The deadline for submitting inputs to the 2026 update of the European Strategy for Particle Physics (ESPP) passed on 31 March. A total of 263 submissions, ranging from individual to national perspectives, express the priorities of the high-energy physics community (see “Community inputs” figure). These inputs will be distilled by expert panels in preparation for an Open Symposium that will be held in Venice from 23 to 27 June (CERN Courier March/April 2025 p11).
Launched by the CERN Council in March 2024, the stated aim of the 2026 update to the ESPP is to develop a visionary and concrete plan that greatly advances human knowledge in fundamental physics, in particular through the realisation of the next flagship project at CERN. The community-wide process, which is due to submit recommendations to Council by the end of the year, is also expected to prioritise alternative options to be pursued if the preferred project turns out not to be feasible or competitive.
“We are heartened to see so many rich and varied contributions, in particular the national input and the various proposals for the next large-scale accelerator project at CERN,” says strategy secretary Karl Jakobs of the University of Freiburg, speaking on behalf of the European Strategy Group (ESG). “We thank everyone for their hard work and rigour.”
Two proposals for flagship colliders are at an advanced stage: a Future Circular Collider (FCC) and a Linear Collider Facility (LCF). As recommended in the 2020 strategy update, a feasibility study for the FCC was released on 31 March, describing a 91 km-circumference infrastructure that could host an electron–positron Higgs and electroweak factory followed by an energy-frontier hadron collider at a later stage. Inputs for an electron–positron LCF cover potential starting configurations based on Compact Linear Collider (CLIC) or International Linear Collider (ILC) technologies. It is proposed that the latter LCF could be upgraded using CLIC, Cool Copper Collider, plasma-wakefield or energy-recovery technologies and designs. Other proposals outline a muon collider and a possible plasma-wakefield collider, as well as potential “bridging” projects to a future flagship collider. Among the latter are LEP3 and LHeC, which would site an electron–positron and an electron–proton collider, respectively, in the existing LHC tunnel. For the LHeC, an additional energy-recovery linac would need to be added to CERN’s accelerator complex.
Future choices
In probing beyond the Standard Model and more deeply studying the Higgs boson and its electroweak domain, next-generation colliders will pick up where the High-Luminosity LHC (HL-LHC) leaves off. In a joint submission, the ATLAS and CMS collaborations presented physics projections which suggest that the HL-LHC will be able to: observe the H → µ+µ– and H → Zγ decays of the Higgs boson; observe Standard Model di-Higgs production; and measure the Higgs’ trilinear self-coupling with a precision better than 30%. The joint document also highlights the need for further progress in high-precision theoretical calculations aligned with the demands of the HL-LHC and serves as important input to the discussion on the choice of a future collider at CERN.
Neutrinos and cosmic messengers, dark matter and the dark sector, strong interactions and flavour physics also attracted many inputs, allowing priorities in non-collider physics to complement collider programmes. Underpinning the community’s physics aspirations are numerous submissions in the categories of accelerator science and technology, detector instrumentation and computing. Progress in these technologies is vital for the realisation of a post-LHC collider, which was also reflected by the recommendation of the 2020 strategy update to define R&D roadmaps. The scientific and technical inputs will be reviewed by the Physics Preparatory Group (PPG), which will conduct comparative assessments of the scientific potential of various proposed projects against defined physics benchmarks.
We are heartened to see so many rich and varied contributions
Key to the ESPP 2026 update are 57 national and national-laboratory submissions, including some from outside Europe. Most identify the FCC as the preferred project to succeed the LHC. If the FCC is found to be unfeasible, many national communities propose that a linear collider at CERN should be pursued, while taking into account the global context: a 250 GeV linear collider may not be competitive if China decides to proceed with a Circular Electron Positron Collider at a comparable energy on the anticipated timescale, potentially motivating a higher energy electron–positron machine or a proton–proton collider instead.
Complex process
In its review, the ESG will take the physics reach of proposed colliders as well as other factors into account. This complex process will be undertaken by seven working groups, addressing: national inputs; diversity in European particle physics; project comparison; implementation of the strategy and deliverability of large projects; relations with other fields of physics; sustainability and environmental impact; public engagement, education, communication and social and career aspects for the next generation; and knowledge and technology transfer. “The ESG and the PPG have their work cut out and we look forward to further strong participation by the full community, in particular at the Open Symposium,” says Jakobs.
A briefing book prepared by the PPG based on the community input and discussions at the Open Symposium will be submitted to the ESG by the end of September for consideration during a five-day-long drafting session, which is scheduled to take place from 1 to 5 December. The CERN Council will then review the final ESG recommendations ahead of a special session to be held in Budapest in May 2026.
