federal funding agencies of particle physics research. P5’s recommendations influence investments for DOE and NSF. HEPAP receives its charge to start the P5 process from DOE and NSF.

• The last P5 process provided its long-range strategy in 2014, for FY 2014-2023.

• The current P5 process started with the last 1-2 year-long particle physics community-wide planning exercise, commonly known as Snowmass, which culminated in July 2022. During Snowmass, the entire particle physics community came together to, 1) identify and document a scientific vision for the future of particle physics in the U.S., 2) define the most important questions for the field, and 3) identify promising opportunities to address them. Other critical sources of input came from the 2024 Committee on Elementary Particle Physics of the National Academies of Sciences, Engineering, and Medicine¹, the 2020 European Strategy for Particle Physics spearheaded by the European Organization for Nuclear Research (CERN), and the CERN member states².

• The latest P5 strategic report for FY 2024-2033³ was released on December 7, 2023, during the HEPAP meeting in Washington DC. The report will serve as a roadmap for U.S. investments in the particle physics field for a prioritized, time-ordered, and globally aware program⁴.

Important Questions for the Particle Physics Field that P5 Aims to Address

Particle physics is the study of the fundamental constituents of matter and their interactions. All ordinary matter, including every atom on the periodic table of elements, consists of only three types of particles: the up and down quarks, which make up the protons and neutrons in the nucleus, and electrons that surround the nucleus. The view of the universe from the behavior of these particles is known as the Quantum Realm, where their interactions are characterized by discrete amounts, or ‘quanta’, of energy. All the known elementary particles and the theories that describe their interactions are summarized by a coherent framework known as the Standard Model (SM) of particle physics. Today, the SM has served as the scientists’ best theory to describe the basic building blocks of matter in the universe. Despite its success at explaining the universe over the last century, the model however has its inherent limits. For example, the SM explains three of the four fundamental forces that govern the universe: electromagnetism, the strong force, and the weak force. It however does not incorporate gravity, nor does it explain the dark matter and dark energy that tends to be prevalent in our universe and which accounts for over 95% of the total matter, resulting in a mass balance issue.

The discovery of new fundamental particles, such as the Higgs Boson and its associated fundamental mass-giving field, revealed how particles have masses. The Higgs boson particle was discovered in 2012 at CERN. Particles get their mass by interacting with the Higgs field; they do not have a mass of their own. The stronger a particle interacts with the Higgs field, the greater its mass. Photons, for example, do not interact with this field and therefore have no mass. Yet other elementary particles, including electrons, quarks, and bosons, do interact and hence have a variety of masses. Bosons, in general, act as force carriers which either give rise to forces between other particles or give rise to the phenomenon of mass.

The most abundant particles in our universe, neutrinos, were discovered in 1956. They have vanishingly small masses and no electrical charge. Neutrinos are produced every time atomic nuclei come together (like in the sun) or break apart (like in a nuclear reactor). Even bananas and avocados emit neutrinos—they come from the natural radioactivity of the potassium in the fruit. Tens of billions of neutrinos emanating from nuclear reactions in the sun’s core pass through Earth’s surface each second without notice. If we understand neutrinos, perhaps we could answer some of the most essential questions in physics at the heart of our very existence about the earliest moments of the universe.

Other fundamental particles relevant to our understanding of the universe are Cosmic Rays. They are the most energetic particles, as they travel across the entire universe in all directions. Cosmic rays are a rare opportunity to study matter that came from beyond our solar system, or even beyond our galaxy. From them, scientists can estimate the amount of matter and different elements and particles in the universe.

Researchers from around the world and theoretical and experimental physicists at the national laboratories supported by DOE continue working together to design experiments to make discoveries and conduct precision tests to further improve measurements and understanding of the properties of particles and their interactions.

Matter, as humanity knows it, everything on Earth, adds up to less than 5% of the universe. It connects the universe with the human reality of the physical world. You and everything around us are made of particles. The future growth of particle physics inherently involves advancing applied fields such as medicine and renewable energy, among others – all of which lead to new and emerging technologies that harness energy and accelerate knowledge and innovation that impact development at industries and advance aspects of our daily lives.

2024 P5 Report Recommendations

The P5 Report emphasizes the necessary level of R&D investment and innovation for the U.S. to be considered a global partner in the field of particle physics.

Summary of recommendations⁵, in the order of priority:

1. As the highest priority, complete construction projects and support their operations:

• High Luminosity-Large Hadron Collider (CERN)
• First phase Deep Underground Neutrino Experiment (DUNE) and Proton Improvement Plan (PIP)-II (Fermi National Accelerator Laboratory)
• Rubin Observatory, Legacy Survey of Space and Time (LSST) (Chile, led by SLAC National Accelerator Laboratory)
• LSST Dark Energy Science Collaboration (SLAC National Accelerator Laboratory)

2. Construct a portfolio of major projects that collectively study fundamental constituents of our universe and their interactions:

• Cosmic Microwave Background Stage 4 (CMB-S4) (CERN)
• Re-envisioned the second phase of DUNE (Fermi National Accelerator Laboratory)
• Offshore Higgs factory, in collaboration with international partners (Lawrence Berkeley National Laboratory)
• Ultimate Generation (G3) dark matter direct detection experiment (TBD lead & location)
• IceCube-Gen2 (TBD lead & location)

3. Create an improved balance between small-, medium-, and large-scale projects to open new scientific opportunities and maximize their results, enhance workforce development, promote creativity, and compete on the world stage,

• Advancing Science and Technology using Agile Experiments (ASTAE)

4. Support a comprehensive effort to develop the resources – theoretical, computational, and technological – essential to our 20-year vision for the field. This includes an aggressive R&D program that, while technologically challenging, could yield revolutionary accelerator designs that chart a realistic path to a 10 TeV parton center-of-momentum (pCM) collider.

5. Invest in initiatives aimed at developing the workforce, broadening engagement, and supporting ethical conduct in the field.

¹ National Academies of Sciences, Engineering, and Medicine. 2023. “Pathways to Discovery in Astronomy and Astrophysics for the 2020s”, Washington, DC: The National Academies Press. https://doi.org/10.17226/26141

² “2020 Update of the European Strategy for Particle Physics”, June 19, 2020, available at: https://cds.cern.ch/record/2721370/files/CERN-ESU-015-2020 Update European Strategy.pdf

³ “Report: Particle Physics Project Prioritization Panel High Energy Physics Advisory Panel”, December 7, 2023, https://science.osti.gov/-/media/hep/hepap/pdf/Reports/P5Report2023_120123-DRAFT-to-HEPAP.pdf

4 “The Path to Global Discovery: U.S. Leadership and Partnership in Particle Physics,” a report from the HEPAP International Benchmarking Subpanel, November 2, 2023, available at: https://science.osti.gov/-/media/hep/hepap/pdf/202203/International_Benchmarking_HEPAP_2023112.pdf

⁵ “Pathways to Innovation and Discovery in Particle Physics One Pager”, https://www.usparticlephysics.org/wp-content/uploads/2023/12/P5Report2023_One-pager_120723-DRAFT.pdf