CERN Large Hadron Collider Enters Long Shutdown 3 for HiLumi Upgrade

CERN has officially switched off the Large Hadron Collider (LHC) to begin Long Shutdown 3 (LS3), a comprehensive maintenance and upgrade program designed to transition the facility into the High-Luminosity LHC (HiLumi LHC). This transition is essential for increasing the collider's luminosity by up to ten times, enabling researchers to collect larger datasets for precision studies of the Higgs boson and the search for physics beyond the Standard Model.

The Transition to High-Luminosity LHC (HiLumi LHC)

The HiLumi LHC, scheduled to begin operations in 2030, will significantly enhance the discovery potential of the accelerator by increasing the rate of particle collisions. This upgrade allows for a deeper exploration of the fundamental laws of nature and more precise measurements of known particles.

Key Infrastructure Upgrades

LS3 is the most extensive intervention on the accelerator complex since the LHC's original construction. The scope of work includes:

• Magnet Replacement: Approximately 1.2 km of magnets and components will be removed and replaced with new equipment.
• Facility Renovation: Projects include the consolidation of the Super Proton Synchrotron (SPS) North Area and the renovation of the ISOLDE facility.
• Target Area Transformation: The CERN Neutrinos to Gran Sasso (CNGS) target area will be dismantled, and the Experimental Cavern North 3 (ECN3) will be converted into a high-intensity fixed-target facility.
• Systems Consolidation: Updates to personnel safety systems, the electrical network, and technical galleries.

Detector Enhancements for ATLAS and CMS

The ATLAS and CMS experiments are undergoing extensive upgrades to handle the increased collision density. While the previous run averaged around 60 proton-proton collisions per bunch crossing, the HiLumi LHC will produce between 140 and 200.

To manage more than five billion interactions per second, these detectors are being equipped with:

• New Trigger Systems: Entirely new systems to identify and select the most promising events for analysis.
• Advanced Tracking: All-silicon tracking systems featuring billions of readout channels.
• High-Precision Timing: Detectors with resolutions measured in tens of picoseconds.
• High-Rate Calorimeters: New systems capable of operating at megahertz rates.

Legacy of the LHC (2008–2026)

Since its first beams in September 2008 and first collisions in 2009, the LHC has served as a primary engine for discovery in high-energy physics. Its most significant achievement was the 2012 discovery of the Higgs boson, confirming a decades-old theoretical mechanism.

Other major scientific contributions include:

• The discovery of more than 85 hadrons.
• Establishing exclusion limits for new particles.
• Research into the matter-antimatter imbalance.
• Exploration of quark-gluon plasma.
• Advancements in superconducting technologies and global computing infrastructure.

Operational Continuity and Public Access

Although no particle beams will circulate until the gradual restart beginning in 2028, scientific activity continues. Researchers are currently analyzing the exabyte-scale datasets accumulated during previous runs to extract new physics results.

Visiting CERN during LS3

Community discussion highlights that Long Shutdowns provide unique opportunities for public access. Because the beams are off, visitors can often access areas usually closed to the public, including the LHC tunnel and elevators to the underground caverns.

"Long Shutdowns are the only moments where a lot of usually inaccessible places can be visited... including going down in the LHC tunnel."

Technical Perspectives and Community Insights

Technical contributors and observers have noted the sheer scale of the data and hardware challenges associated with the upgrade. One contributor noted the massive jump in channel capacity for the ATLAS Inner Tracker (ITK), moving from 8 million channels in previous iterations to 5 billion channels for the new system.

From a historical perspective, some observers have compared the LHC's trajectory to the cancelled Superconducting Super Collider (SSC). While the SSC would have had higher beam energy (20TeV vs 7TeV), the LHC's successful international collaboration and long-term funding model ensured the discovery of the Higgs boson and the continued evolution of particle physics.