The LHC is the largest and most powerful particle accelerator ever built. It took international teams numbering in the thousands to design, construct and operate the 27 km collider and its four giant particle detectors. Collider features authentic objects from CERN, home of the LHC, from huge superconducting magnets to incredibly precise detectors used to record the passage of the tiny, invisible particles.
Inside the LHC videos | The collider | The detectors
What is a collider?
The LHC tunnel. © CERN
In some ways, a collider is a simple and rather brutal device. They take tiny particles, accelerate them to tremendous speeds, and smash them into each other. The reason is not to break particles apart to see what they are made of, but to create altogether new particles from the energy of the collisions.
In this sense colliders aren’t 'atom-smashers', but particle factories, creating new and exotic matter from pure energy. By studying these new particles, physicists learn more about the physical laws that govern our universe at the most fundamental level.
Colliders vary in design. Some, like enormous rifles, fire particles at each other in straight lines, while others whirl them round a ring. The type of particle selected for colliding also differs, ranging from everyday electrons to more exotic particles of antimatter.
CERN’s Large Hadron Collider (LHC) is the largest and most powerful particle collider ever built. Housed in a 27-km tunnel beneath the Swiss-French border, it fires two beams of hadrons around and around in a circle before colliding them into each other. A hadron is a category of particle, and includes the stuff that makes up the atomic nucleus: neutrons and protons. The LHC mostly uses protons, but occasionally picks much larger projectiles, such as lead nuclei.
Making a particle beam
The LHC’s protons are sourced from a bottle of hydrogen gas. Hydrogen atoms are desirable as they consist of just one proton with an orbiting electron, which is easy enough to strip away – a process known as ionisation. CERN’s impressively-named 'duoplasmatron' does this ionising deed, zapping the hydrogen with electric fields, which results in a fine supply of pure, unadulterated protons.
Before the protons enter the LHC’s main circuit, they are sent through a series of smaller, preliminary accelerators – some in their glory days used to be the chief collider at CERN. Among these recycled machines is CERN’s very first accelerator, the proton synchrotron, which began operation back in 1959.
The Hydrogen bottle and the duoplasmatron where the protons are sourced. © CERN and Injection of protons, graphic from the Collider exhibition. © Science Museum / Northover Brown
Swarms of protons, called 'bunches', are gradually injected into the LHC ring, building up the two 27-km clockwise and anticlockwise beams piece by piece. By the time the protons reach the LHC they are already travelling at 99.9998% of the speed of light.
Along one of the LHC’s straights, sit sixteen radio-frequency cavities that each give the beams a 2-million-volt energy boost every time they pass (a whopping 11,000 times a second).
Eventually, the protons reach 4-trillion electron-volts, travelling at 99.999997% the speed of light. At that speed, persuading the protons to go in a circle, even around a curve as gentle as the 27 km LHC ring, requires applying an absolutely enormous force.
The LHC’s line of radio-frequency accelerating cavities. © CERN
Steering and focussing
Most of the LHC ring is made up of magnets, which have the formidable task of controlling the highly-energetic particle beams. All the magnets are superconducting, a miraculous property which means they offer no electrical resistance, allowing the generation of tremendously powerful magnetic fields. The magnets only superconduct at extremely low temperatures, and must be cooled using liquid helium to -271.25°C – colder than deep space, and two degrees above the lowest temperature possible, absolute zero.
The most common magnets are dipoles - so called because they have two poles, north and south. They have the job of steering the beams around the circuit. Covering just over 20-km of the ring, there are 1,232 dipoles in total. Each individual piece is 16.5 meters long, which was limited by the maximum vehicle length allowed on European roads.
Transporting the first dipole magnet. © CERN
Squeezing the beam into a space narrower than a human hair is achieved with quadropole magnets that contain an additional set of north-south poles. The bunches of protons must be very precisely focussed to obtain the highest possible collision rate.
At four points around the ring, special magnets are used to focus the beams onto a collision point. When the beams collider their vast energy is released in the form of new particles which are studied by cathedral-sized particle detectors.
An engineering masterpiece
The LHC was designed and built on the back of decades’ worth of expertise, technology and machinery – both from CERN’s own past and from the wider international physics community. A decade in the construction, the LHC stands as a monument to human ingenuity and ambition.