On 8 December, the high-energy physics advisory panel to the US Department of Energy and National Science Foundation released a 10-year strategic plan for US particle physics. The Particle Physics Project Prioritization Panel (P5) report recommends projects across high-energy physics for different budget scenarios. Extensive input from the 2021 Snowmass exercise and other community efforts was distilled into three overarching themes: decipher the quantum realm; explore new paradigms in physics; and illuminate the hidden universe, each of which has been linked to science drivers that represent the most promising avenues of investigation for the next 10 years and beyond.
“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,” said panel chair Hitoshi Murayama (UCB). “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&D needed to build the next-generation particle collider.”
Independent of the budget scenario, realising the full scientific potential of existing projects is the highest P5 priority, including the High-Luminosity LHC, DUNE and PIP-II, and the Vera C Rubin Observatory. In addition, the panel recommends continued support for the medium-scale experiments NOvA, SBN, T2K and IceCube; DarkSide-20k, LZ, SuperCDMS and XENONnT; DESI; Belle II and LHCb; and Mu2e.
On the hot topic of future colliders, the P5 report endorses an off-shore Higgs factory, naming FCC-ee and ILC, to advance studies of the Higgs boson following the HL-LHC. The US should actively engage in design studies to establish the technical feasibility and cost of Higgs factories and convene a targeted panel to make decisions in US accelerator physics at the time when major decisions concerning an off-shore Higgs factory are expected, at which point the US should commit funds commensurate with its involvement in the LHC and HL-LHC. Looking further into the future “and ultimately aim to bring an unparalleled global facility to US soil”, the P5 report supports vigorous R&D toward a 10 TeV parton-centre-of-momentum collider, including a targeted programme to establish the feasibility of a 10 TeV muon collider at Fermilab – dubbed “our Muon Shot”.
Astro-matters
Looking outward, the panel identified several critical areas in cosmic evolution, neutrinos and dark matter where next-generation facilities could make a dramatic impact. Topping the list are: CMB-S4, which will use telescopes in Chile and Antarctica to study the cosmic microwave background (CERN Courier March/April 2022 p34); early implementation of a planned accelerator upgrade at Fermilab to advance the timeline of DUNE (in addition to a re-envisioned second phase of DUNE and R&D towards an advanced fourth detector); and a comprehensive Generation-3 dark-matter experiment to be coordinated with international partners and preferably sited in the US. Here, states the report, the impact of the more constrained budget scenario is severe, and could force the US to cede leadership in Generation-3 and to descope or delay elements of DUNE: “Limiting of DUNE’s physics reach would negatively impact the reputation of the US as an international host, and more limited contributions to an off-shore Higgs factory would tarnish our standing as a partner for future global facilities.”
Multi-messenger observatories with dark-matter sensitivity, including IceCube Gen-2 for the study of neutrino properties, and small-scale dark-matter experiments employing innovative technologies, are singled out for support. In addition, the panel recommends that the Department of Energy create a new competitive programme to support a portfolio of smaller, more agile experiments in high-energy physics.
The P5 report supports vigorous R&D toward a 10 TeV parton-centre-of-momentum collider
Investing in the scientific workforce and enhancing computational and technological infrastructure are described as “crucial”, with increased support for theory, general accelerator R&D, instrumentation and computing needed to bolster areas where US leadership has begun to erode. The report also urges broader engagement with and support for the workforce, suggesting that all projects, workshops, conferences and collaborations incorporate ethics agreements that detail expectations for professional conduct and establish mechanisms for transparent reporting, response and training.
“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 US community with a 10-year budgetary timeline and a 20-year context,” said P5 panel deputy chair Karsten Heeger (Yale). “The panel thought about where the next big discoveries might lie and how we could maximise impact within budget, to support future discoveries and the next generation of researchers and technical workers who will be needed to achieve them.”
