The Large Hadron Collider

“Large” refers to its size, approximately 27km in circumference

“Hadron” because it accelerates protons or ions, which belong to the group of particles called hadrons

“Collider” because the particles form two beams travelling in opposite directions, which are made to collide at four points around the machine

The LHC is a machine designed to advance knowledge in fundamental physics. Since the early 1970s, our knowledge is encompassed in the Standard Model of particle physics, a theory that describes the fundamental particles and their interactions. The Standard Model has been remarkably successful, but it leaves many questions open that the LHC aims to address. The  announce of the discovery of the Higgs boson in July 2012 confirmed the mechanism predicted in 1964 and through which fundamental particles acquire mass. However, this particle, unique in the particle physics landscape, holds the key to many mechanisms that physicists seek to understand by analysing it. Other questions include the nature of dark matter (which makes 80% of the matter of the Universe, the matter-antimatter asymmetry, how ordinary matter formed in the first instants of the Universe.

Scientists started thinking about the LHC in the early 1980s, when the previous accelerator, the LEP, was not yet running. The idea was to reuse the LEP tunnel to host a machine able to collide protons instead of electrons. In December 1994, the CERN Council voted to approve the construction of the LHC and in October 1995, the LHC technical design report was published.

Contributions from Japan, the USA, India and other non-Member States accelerated the process. Between 1996 and 1998, four experiments (ALICE, ATLAS, CMS and LHCb) received official approval and construction work started on the four sites. The construction and assembly of the LHC span from 1996 to 2008. It started on 10 September 2008. It was a global and complex adventure, involving several thousand people and companies all around the world.

Superconducting magnets

The LHC occupies the same tunnel originally built for its predecessor, the Large Electron-Positron Collider (LEP). Unlike the LEP, which collided electrons, the LHC accelerates protons, roughly 2 000 times more massive. For steering them along the 27-kilometer circular path, the LHC required magnets far more powerful than conventional electromagnets. It is composed of 9 000 superconducting magnets, capable of generating much higher magnetic fields. These magnets represent one of the most formidable technological challenges of the project. 

Cryogenics

 The superconducting magnets are cooled down to 1.9 K (-271°C), colder than outer space, using the world’s largest cryogenic systems, which chills 23 km of the accelerator’s 27-km. This system relies on superfluid helium, a refrigerant with a very efficient heat-transfer property.

Vacuum

 In the LHC, particles circulate in pipes that have been emptied of air. Within the vacuum chambers, the pressure is ten million million times lower than outside at sea level:
10^-13 bar. This is almost as low as the pressure on the moon. 

Accelerating cavities

The LHC uses eight superconducting cavities per beam, each delivering 2 megavolts at 400 MHz to accelerate the beams.

Hadrons (protons or heavy ions) are accelerated in a chain of four accelerators before being injected into the LHC. They are then accelerated in the LHC for about 20 minutes before reaching their nominal energy and being brought into collisions at the centre of four large experiments. The beams circulate for between 10 and 15 hours before being dumped when they do not contain enough particles.

Each beam is structured in around 2500 bunches, each bunch containing 100 billion protons. The bunches are usually spaced 25 nanoseconds apart (or about 7 metres), but for practical reasons, there are several bigger gaps.  

The energy of the proton collision is 13.6 TeV, which is the highest energy ever reached in a Lab. However, it’s a very small amount of energy on the human scale, equivalent to that of a needle falling to the ground from a height of a few centimetres! But, on the tiny scale of particles, an accelerator concentrates this energy into a space that’s ten trillion times smaller than a needle, making it possible to obtain very high energy densities and to create particles. 

The energy of the lead-ion collision is 5.36 TeV per nucleon pair; it corresponds to the energy of two nucleons (proton or neutron) colliding.

The LHC can only reproduce phenomena that already happen naturally all around us, on Earth and in outer space. The Universe as a whole produces more than 10 million million LHC-like experiments per second. If such phenomena were dangerous or destructive, it would contradict what we see: stars, galaxies and the Earth still exist.

A commission of experts has assessed the risks of running LHC and concluded it presents no danger: you can read the full report here. The report was independently reviewed and endorsed by three other societies of physicists – the American Physical Society, the UK Institute of Physics, and the German Physical Society – that also concluded that LHC collisions are perfectly safe.

According to Einstein’s theory of relativity, it is impossible for LHC to produce black holes. However, some speculative theories predict the production of microscopic black holes. All these theories predict that these particles would disintegrate immediately. Black holes, therefore, would have no time to start accreting matter and to cause macroscopic effects.

Construction costs (MCHF)

*This includes: Machine R&D and injectors, tests and pre-operation.
** Contains infrastructure costs (such as caverns and facilities). The total cost of all LHC detectors is about 1500 MCHF

The experimental collaborations are individual entities, funded independently from CERN. CERN is a member of each experiment and contributes to the maintenance and operation budget of the LHC experiments.

The total power consumption of the LHC, its experiments and its general services is around to 600 GWh per year, with a maximum of 695 GWh in 2024. 
The total CERN energy consumption is around 1.3 TWh per year while the total electrical energy production in the European Union 3000 TWh, in France around 450 TWh, and in Geneva canton is around 3 TWh.

For more information, see CERN environment report 2023-2024.

The LHC circulated its first protons on 10 September 2008. Nine days later, a faulty electrical connection between two magnets damaged a sector of the accelerator, requiring several months of repairs. By the end of 2009, the accelerator restarted, producing its first collisions on 23 November 2009, and its first high-energy collisions (7 TeV) on 30 March 2010.  On 8 November 2010, the accelerator collided lead ions for the first time. Since then, the LHC has completed three operational run, increasing both energy and luminosity. It reached 13 TeV in 2015, at the start of its second run, and 13,6 TeV in 2022, at the start of its third run. The LHC has produced a lot of physics results, including the discovery of the long-sought Higgs boson in 2012.

More details on the LHC Milestones page

Luminosity is a measure of the rate of collisions. Instantaneous luminosity describe the number of collisions happening in a unit of time (for example, every second). Integrated luminosity measeg the total number of collisions produced over a period of time. Integrated luminosity is usually expressed in units of “inverse femtobarns” (fb-1). A femtobarn is a unit of cross-section, a measure of the probability for a process to occur in a particle interaction. One inverse femtobarn corresponds to around 100 million million collisions.

The LHC will operate until the Long Shut Down 3, starting in July 2026. During the LS3, the LHC will undergo a major upgrade in order to increase its luminosity, i.e. the number of collisions. The upgraded machine, the High-Luminosity LHC (HL-LHC), which is due to restart mid-30 will run at least another decade.

See the HiLumi LHC page