The European Strategy for Particle Physics is the cornerstone of Europe’s decision-making process for the long-term future of the field. Mandated by the CERN Council, it is formed through a broad consultation of the grass-roots particle physics community, it actively solicits the opinions of physicists from around the world, and it is developed in close coordination with similar processes in the US and Asia to ensure coordination between regions and optimal use of resources globally.
The European Strategy process was initiated by the CERN Council in 2005, resulting in a document being adopted by the Council in 2006. This document placed the LHC at the top of European particle physics’ scientific priorities, with a significant luminosity upgrade already being mooted. A ramp-up of R&D into future accelerators also featured high on the priority list, followed by coordination with a potential International Linear Collider (ILC), and participation in a global neutrino programme. The original Strategy also foresaw increased collaboration with neighbouring fields, such as astro-particle and nuclear physics, and recognised the importance of complementary issues such as communications and technology transfer.
First update
The original European Strategy prescribed regular updates to take into account the evolution of the field. The first of these was prepared in 2012 and adopted in 2013. By this time, the LHC had proved its capacity with the discovery of the long-sought Higgs boson, evidence for the Brout-Englert-Higgs mechanism through which fundamental particles acquire their mass.
Again, it came as no surprise that the LHC topped the list of scientific priorities for European particle physics, with the high-luminosity upgrade (HL-LHC) increasing in importance, and preparations for the post-LHC future taking shape. “Europe”, said the Strategy document, “needs to be in a position to propose an ambitious post-LHC accelerator project at CERN by the time of the next Strategy update.”
Second update
As the second update of the European Strategy got underway, in 2018, the stakes were high. Europe, in collaboration with partners from around the world, was engaged in R&D projects for a range of ambitious post-LHC facilities at CERN under the Compact Linear Collider (CLIC) and Future Circular Collider (FCC) umbrellas. It was time to check progress on these, matching their expected performance to physics needs as well as compared to other projects being considered beyond Europe: the ILC project in Japan, and the Circular Electron-Positron Collider (CEPC) in China.
In other areas of particle physics, much had changed since the first strategy update. Europe, through CERN, was contributing fully to a globally coordinated neutrino programme with experiments to be carried out in the US and Japan. The ILC, which would be complementary to the LHC, remained on the table with a site having been identified in Japan and a decision on whether to go forward eagerly anticipated. There were ambitious plans to build a large collider in China. And at CERN, a study to investigate the potential for physics beyond colliders, maximising the potential for CERN’s unique accelerator complex, was launched in 2016.
A call for input from the entire physics community received nearly 200 submissions. These were discussed during an Open Symposium in Granada in May 2019, distilled into a Physics Briefing Book, and turned into final recommendations during a week-long drafting session held in Germany in January 2020. The Strategy was approved by the CERN Council in June 2020. It placed priority on the successful completion of the HL-LHC over the coming decade and began to map out the potential landscape for research in Europe in the post LHC era. The Strategy update recommended a so-called Higgs factory as the highest priority to follow the LHC, while pursuing a technical and financial feasibility study for a next-generation hadron collider in parallel, in preparation for the long-term. Maintaining existing European support for neutrino physics in the US and Japan was strongly recommended, and the comprehensive report also covered areas of synergy with neighbouring fields and addressed societal aspects ranging from training and knowledge transfer to minimising the environmental footprint of future facilities.
Third update
The third update of the European Strategy for Particle Physics was launched by the CERN Council in June 2024. One of its main goals is to develop a visionary and concrete plan that greatly advances knowledge in fundamental physics through the realisation of the next flagship collider at CERN, and to prioritize alternative options to be pursued if the preferred plan turns out not to be feasible or competitive.
Since the last update, excellent progress has been made at CERN and beyond in preparation for future colliders. A Feasibility Study for the realisation of the FCC presented a mid-term report in March 2024 and a final report is expected to become available in early 2025. In addition, there is a clearer view of the international landscape for future colliders: in December 2023 the US concluded a similar (“P5”) prioritisation process and a Technical Design Report for the CEPC project in China was released. The ILC project has established an International Technology Network.
Given the long timescales involved in realizing new large-scale projects and the importance of long-term community engagement, the next update of the European Strategy process is timely. Through close consultation of the particle-physics community across universities, laboratories and national institutes, the third Strategy -- outlining the brightest future for the field -- is expected to conclude in January 2026.