Why the ESPP and P5 strategy process are important to us

14 DECEMBER, 2023 | By Markus Brugger and Roberto Losito

Throughout the past century, institutions like ours have used experiments of increasing energy and precision to uncover how the universe works at the smallest scales. Their efforts have led to the Standard Model of particle physics, which describes the most fundamental ingredients of matter (leptons and quarks) and the forces that control them (the electromagnetic, strong and weak forces). At the same time, scientists have also developed a standard model of cosmology to account for the largest scales. This is often quoted as a literal recipe for the evolution of the universe, from the beginning of time to today (and beyond): “start with the big bang; add one-part dark matter, three parts dark energy and just a dash of matter; let simmer for circa 13.8 billion years…”.

Joke aside, nature challenges us and hides the secrets of the fundamental physical laws in the tiniest corners of space and time. By developing technologies to probe ever-higher energy and thus smaller distance scales, our field of science has made discoveries that have transformed the scientific understanding of the world. Nevertheless, as we already mentioned in previous articles, many of the mysteries about the universe, such as the nature of dark matter, and the preponderance of matter over antimatter, are still to be explored.

So, what are the scientific priorities and future objectives of particle physics? Which instruments offer the best potential to achieve these objectives? And which technologies need to be developed to build such instruments?

These are the main questions addressed by the regular updates (though not timewise synchronized) of the European Strategy for Particle Physics (ESPP) and the US Snowmass/P5 process.

Their main deliverable is to propose a vision for both the near-term and the long-term future of our field of science. It aims to significantly extend knowledge beyond the current limits, to drive innovative technological development, and to maintain Europe's and respectively US important role in particle physics, both taking carefully into account the global context.

These strategies are above all science driven, leading a strategic reflection to which hundreds of physicists contributed. The scientific vision outlined in the respective deliberation documents then serves as an important guideline for both Europe’s and US science policy and funding agencies, thus most importantly also providing key input, and often driving decisions, for our CERN mid-/long-term strategy.

The last ESPP iteration was completed in 2020 and provided key conclusions, with CERNs scientific objectives being aligned to study in detail the Higgs boson and the exploration of the high-energy frontier. In this, also highlighting the ongoing and successful completion of the High-Luminosity LHC project, to remain the focal point of European particle physics. Here our group contributes through the Machine Detector Interface (MDI) work package (WP8), moving now into the final production phase. Furthermore, our involvement in the CERN Hostlab project accompanying the major upgrades of the LHC experiments, and our related technical services form an important cornerstone of our group activities.

The ESPP 2020 strategy further emphasized the importance of ramping up research and development (R&D) for advanced accelerator, detector, and computing technologies, as a necessary prerequisite for all future projects. Here our consolidated, and to be consolidated, test beam and user areas are essential, and world-wide unique places where detector technologies can be developed, validated and calibrated.

Moving on with the last ESPP update, the Higgs boson discovered in 2012 is expected to be a powerful tool to look for physics beyond the Standard Model, thus the strategy also highlights the need to pursue an electron-positron collider acting as a “Higgs factory” as the highest-priority facility after the LHC. Such a machine would produce copious amounts of Higgs bosons in a very clean environment, allowing to probe nature at highest precision. Similarly, the exploration of significantly higher energies than the LHC will allow new discoveries to be made and the answers to existing mysteries, such as the nature of dark matter, to potentially be found, thus requiring a technical and financial feasibility study for a next-generation hadron collider at the highest achievable energy, with the above-mentioned electron-positron collider as a possible first stage. Here only recently the FCC feasibility study concluded its mid-term review. In this last phase of the feasibility study, and later when the experiments and related services will be further detailed, again MDI and required infrastructure will provide opportunities for our groups’ involvement.

Furthermore, the ESPP addresses the context of the quest for dark matter and the exploration of flavor and fundamental symmetries as crucial components of the search for new physics, the importance of a broad physics program has been highlighted, referring to precision measurements of flavor physics and electric or magnetic dipole moments, and searches for axions, dark sector candidates and feebly interacting particles. In this context, the ESPP emphasizes the importance of a diverse physics program that is thus considered complementary to the energy frontier. Here our strong involvement in the many fixed target experiments, the related consolidation projects of our experimental areas, as well as important upgrade programs, are at the heart of our group activities, across physics studies, beamline developments and operation, equipment responsibilities, key services, coordination, and project activities.

Most importantly, developed and strengthened over the last eight years, our internal roadmap now provides us with NA-CONS, PBC and ECN3-HI a vision into the 2040s!

Why is all this important? History has shown that fundamental research is a driver of innovation, and accelerator-based particle physics has spawned numerous technological advances. Our understanding of particle physics and cosmology has today reached an unprecedented level of maturity, and this has dramatically changed the targets of research, allowing us to rely on precise models and equations that describe most observed phenomena. Nevertheless, many principles underlying those equations remain unknown, and so are the many open questions as mentioned already in previous articles.

The urge to seek answers to such questions is part of what defines us as humans. The ambitious task that lies ahead entails global collaboration on a courageous experimental venture, involving high-energy colliders, low(er)-energy precision experiments, observational cosmology, cosmic rays, dark-matter searches, gravitational waves, terrestrial and cosmic neutrinos, and much more.

Very exciting times, and where we have a vast playground, literally in front of us!

Moving now from the ESPP to the comparable process in the US where only last week, on December 7th the Particle Physics Project Prioritization Panel (P5), a committee of experts that convenes roughly once a decade, started to present its 2023 report, which charts a course for U.S. particle physics across the next one to two decades. On Monday hundreds of researchers gathered at Fermilab, the locus of U.S. particle physics. They discussed the report and its implications, with hours of questions and comments ranging from technical and scientific queries. Today, Vladimir Shiltsev gave a respective delightful ATS seminar, discussing the major recommendations of the Snowmass Accelerator Frontier.

Similarly as the ESPP process above, now three years later, the P5 report includes an enormous amount of valuable details, comparative summaries and made a large number of well reflected recommendations, including the above mentioned HL-LHC and its completion, stages of the Deep Underground Neutrino Experiment (DUNE) project, a challenging multi-billion effort in South Dakota that is presently the US’s largest particle physics project, as well as he Vera C. Rubin Observatory, all three as first recommendation and thus highest priority. Here we are involved through the H2 and H4 beamline serving the Neutrino Platform and its installation in the EHN1 extension.

The second recommendation refers to a portfolio of major projects starting with new next generation telescopes and aiming for building the Cosmic Microwave Background Stage IV (CMB-S4) project—a series of telescopes to study the early universe. The third bullet point highlights here the US involvement in an off-shore Higgs factory to be realized in collaboration with international partners, where the US should actively engage in respective feasibility and design studies and get further involved once a specific project is deemed feasible and well-defined, evidently while maintaining a healthy US on-shore program in particle physics. This linking again to the above discussed FCC feasibility study.

The third recommendation refers to Belle II upgrades among others, as well as contributions towards SuperKEKB, where we recently discussed in our Advent Stories also CERNs’ respective involvement, e.g. through NA61 or WCTE.

The forth and fifth high-level recommendation discusses the required R&D programs considered as essential for a 20-year vision for the field of accelerators and detectors, considering major test facilities and demonstrator facilities within the next 10 years, here in particular offering strong support for pursuing R&D for a muon collider, a technically intimidating goal that could pay off with discoveries of new particles and interactions. Here together with Roberto and Prof. Donatella Lucchesi we had very recently an interesting discussion concerning required MDI studies, perhaps leading to a first respective involvement from our side.

For proponents of a muon collider, this P5 exercise was a bit like Christmas come early: Language in the report called for a “muon shot” akin to the “moonshot” of the U.S. Apollo program in the 1960s. Using similar language, early career researchers are excited about a muon collider, “not only because it is a paradigm shift in particle physics but also because it is a hard problem to solve.”

Confirming that our science community is not shy of humor, Vladimir told us that after the questions at the P5 meeting ended, the physicists made their way to a reception where the hosts cut into a cake decorated with a clear message: “SHOOT FOR THE MUON.”

Clearly very exciting times ahead, with our CERN Council starting first discussions on the next European Strategy for Particle Physics process.