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.
Snapshots of the ongoing upgrades
So long and thanks for all the collisions!
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.
First petals installed in the ATLAS Inner Tracker Strips Endcap
A major milestone has been reached in the construction of the ATLAS Inner Tracker (ITk), with the installation of the first nine ‘petals’ of the ITk Strips Endcap at DESY in Germany. Installation began on 14 September, and on 28 September the ninth petal was successfully inserted into one of the endcap structures, marking the launch of the full-scale integration phase. Each wedge-shaped petal is around 60 cm long and carries six silicon detector modules on each side, with elements facilitating electronic, cooling and other infrastructure all mounted on one lightweight structure. A key part of the ITk detector, the Strips subsystem will feature two endcaps, each containing 192 petals arranged across six disks. The first nine petals that were installed in the endcap were assembled at DESY, with 184 remaining assembled petals due to be integrated. The second endcap is being assembled at Nikhef in the Netherlands, where a further 192 petals will be installed. Petal by petal, the endcaps are beginning to bloom – bringing the ITk closer to taking its place at the heart of ATLAS to explore the physics of HiLumi LHC.
890 metres of beam vacuum piping dismantled for HiLumi LHC
In August and September, CERN’s Beam Vacuum Operation team carried out the complete dismantling of all beam vacuum lines in sections of the LHC by ATLAS and CMS, to make space for the new HiLumi LHC. The teams used huge pliers to cut the piping into smaller pieces to allow for the removal, handling and sorting of the various components. This was done in close coordination with CERN’s Fire Brigade, particularly for the safe cutting and sorting of the approximately 7 m-long copper beam pipes. In parallel, close support was provided by the Transport team for the removal and transportation of 32 vacuum chambers, each approximately 116 kg and 6 m long. The teams dismantled more than 1,300 components, corresponding to a length of approximately 890 metres of beam vacuum piping! The removal of the complete vacuum system in these sections of the tunnel – including the beam vacuum piping but also its associated parts, like the connecting modules, support system, instrumentation and valves – forms part of the Vacuum group’s wider mission for the HiLumi LHC, which involves everything from ensuring an intact beam and insulation vacuum to carbon coating of many sections in contact with the beam.
ISOLDE prepares for a higher energy proton beam
At ISOLDE – CERN’s Isotope Separator On-Line facility – a major project of Long Shutdown 3 is to consolidate and upgrade the transfer line connecting the Proton Synchrotron Booster to ISOLDE. The transfer line was originally designed to operate with 1 GeV protons but has operated at 1.4 GeV since the 2000s. It is now being reconfigured to deliver 2 GeV proton beams to ISOLDE to increase exotic-isotope production yields. Some existing magnets will be reused, but new magnets are required to meet the upgraded beam-energy requirements. Several magnets have therefore been removed from the line – a handling operation that presented many challenges. After more than three decades of operation, the target area had accumulated shielding, services and handling systems, leaving very limited space and making some transport routes unusable. In addition, residual dose rates remained in some areas, requiring movable shielding and carefully planned handling procedures. Thanks to extensive preparation, ten magnets – ranging from smaller units to 8.5-tonne dipoles – were already successfully extracted. The photos show an 8.5-tonne dipole being prepared and transported on electrically powered rollers through a very narrow passage.
Upgrading the CMS muon chamber electronics
Work on the CMS muon system upgrade is now underway, with teams starting to install the new electronic system for the Drift Tube (DT) chambers. By 2028, all the 250 muon chambers in the CMS barrel will be equipped with the new mini crates housing the electronic boards to read out and control the DT chambers These boards digitise signals from muons passing through the DT with nanosecond-level precision and transmit the information via high-speed optical links to the backend electronics. The new architecture will contribute to improve the performance of the CMS trigger system for the High-Luminosity LHC era, which selects interesting collisions among the billions that occur every second at the heart of the detector. Without even waiting for collisions to return, the first DT chamber equipped with the new electronics has already recorded first real signals from muons produced by cosmic rays, particles that strike Earth’s atmosphere and generate showers of secondary particles. These represent the first real measurements from the future CMS DT system.
CMS BRIL subsystems successfully removed
All four CMS Beam Radiation, Instrumentation, and Luminosity (BRIL) subsystems have been fully extracted from the heart of CMS, following several years of reliable operation. Known as Margherita, Capriocciosa, Hawaii and Calzone, the modules had been placed around the beampipe to measure luminosity and beam conditions. The BRIL subsystems will be replaced with next-generation instrumentation designed to meet the challenging conditions of HiLumi LHC. In particular, the Fast Beam Condition Monitor (FBCM) will take their place as a standalone luminosity detector while also providing precise timing information, enabling improved measurement of background conditions associated with incoming particle beams. This will help enhance the quality of the data used by CMS researchers to investigate the particle collisions that take place inside the detector. Their removal also makes way for the extraction of the Pixel and Strip detectors due to take place over the coming weeks.
ElQA: ensuring electrical integrity in the LHC
The LHC has more than 1600 superconducting electrical circuits powering the magnets guiding the particle beams around the 27 km circumference of the accelerator. Every time the LHC stops for a major shutdown, all these circuits must be carefully checked for any problems. This is the job of the Electrical Quality Assurance (ElQA) team. The work consists of three phases. During the first phase, the sectors are tested under cryogenic conditions, carefully monitored during the warm-up and tested again at room temperature to assess the impact of several years of operation and the subsequent warm-up. In the second phase, the ElQA team supports LS3 activities in the LHC, notably the replacement of four magnets, the repair of 11 cold bypass diodes, and the installation of the new High-Luminosity magnets around the ATLAS and CMS experiments. Finally, at the end of LS3, each sector will again be tested before, during and after the cool-down of the magnets in order to find and fix any electrical problem as early as possible. The circuits will then be ready for hardware commissioning before restarting operation with beam.
77 Tonnes of ATLAS Shielding on the Move
On 22 July, CERN teams successfully completed the first major transport operation of LS3. A 77-tonne shielding component, known as the JFSU, was moved from the surface hall above the ATLAS experiment to Building 191 on CERN’s main site. The JFSU is a massive metal structure that forms part of the ATLAS radiation shielding. Two JFSUs are installed around the particle beamline on either side of the ATLAS detector, helping to protect muon chambers from background particles produced in secondary particle interactions. As part of the High-Luminosity LHC (HiLumi LHC) upgrade, the JFSU must be specially modified. From January 2027, a CERN robotic team will precision-machine additional slots into the component to route services to new vacuum units. Approximately one tonne of material will be removed during a three-month operation, with this first JFSU scheduled to return to ATLAS in April 2027. This complex logistical operation marked the start of the Collider-Experiment Interface activities (WP8) during Long Shutdown 3 (LS3) and brought together teams from WP8, the ATLAS Collaboration, CERN Radiation Protection, CERN Safety and CERN Heavy Transport. Similar transport operations will take place in coming years for the second JFSU and two other shielding elements.
The LHC is being warmed up from near absolute zero to room temperature
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
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
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
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
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
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
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
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
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!
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.
First petals installed in the ATLAS Inner Tracker Strips Endcap
A major milestone has been reached in the construction of the ATLAS Inner Tracker (ITk), with the installation of the first nine ‘petals’ of the ITk Strips Endcap at DESY in Germany. Installation began on 14 September, and on 28 September the ninth petal was successfully inserted into one of the endcap structures, marking the launch of the full-scale integration phase. Each wedge-shaped petal is around 60 cm long and carries six silicon detector modules on each side, with elements facilitating electronic, cooling and other infrastructure all mounted on one lightweight structure. A key part of the ITk detector, the Strips subsystem will feature two endcaps, each containing 192 petals arranged across six disks. The first nine petals that were installed in the endcap were assembled at DESY, with 184 remaining assembled petals due to be integrated. The second endcap is being assembled at Nikhef in the Netherlands, where a further 192 petals will be installed. Petal by petal, the endcaps are beginning to bloom – bringing the ITk closer to taking its place at the heart of ATLAS to explore the physics of HiLumi LHC.
