The road to HiLumi

As of mid-2026, the accelerator complex and experimental facilities at CERN are undergoing major maintenance and upgrade work. This phase, known as “Long Shutdown 3” or “LS3”, is expected to last up to four years and will involve thousands of experts across CERN’s sites and tunnels. The work will cover civil engineering, infrastructure consolidation, upgrades of facilities and the installation of cutting-edge technology.

The flagship project is the High-Luminosity Large Hadron Collider (HiLumi LHC), a major upgrade to dramatically improve the performance of the LHC, CERN’s largest accelerator. Over 1.2 kilometres of the LHC are being dismantled and replaced by new systems to increase the luminosity (the number of particle collisions).

The LHC experiments will be greatly improved with the installation of new subdetectors and revolutionary new systems, ready for the challenges and opportunities that the increased number of collisions will bring.

The entire accelerator complex will be shut down for maintenance, consolidation and upgrades. This work will ensure the high level of availability and reliability of particle beams that is required to meet the ambitious physics objectives of the next operating period. Several experiment facilities are being renovated, such as the North Area Hall, which houses many fixed-target experiments supplied by the SPS, CERN’s second largest accelerator. A significant improvement programme is also under way at ISOLDE, the nuclear physics facility. The accelerator complex and its experiment facilities will gradually resume operation from mid-2028 onwards.

The whole undertaking is a major logistical challenge, and CERN’s underground facilities will be with a hive of activity. Safety remains the top priority throughout this busy period.

Planned shutdowns like the LS3 are an integral part of the lifecycle of particle accelerators, especially those that operate at near absolute-zero temperatures. These periods group allow preventive and corrective maintenance, consolidation and upgrades to be carried out at the same time.

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HiLumi LHC

Accelerator complex

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Snapshots of the ongoing upgrades

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So long and thanks for all the collisions!

Date: 29 June 2026

On Saturday 27 June at 6 am, the LHC’s Page 1 — the accelerator’s dashboard — said “goodbye” to collisions. The LHC operators dumped the last beams before the accelerator’s metamorphosis. The LHC is now entering a major upgrade phase: four years during which the world’s most powerful collider will be transformed into an even better performing machine. The other CERN accelerators will continue running until the end of August before entering their third long shutdown.

The LHC is being warmed up from near absolute zero to room temperature

Date: 05 August 2026

For only the third time since it began operating in 2008, the Large Hadron Collider (LHC) is slowly being returned to room temperature after being kept at close to -271°C, just above absolute zero. With the LHC at room temperature, technicians and engineers will be able to work on the accelerator as it is transformed into the HiLumi LHC during Long Shutdown 3 (LS3). Warming up more than 36 000 tonnes of cryogenic equipment across 27 kilometres of tunnel requires meticulous planning at every step. The cryogenics team has been running the warm-up sequence since LS3 began on 29 June 2026. Once the electrical lockout of the LHC had been successfully completed, the first step was to remove the liquid helium stored inside the magnets and the cryogenic line. The technicians used heaters to evaporate the helium, which was then sent to the surface to be liquefied and stored ready to be used again once the accelerator resumes operation in mid-2030. With the helium removed, large heaters are now gradually raising the temperature of the magnets while a control system keeps the process safe.

First major structure of the future CMS tracker arrives at CERN

Date: 04 August 2026

The first major mechanical structure of the future CMS tracker has arrived at CERN, as preparations continue for the High-Luminosity LHC (HiLumi LHC). The current tracker needs replacing to prepare the detector for the higher collision rates expected at the HiLumi LHC. Built and assembled by teams at the IPHC (CNRS/University of Strasbourg) in France, the Tracker Barrel 2S (TB2S) wheel is a 2.3-metre-long structure that forms the outer mechanical framework of the CMS detector’s future tracker. Designed from lightweight carbon-fibre composite materials, the TB2S wheel combines high mechanical precision with the strength needed to support hundreds of kilograms of detector components. Starting from 2027, more than 4 thousand 2S-type silicon sensor modules, each consisting of two silicon microstrip sensors separated by 1.8 mm, will be installed on the structure. The project was launched in 2017, with the journey from Strasbourg to CERN taking place in July in a specially designed transport container. It will now undergo further measurements before silicon modules are integrated. This structure will become the backbone of the new CMS tracker, enabling the experiment to record the unprecedented volume of data that will be produced in the HiLumi LHC era.

HGCAL keeps its cool in major testing milestone

Date: 28 July 2026

One of the largest structures of CMS’s future High Granularity Calorimeter (HGCAL) has successfully completed a demanding series of tests, marking an important milestone on the road to the High-Luminosity LHC and the next era of particle physics research. HGCAL will replace the existing endcap calorimeters of the CMS experiment, allowing particle showers to be reconstructed in unprecedented detail despite the challenging conditions expected. The ‘cold tests’ on the main supporting structure of the HGCAL went smoothly, with no overheating, condensation, or leaks recorded! This structure will function like a giant, highly controlled freezer: the inside of the structure must be kept at -35°C while the outside should remain close to room temperature. The HGCAL active elements, called cassettes, will be slotted into the large absorber structures made of stainless steel plates and will need this much colder temperature to function optimally for more than a decade. This is the first of two absorber structures to undergo these rigorous temperature tests.

ATLAS beam vacuum assembly enters retirement

Date: 24 July 2026

After more than a decade of service, one of the LHC’s key components for guiding particle beams through the ATLAS experiment has been removed, making way for the High-Luminosity LHC era. Installed during Long Shutdown 1, this 5.8-metre-long aluminium assembly connected the ATLAS big muon wheel to the endcap toroid, providing the vacuum environment the particle beams needed to travel between them. It replaced an earlier stainless-steel design, helping to reduce radiation exposure and minimise unwanted background signals in the ATLAS experiment. Extracting the 300 kg assembly was a delicate operation. Suspended from the surface hall crane over 80 metres above it, the assembly had to be carefully manoeuvred through the experiment with only a few centimetres of clearance from surrounding components, all while teams worked in a radiologically challenging environment. As this second-generation assembly retires, a new design developed for the High-Luminosity LHC will take its place. Its removal marks the end of a successful chapter for both ATLAS and the LHC, celebrating more than a decade of the assembly’s reliable operation and the efforts of the many people who brought it from design to reality.

New Faraday cages for HiLumi sensitive electronics

Date: 23 July 2026

Four Faraday cages have been installed in the HiLumi LHC underground galleries to host the electronics that control and drive the new crab cavities (devices to tilt the beams before the interaction points to maximise crossing). As this system needs to react to the picosecond scale, the utmost precision is required and this can only be reached in an environment without disruption from electromagnetic fields coming from other systems. The Faraday cages provide such shielding, allowing the control electronics to work without external perturbations. As well as the metallic panels to shield against electromagnetic fields, there are two other insulating layers: one made of rock wool for insulating temperature to keep radio frequency signals stable, and a sound-absorbing layer.

ATLAS High-Level Trigger farm on the move

Date: 15 July 2026

A major relocation campaign has been launched to dismantle one of the ATLAS experiment’s key computing systems and transport it from LHC Point 1 to its new home at LHC Point 8, the site of the LHCb experiment. The ATLAS HLT farm is a dense computing installation where hundreds of servers process the vast amounts of data ATLAS produces. Teams coordinated by the CERN IT department are now dismantling these units, securing them to pallets and lifting them by crane through the intricate network of cables and services supporting the experiment. The move is driven by the extensive infrastructure upgrades required for HiLumi LHC. As Point 1 undergoes major reconfiguration, space and power constraints mean that the HLT computing infrastructure must find a new home. Through a collaboration between ATLAS, LHCb and the CERN IT department, the servers will be installed in a dedicated container at Point 8, extending the lifetime of valuable equipment and reducing electronic waste. 240 of the 416 multi-node server chassis have already been removed, marking the start of an operation to relocate all units by the end of July. The next phase will focus on installation and preparing the system for network commissioning in autumn.

ISOLDE beam dumps successfully dismantled

Date: 09 July 2026

ISOLDE has reached a major milestone in its ISOLDE Improvement Programme with the dismantling of its two beam dumps. Designed in the early 1990s, this equipment had seen the facility through more than three decades of operation. As the energy and intensity of the beam delivered by the Proton Synchrotron Booster gradually increased, the dumps were pushed to their thermal and mechanical limits, making their replacement essential. Installed downstream of the target stations, the dumps consisted of massive steel blocks shielded by concrete blocks and 10 metres of earth. Extracting them was no mean feat: not only were they highly radioactive, but they were also buried under 8000 m3 of earth. Over the course of three years of studies carried out in collaboration with several groups, the radiological characterisation of the soil was completed (HSE-RP) and excavation and dismantling protocols were drawn up (SCE-PPM and SY-STI, respectively). The most radioactive components were then successfully removed using heavy-duty handling equipment (EN-THE) and robotic systems (BE-CEM). Work will now focus on the construction of an underground technical building that will provide access to the new water-cooled beam dumps, which are currently in production.

Delivering power to the HiLumi magnets

Date: 07 July 2026

The HiLumi LHC’s new magnets will be powered by very high currents. These currents need to be transferred without losses between power converters operating at room temperature in underground galleries and superconducting magnets at extremely cold temperatures. This is achieved through an innovative system called the Cold Powering System which uses high-temperature and intermediate-temperature superconductors to transport the current over around 100 metres. Discover how it works.

Safety kicks off the LHC’s transformation

Date: 30 June 2026

On 29 June, the doors of the LHC opened for a major phase of work that will transform the collider into the HiLumi LHC, an even better performing machine. The initial interventions focus on securing the accelerator, particularly from an electrical standpoint. To minimise risk as much as possible, the electrical circuits are locked out that is, powered down and secured. A team intervened as early as this afternoon at Point 1 of the LHC to carry out this safety procedure. Similar interventions will take place across the entire LHC in the coming days, and then, in September, throughout the rest of the accelerator complex.

So long and thanks for all the collisions!

Date: 29 June 2026

The LHC is being warmed up from near absolute zero to room temperature

Date: 05 August 2026

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