The accelerator complex at CERN consists of a series of machines that progressively increase the energy of particles. Each machine accelerates the particle beam to higher energy levels before passing it on to the next machine in the sequence. In the Large Hadron Collider (LHC), the final stage in this chain, particle beams are accelerated to a record energy of 6.8 TeV per beam.

The LHC is situated approximately 100 meters underground on average and spans a circumference of 26.7 kilometers. It houses four major experiments designed to explore fundamental physics. The LHC features two separate beam pipes, each running through the same cold mass, with a gap of just 19.4 cm between them. To maintain the superconducting state of the magnets, the LHC requires 150 tons of liquid helium, which is used to keep the magnets at extremely low temperatures, approximately 1.9 Kelvin (-271.25°C ), which is just a few degrees above absolute zero.

To fill the Large Hadron Collider (LHC) with protons and ions, multiple accelerators are used in sequence. For protons, the process begins with Linear Accelerator 4 (Linac4), which accelerates negative hydrogen ions (H-) to 160 MeV. These ions are then injected into the Proton Synchrotron Booster (PSB), where they are stripped of their electrons, leaving only protons. The protons proceed through the Proton Synchrotron (PS) and the Super Proton Synchrotron (SPS) before reaching the LHC for high-energy collisions. For heavy ions, the process starts with Linac3, which accelerates lead ions and injects them into the Low Energy Ion Ring (LEIR). After passing through the PS and SPS, the ions are injected into the LHC.

CERN's long-term schedule includes a series of planned accelerator runs, technical stops, and long shutdowns for maintenance and upgrades. CERN has recently revised its schedule for the accelerator complex, extending the LHC's third physics data-taking period (Run 3) until July 2026. Following this, the third Long Shutdown (LS3) will begin, delayed by seven and a half months, to accommodate additional civil engineering work for the High-Luminosity LHC (HL-LHC). The LS3 will involve the installation of key upgrades, including more powerful focusing magnets, crab cavities, superconducting transmission lines, and an enhanced protection system. During this time, the ATLAS and CMS experiments will replace significant portions of their detectors and electronics. These changes will ensure that the HL-LHC, which will enable higher luminosity and more collisions, is ready to begin operations in June 2030.

CERN’s long-term schedule includes periodic technical stops to maintain its accelerator complex. During the second quarter (Q2), a one-week technical stop is scheduled to carry out interventions, for example on the RF cavities. These interventions typically involve emptying the cavities of liquid helium, refilling them, and cooling them again. During the same period, there is also a 30-hour technical stop in the injectors. In the fourth quarter (Q4), the focus shifts to a technical stop solely in the Large Hadron Collider (LHC). This stop is necessary to convert the experimental setups from proton running to ion running, with the last few weeks of the year dedicated to running lead ions in the LHC. Meanwhile, proton beams continue to be accelerated in the injector complex.

The Large Hadron Collider (LHC) is filled with beams in a precise manner to ensure high-energy collisions. Each bunch of beams contains 160 billion particles, with a 25-nanosecond bunch spacing, meaning that the bunches are spaced 25 nanoseconds apart as they travel around the collider ring. However, only a small fraction of these particles-typically around 60 out of the 160 billion— actually collide when the beams cross. These collisions produce a variety of secondary particles, which are then detected and analyzed to study fundamental physics. After each collision, the number of particles decreases, resulting in fewer particles remaining in the beam for the following collisions.

The typical operation of the LHC consists of several phases, each with its own purpose and duration. The injection phase lasts 30 to 45 minutes, during which the particles are injected into the accelerator ring and accelerated to the required energy. The ramp phase follows, lasting about 30 minutes, during which the energy of the particle bunches is gradually increased to the desired level. The squeeze and adjustment phase takes around 10 minutes, during which the beams are focused to the smallest possible size to maximize collision rates. After this, the system enters the stable beam operation phase, lasting about 10 hours, during which the beams are kept stable for experiments. The turnaround time, defined as the time between dumping the beam and the next collision, is a critical performance metric, with the current record time standing at 1 hour and 53 minutes.