The Particle Physics Project Prioritization Panel (P5) is a scientific advisory panel tasked with recommending plans for U.S. investment in particle physics research over the next ten years, on the basis of various funding scenarios. The P5 is a temporary subcommittee of the High Energy Physics Advisory Panel (HEPAP), which serves the Department of Energy's Office of Science and the National Science Foundation. In 2014, the panel was chaired by Steven Ritz of the University of California, Santa Cruz. In 2023, the panel was chaired by Hitoshi Murayama of the University of California, Berkeley.
2014 report In 2013, HEPAP was asked to convene a panel (the P5) to evaluate research priorities in the context of anticipated developments in the field globally in the next 20 years. Recommendations were to be made on the basis of three funding scenarios for high-energy physics:
A constant funding level for the next three years followed by an annual 2% increase, relative to the FY2013 budget A constant funding level for the next three years followed by an annual 3% increase, relative to the proposed FY2014 budget An unconstrained budget
Science drivers In May 2014, the first P5 report since 2008 was released. The 2014 report identified five "science drivers"—goals intended to inform funding priorities—drawn from a year-long discussion within the particle physics community. These science drivers are:
Use of the Higgs boson as a tool for further inquiry Investigation of the physics of neutrino mass Investigation of the physics of dark matter Investigation of the physics of dark energy and cosmic inflation Exploration of new particles, interactions, and physics principles
Recommendations In pursuit of the five science drivers, the 2014 report identified three "high priority large category" projects meriting significant investment in the FY2014–2023 period, regardless of the broader funding situation: the High Luminosity Large Hadron Collider (a proposed upgrade to the Large Hadron Collider located at CERN in Europe); the International Linear Collider (a proposed electron-positron collider, likely hosted in Japan); and the Long Baseline Neutrino Facility (an expansion of the proposed Long Baseline Neutrino Experiment (that was renamed the Deep Underground Neutrino Experiment), to be constructed at Fermilab in Illinois and at the Homestake Mine in South Dakota). In addition to these large projects, the report identified numerous smaller projects with potential for near-term return on investment, including the Mu2e experiment, second- and third-generation dark matter experiments, particle-physics components of the Large Synoptic Survey Telescope (LSST), cosmic microwave background experiments, and a number of small neutrino experiments. The report made several recommendations for significant shifts in priority, namely:
An increase in the proportion of the high-energy physics budget devoted to construction of new facilities, from 15% to 20%-25% An expansion in scope of the Long Baseline Neutrino Experiment to a major international collaboration, with redirection of resources from other R&D projects to the development of higher powered proton beams for the neutrino facility Increased funding for second-generation dark matter detection experiments Increased funding of cosmic microwave background (CMB) research The panel stressed that the most conservative of the funding scenarios considered would endanger the ability of the U.S. to host a major particle physics project while maintaining the necessary supporting elements.
Impact and outcomes since 2014 A goal of the 2014 P5 exercise was to provide Congress with a science-justified roadmap for project funding. Five years later, in 2019, the Department of Energy Office of Science declared: "Congressional appropriations reflect strong support for P5. Language in appropriations reports have consistently recognized community’s efforts in creating and executing the P5 report strategy" and "P5 was wildly successful." From 2016 to 2020, the High Energy Physics (HEP) budget grew from less than $800 million to more than $1 billion. However, members of the high energy physics community were concerned because the increased funding went primarily toward projects, while funding for core research and technology programs, which was also supported by P5, declined from $361 million to $316 million. In 2020, an assessment of progress of the P5-defined program produced by the High Energy Physics Advisory Panel (HEPAP) concluded: "While investments over the past 5 years have focused on project construction, it will be fundamentally important to balance the components of the [High Energy Physics] budget to continue successful execution of the P5 plan. Operations of the newly constructed experiments require full support to reap their scientific goals. The [High Energy Physics] research program also needs strong support to fully execute the plan, throughout the construction, operations, and data analysis phases of the experiments, and to lay a foundation for the future." As of 2022, several of the "Large Projects" identified as priorities by the 2014 P5 had fallen considerably behind schedule or been affected by cost gaps, including:
The Deep Underground Neutrino Experiment, has been reduced in scope, with start-up delayed from 2027 to 2032. The mu2e experiment was delayed from 2020 to 2026. The PIP-II project start-up was pushed back from 2020 to 2028. The High Luminosity LHC contributions from Fermilab faced a $90M cost gap in 2021. The International Linear Collider (ILC), proposed for construction in Japan, was "shelved".
Prelude to the 2023 report
Issues The P5 process occurred in spring 2023 and was informed by the outcomes of the 2021 Snowmass Process finalized in summer 2022. The Snowmass 2021 study identified two existential threats to the field that P5 must address:
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