The third update of the European strategy for particle physics, launched by the CERN Council on 21 March, is getting into its stride. At its June session, the Council elected former ATLAS spokesperson Karl Jakobs (University of Freiburg) as strategy secretary and established a European Strategy Group (ESG), which is responsible for submitting final recommendations to Council for approval in early 2026. The aim of the strategy update, states the ESG remit, is to develop “a visionary and concrete plan that greatly advances human knowledge in fundamental physics through the realisation of the next flagship project at CERN”.
“Given the long timescales involved in building large colliders, it is vital that the community reaches a consensus to enable Council to take a decision on the next collider at CERN in 2027/2028,” Jakobs told the Courier. To reach that consensus it is important that the whole community is involved, he says, emphasising that, compared to previous strategy updates, there will be more opportunities to provide input at different stages. “There is excellent progress with the LHC and no new indication that would change our physics priorities: understanding the Higgs boson much better and exploring further the energy frontier are key to the next project.”
The European strategy for particle physics is the cornerstone of Europe’s decision-making process for the long-term future of the field. It was initiated by the CERN Council in 2005, when completing the LHC was listed as the top scientific priority, and has been updated twice. The first strategy update, adopted in 2013, continued to prioritise the LHC and its high-luminosity upgrade, and stated that Europe needed to be in a position to propose an ambitious post-LHC accelerator project at CERN by the time of the next strategy update. The second strategy update, completed in 2020, recommended an electron–positron Higgs factory as the highest priority, and that a technical and financial feasibility study for a next-generation hadron collider should be pursued in parallel.
Significant progress has been made since then. A feasibility study for the proposed Future Circular Collider (FCC) at CERN presented a mid-term report in March 2024, with a final report expected in spring 2025 (CERN Courier March/April 2024 pp25–38). There is also a clearer view of the international landscape. In December 2023 the US “P5” prioritisation process stated that the US would support a Higgs factory in the form of an FCC-ee at CERN or an International Linear Collider (ILC) in Japan, while also exploring the feasibility of a high-energy muon collider at Fermilab (CERN Courier January/February 2024 p7). Shortly afterwards, a technical design report for the proposed Circular Electron Positron Collider (CEPC) in China was released (CERN Courier March/April 2024 p39). The ILC project has meanwhile established an international technology network in a bid to increase global support.
Alternative scenarios
In addition to identifying the preferred option for the next collider at CERN, the strategy update is expected to prioritise alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive. “That we should discuss alternatives to the chosen baseline is important to this strategy update,” says Jakobs. “If the FCC were chosen, for example, a lower-energy hadron collider, a linear collider and a muon collider are among the options that would likely be considered. However, in addition to differences in the physics potential we have to understand the technical feasibility and the timelines. Some of these alternatives may also require an extension of the physics exploitation at the HL-LHC.”
Given the long timescales involved in building large colliders, it is vital that the community reaches a consensus
The third strategy update will also indicate physics areas of priority for exploration complementary to colliders and add other relevant items, including accelerator, detector and computing R&D, theory developments, actions to minimise environmental impact and improve the sustainability of accelerator-based particle physics, initiatives to attract, train and retain early-career researchers, and public engagement.
The particle-physics community is invited to submit written inputs by 31 March 2025 via an online portal that will appear on the strategy secretariat’s web page. This will be followed by a scientific open symposium from 23 to 27 June 2025, where researchers will be invited to debate the future orientation of European particle physics. A “briefing book” based on the input and discussions will then be prepared by the physics preparatory group, the makeup of which was to be established by the Council in September before the Courier went to press. The briefing book will be submitted to the ESG by the end of September 2025 for consideration during a five-day-long drafting session, which is scheduled to take place from 1 to 5 December 2025. To allow the national communities to react to the submissions collected by March 2025 and to the content of the briefing book, they are offered further opportunities for input both ahead of the open symposium (with a deadline of 26 May 2025) and ahead of the drafting session (with a deadline of 14 November 2025). The ESG is expected to submit the proposed strategy update to the CERN Council by the end of January 2026.
“The timing is well chosen because at the end of 2025 we will have a lot of the relevant information, namely the final outcome of the FCC feasibility study plus, on the international scale, an update about what is going to happen in China,” says Jakobs. “The national inputs, whereby national communities are also invited to discuss their priorities, are considered very important and ECFA has produced guidelines to make the input more coherent. Early-career researchers are encouraged to contribute to all submissions, and we have restructured the physics preparatory group such that each working group has a scientific secretary who is an early-career researcher. We look forward to a very fruitful process over the forthcoming one and a half years.”
