The LHC is shut down because CERN is rebuilding it into the High Luminosity LHC. The stop is planned, not a failure. Run 3 ended on 27 June 2026, Long Shutdown 3 began in early July 2026, and the upgrade work runs about four years until the restart targeted for June 2030.
The Last Day of Run 3
The LHC's third run began in July 2022 and closed on 27 June 2026, when the final proton beams were dumped after a deliberately intense last season. Run 3 ran at a record collision energy of 13.6 TeV and delivered more collision data than the first two runs combined. That dataset will keep the analysis teams busy for years, which matters, because the machine itself now goes quiet for a while.
What "Long Shutdown 3" Means
CERN alternates running periods with long shutdowns. Run 1 was followed by Long Shutdown 1 in 2013, Run 2 by Long Shutdown 2 in 2019. Each pause is used for repairs, consolidation and upgrades that cannot be done while 36,000 tonnes of equipment sits at 1.9 kelvin. LS3 is the third and by far the largest of these. Its defining job is installing the High Luminosity upgrade, a project that has been in design and construction since the 2010s.
The Work List
A short version of what happens underground between 2026 and 2030:
- Warm up. The full cryogenic chain, more than 36,000 tonnes of it across 27 kilometres, is brought from 1.9 K to room temperature so people can work on it.
- Dismantle and rebuild 1.2 km of accelerator. The insertion regions around ATLAS and CMS come out. New niobium tin focusing magnets, new service galleries and new superconducting power links go in.
- Install crab cavities. Superconducting cavities that tilt the proton bunches at the collision points, recovering overlap that would otherwise be lost to the beam crossing angle.
- Upgrade protection systems. New collimators to absorb stray particles, because the upgraded beams carry roughly 700 megajoules, about double what Run 3 handled.
- Rebuild the big detectors. CMS replaces its entire silicon tracker with more than four thousand new sensor modules and adds a high granularity endcap calorimeter. ATLAS installs an all new inner tracker. Both add precision timing detectors to cope with the pileup of overlapping collisions.
- Consolidate everything else. Thousands of routine maintenance jobs across the injectors, electrical systems, ventilation and safety infrastructure that only a long stop makes possible.
Why It Takes Four Years
Two reasons: thermodynamics and scale. You cannot open a superfluid helium cryostat on a whim. Warming the machine takes months, cooling it back down takes months more, and everything in between has to happen in that window. The scale is the other half. The HL-LHC components were built over a decade by labs on three continents, and installing them means civil works, magnet transport through narrow shafts, kilometres of new cabling, and tens of thousands of high precision connections, every one of which gets tested before cooldown.
For comparison, LS1 took two years mostly to fix magnet interconnects. LS2 took three to upgrade the injectors. LS3 rebuilds the collider's heart. Four years is the compressed version of that job.
Is Anything Still Running at CERN?
Yes. The laboratory does not sleep during a shutdown. Data analysis from Run 3 continues at full speed, fixed target experiments and test facilities keep their own schedules, and from mid 2028 the injector complex restarts to recommission beams well before the LHC needs them. The shutdown is quiet only in the tunnel itself, and even there it is the opposite of quiet.
How to Follow the Shutdown
LS3 will mostly be invisible from the surface, but it is trackable if you know where to look. CERN publishes regular accelerator reports describing progress in the tunnel, the experiments post updates as detector components arrive and get lowered down their shafts, and the big milestones, first magnet out, first triplet in, start of cooldown, tend to get proper announcements. Our approach on this site is to keep the countdown and shutdown progress bar current and to reflect schedule changes the day they are published rather than speculating between announcements.
The years to watch: 2027 and 2028 are peak installation, mid 2028 brings the injector restart and the first beams anywhere in the complex since the stop, and 2029 into 2030 is cooldown and powering tests, the quiet months where four years of work either proves itself or sends engineers back down the shaft.
When It Ends
LS3 ends when the machine is cold, powered and ready for beam, currently targeted for June 2030. From that point the LHC is no longer the machine that found the Higgs boson in 2012. It is the High Luminosity LHC, designed to produce collision data about ten times faster over its lifetime. The countdown on our homepage tracks exactly this moment, and the restart guide covers what first beams will look like.