The Chamonix Workshop 2025, held from 27 to 30 January, brought together CERN's accelerator and experimental communities to reflect on achievements, address challenges and chart a course for the future.
As the discussions made clear, CERN is at a pivotal moment. The past decade has seen transformative developments across the accelerator complex, while the present holds significant potential and opportunity.
Reviewing accelerator operations
The workshop opened with a review of accelerator operations, supported by input from December's Joint Accelerator Performance Workshop. Maintaining current performance levels requires an extraordinary effort across all the facilities. Performance data from the ongoing Run 3 shows steady improvements in availability and beam delivery. These results are driven by dedicated efforts from system experts, operations teams and accelerator physicists, all working to ensure excellent performance and high availability across the complex.
Attention is now turning to Run 4 and the High-Luminosity LHC (HL-LHC) era. Several challenges have been identified, including the demand for high-intensity beams, electron cloud effects and radiofrequency (RF) power limitations. Measures to address these issues will be implemented during Long Shutdown 3 (LS3), ensuring CERN's accelerators continue to meet the demands of its diverse physics community.
Preparing for Long Shutdown 3 and the High-Luminosity LHC
LS3 will be a crucial period for CERN. In addition to the deployment of the HL-LHC and major upgrades to the ATLAS and CMS experiments, it will see a widespread programme of consolidation, maintenance and improvements across the accelerator complex to secure future exploitation over the coming decades.
Progress on the HL-LHC upgrade was reviewed in detail, with a focus on key systems - magnets, cryogenics and beam instrumentation - and on the construction of critical components such as crab cavities. The next two years will be decisive, with significant system testing scheduled to ensure these technologies meet ambitious performance targets.
Planning for LS3 is already well advanced. Coordination between all stakeholders has been key to aligning complex interdependencies, and the experienced teams are making strong progress in shaping a resource-loaded plan. The scale of LS3 will require meticulous coordination, but it also represents a unique opportunity to build a more robust and adaptable accelerator complex for the future.
A diverse and evolving physics programme
Looking beyond LS3, CERN's unique accelerator complex is well positioned to support an increasingly diverse physics programme. This diversity is one of CERN's greatest strengths, offering complementary opportunities across a wide range of fields.
The high demand for beam time at ISOLDE, n_TOF, AD-ELENA and the North and East Areas underscores the need for a well-balanced approach that supports a broad range of physics. The discussions highlighted the importance of balancing these demands while ensuring that the full potential of the accelerator complex is realised.
Future opportunities such as those highlighted by the Physics Beyond Colliders study will be shaped by discussions being held as part of the update of the European Strategy for Particle Physics (ESPP). Defining the next generation of physics programmes entails striking a careful balance between continuity and innovation, and the accelerator community will play a central role in setting the priorities.
The Future Circular Collider and long-term vision
A forward-looking session at the workshop focused on the Future Circular Collider (FCC) Feasibility Study and the next steps. The physics case was presented alongside updates on territorial implementation and civil engineering investigations and plans. How the FCC-ee injector complex would fit into the broader strategic picture was examined in detail, along with the goals and deliverables of the pre-technical design report (pre-TDR) phase that is planned to follow the Feasibility Study's conclusion.
While the FCC remains a central focus, other future projects were also discussed in the context of the ESPP update. These include mature linear collider proposals, the potential of a muon collider and plasma wakefield acceleration. Development of key technologies, such as high-field magnets and superconducting RF systems, will underpin the realisation of future accelerator-based facilities.
Looking to the future
The next steps - preparing for Run 4, implementing the LS3 upgrade programmes and laying the groundwork for future projects - are ambitious but essential. CERN's future will be shaped by how well we seize these opportunities.
The shared expertise and dedication of CERN's personnel, combined with a clear strategic vision, provide a solid foundation for success. The path ahead is challenging, but with careful planning, collaboration and innovation, CERN's accelerator complex will remain at the heart of discovery for decades to come.
