The Long Shutdown 3

Countdown to LS3

By Jean-Philippe Tock for the LS3 Coordination team

Countdown to LS3 (3-4 September 2025): Overview · Indico

The third Long Shutdown (LS3) marks a critical milestone for CERN, enabling installation and commissioning of the High-Luminosity LHC equipment for the accelerator and the phase II upgrades of the ATLAS and CMS experiments. This period will also see a range of essential projects: treating LHC beam screens to mitigate heat from electron clouds, consolidating the North Area, preparing for the SHiP detector installation, dismantling the CNGS Target Area for the next AWAKE runs, and replacing the ISOLDE beam dumps. In addition, a comprehensive maintenance and consolidation programme will address all systems, with a focus on the Personal Safety Systems and technical infrastructure.

On 3–4 September, around 200 physicists, engineers, and technicians from all groups involved in LS3 gathered for the “Countdown to LS3” event to review progress and preparedness. https://indico.cern.ch/event/1486762/

This followed the LS3 Readiness Review held a year earlier: https://indico.cern.ch/event/1387050/, which revealed that the estimated duration of LHC interventions exceeded the allocated time. A delay of about one year for the start of Run 4 was requested by ATLAS and CMS to have a robust schedule. After analysing all inputs, the long-term schedule was reworked: through a CERN-wide effort, the LS3 schedule for the LHC has since been optimised to fit within 47 months, as confirmed at the recent event, though some challenges remain. Similar optimisation work is ongoing for the injector complex.

Safety has been a top priority, with a firm and pragmatic approach to reinforcing safety culture and ensuring compliance with CERN’s LS3 objectives. Significant progress in space and logistics management will be reviewed in December to ensure safe and efficient operations.

As the start of LS3 approaches, the scale of preparation is unprecedented. Success will depend on meticulous planning, resource allocation, and the flexibility and commitment of all stakeholders; CERN staff, collaborators, suppliers, and contractors. The journey to Run 4 will be demanding, but the goal is within reach.

As Mike Lamont, ATS Director, emphasised in his opening talk: “LS3 will secure the accelerator complex for the next generation, with the HL-LHC as its flagship into the 2040s — ensuring a resilient, high-performing complex and a rich, diverse physics programme.”

More information can be found on the CERN Long Shutdown 3 Website: https://ls3c.web.cern.ch/