The Large Hadron Collider (LHC) is the most powerful particle accelerator ever built. Based at the European particle physics laboratory CERN, near Geneva in Switzerland, it is the world’s largest laboratory and is dedicated to the pursuit of fundamental science.
The LHC allows scientists to reproduce the conditions that existed within a billionth of a second after the Big Bang. This is the moment, around 14 billion years ago, when the Universe is believed to have started with an explosion of energy and matter. During this first moment of time the particles and forces that shaped our Universe came into existence.
Scientists recreate these conditions by colliding beams of high-energy protons or ions at close to the speed of light. This takes place inside the LHC’s 27km circular accelerator 100m below the ground.
On 4 July 2012 two of the experiments on the LHC, ATLAS and CMS, announced that they had detected a Higgs-like Boson. Further announcements since have confirmed that it is the boson physicists have been looking for since it was predicted 50 years ago. This is one of the greatest discoveries of our time made possible by the unique conditions of the LHC.
The LHC is exactly what its name suggests – a large collider of hadrons. Strictly, LHC refers to the collider; a machine that deserves to be labelled ‘large’, it not only weighs more than 38,000 tonnes, but runs for 27km (16.5m) in a circular tunnel 100 metres beneath the Swiss/French border at Geneva.
However, the collider is only one of three essential parts of the LHC project. The other two are:
- the detectors, which sit in 4 huge chambers at points around the LHC tunnel
- the GRID, which is a global network of computers and software essential to processing the data recorded by LHC’s detectors
The LHC’s 27km loop in a sense encircles the globe, because the LHC project is supported by an enormous international community of scientists and engineers. Working in multinational teams, at CERN and around the world, they are building and testing LHC equipment and software, participating in experiments and analysing data. The UK has a major role in leading the project and has scientists and engineers working on all the main experiments.
How it Works??
Accelerators were invented in the 1930s to provide energetic particles to investigate the structure of the atomic nucleus. Since then, they have been used to investigate many aspects of particle physics. Their job is to speed up and increase the energy of a beam of particles by generating electric fields that accelerate the particles, and magnetic fields that steer and focus them.
An accelerator comes either in the form of a ring (a circular accelerator), where a beam of particles travels repeatedly round a loop, or in a straight line (a linear accelerator), where the particle beam travels from one end to the other. At CERN a number of accelerators are joined together in sequence to reach successively higher energies.
The type of particle used depends on the aim of the experiment. The Large Hadron Collider (LHC) accelerates and collides protons, and also heavy lead ions. One might expect the LHC to require a large source of particles, but protons for beams in 27-kilometre ring come from a single bottle of hydrogen gas, replaced only twice per year to ensure that it is running at the correct pressure.
How to accelerate protons
In the first part of the accelerator, an electric field strips hydrogen nuclei (consisting of one proton and one electron) of their electrons. Electric fields along the accelerator switch from positive to negative at a given frequency, pulling charged particles forwards along the accelerator. CERN engineers control the frequency of the change to ensure the particles accelerate not in a continuous stream, but in closely spaced “bunches”.
Radiofrequency (RF) cavities – specially designed metallic chambers spaced at intervals along the accelerator – are shaped to resonate at specific frequencies, allowing radio waves to interact with passing particle bunches. Each time a beam passes the electric field in an RF cavity, some of the energy from the radio waves is transferred to the particles, nudging them forwards.
It’s important that the particles do not collide with gas molecules on their journey through the accelerator, so the beam is contained in an ultrahigh vacuum inside a metal pipe – the beam pipe.
Various types of magnet serve different functions around a circular accelerator. Dipole magnets, for example, bend the path of a beam of particles that would otherwise travel in a straight line. The more energy a particle has, the greater the magnetic field needed to bend its path. Quadrupole magnets act likes lenses to focus a beam, gathering the particles closer together.
Collisions at accelerators can occur either against a fixed target, or between two beams of particles. Particle detectors are placed around the collision point to record and reveal the particles that emerge from the collisions.