In the September CERN Council Session, we presented a revised schedule for Long Shutdown 3 (LS3) of the Large Hadron Collider (LHC) and its injector complex. This new schedule includes a delayed start and an increased duration for LS3.

LS3 for the LHC is now scheduled to begin at the start of July 2026, seven and a half months later than planned. The overall length of the shutdown will increase by around four months. Combined, these measures will shift the start of the High-Luminosity LHC (HL-LHC) (Run 4) by approximately one year, to June 2030. The injectors will continue to operate throughout July and August 2026 and start their extensive programme of work in September 2026. The injectors will gradually restart operations in 2028; a more detailed plan will be established in the coming weeks.

Complex challenges and collaborative solutions

The delayed start of LS3 is primarily due to significant challenges encountered during the Phase II upgrades of the ATLAS and CMS experiments, which have led to the erosion of contingency time and introduced considerable schedule risks. The challenges faced by the experiment teams include COVID-19 and the impact of the Russian invasion of Ukraine. For ATLAS, the new Inner Tracker (ITk) is on the critical path and, for CMS, the tracker and HGCAL projects are currently operating with low schedule contingency.

Preparing for high luminosity

On the machine side, a key activity during LS3 is the drilling of 28 vertical cores to link the new HL-LHC technical galleries to the LHC tunnel. Although initially expected to take six months, this timeframe was reduced to two months in 2021 to optimise the schedule. However, challenges encountered during the tendering process and in subsequent consultations with specialists necessitated a return to the original six-month timeline for core excavation.

The decision to shift the start of the HL-LHC by approximately one year and increase the length of the shutdown reflects a consensus supported by our scientific committees. The decision was reached through careful dialogue that began in the middle of this year, involving machine teams, experiments and the CERN Directorate.

Major upgrades: paving the way for future exploitation

LS3 represents a pivotal phase in enhancing CERN’s capabilities, involving substantial improvements aimed at transforming the LHC into the HL-LHC, as well as major upgrades to both the ATLAS and CMS experiments. These upgrades are designed to significantly enhance the luminosity and data collection capabilities of the experiments. They will enable a diverse and extensive physics programme that will ensure that CERN remains at the forefront of high-energy physics into the early 2040s.

In addition to high-luminosity enhancements, LS3 will involve a huge programme of work across the accelerator complex and encompass numerous critical projects, including:

• Phase 1 of the North Area consolidation project to enhance our long-term fixed target research capabilities. Work will start on transforming ECN3 into a high intensity fixed target facility.
   
• CNGS target dismantling to pave the way for enhanced wakefield acceleration research at AWAKE.
   
• ISOLDE improvements, including replacing ageing beam dumps, to enable long-term exploitation and boost the facility’s nuclear studies potential.
   
• Extensive maintenance and consolidation across all machines and facilities, including renovation of personnel safety systems and technical galleries, to ensure operational safety, longevity and availability.

All these activities are essential to ensuring the medium-term future of the Laboratory and allowing full exploitation of its remarkable potential in the coming decades.

The LS3 Readiness Review, which took place from 11 to 13 September, underscored the scale of this endeavour and our advanced state of preparedness, leveraging considerable experience from previous shutdowns, committed teams and stakeholder engagement to ensure the success of all these extensive upgrades and modifications.

As we reflect on the exceptional performance of our entire accelerator complex and its technical infrastructure – highlighted by the 110 fb^-1 collected by ATLAS and CMS so far this year – it is important to acknowledge that the extensive work performed during long shutdowns and technical stops is fundamental to our continued success.