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What Happens at the LHC?

The Large Hadron Collider (LHC) is the last in a ‘ladder’ of accelerators that are used in sequence to accelerate particles up to the LHC’s maximum energy. Each accelerator builds up the particles to their maximum energies before introducing it to the next biggest accelerator. The particles the LHC accelerates and collides are protons or lead nuclei, both have positive charges and this means that they can be steered by use of magnetic fields. There are 9300 superconducting magnets, of various types, that are used to steer and focus beams of particles as they race around the colossal loop of the LHC.

Figure 1: LHC pipeline
(Credit: CERN)

The LHC accelerates two thin beams of atomic particles travelling in opposite directions around the 27km collider, each about 2mm across (small enough to pass through the zero on a 20p piece). Each beam sits inside its own pipe, where the beam pipes are enclosed in a sheath of superconducting magnets and all of this is bathed in super-cooled liquid helium at 1.8K (-271°C). At the centre of each of the four detectors the beams briefly share the same pipe as the magnets direct them to collide head-on.

Figure 2: LAC simulation of a single collision.
(Credit: CERN)

The principles behind a detector are extremely simple, but on this large scale it becomes very difficult. A detector is designed to track the motion and measure the energy and charge of all the particles thrown out from each collision. They are able to follow the millions of particles produced and identify the distinctive behaviour of interesting new particles from among haystack. The LHC detectors are very large (ATLAS alone is the size of a 5 storey building!), their great size is necessary to trap high energy particles travelling at tremendous speeds and to allow the tracks of charged particles to be measurably curved by the detectors magnets.

Figure 3: Schematic of the ATLAS detector
(Credit: CERN)

The layers nearest to the collision point are designed to very precisely track the movement of charged particles. Of special interest are tracks from the decays of short-lived particles that and the most interesting to the researchers. Subsequent layers track the movement, slow down and stop longer-lived and more energetic particles, as these particles are slowed down they release energy that can be measured.

It is impossible for any computer to collect and process all the raw data that each detector creates at any point in time, just over one petabyte (~ 1,050,000 gigabytes) per second. Therefore, a lot of the data is instantly filtered out by the so-called trigger systems of the experiments. The trigger systems run complicated algorithms designed to find the interesting science and write the selected data to computer storage. The information saved is then processed more slowly by other algorithms designed to determine the full details of the particles and so find the, often very rare, decays that are of especial interest. This monumental task is handled by “The Grid”, the Grid has many applications, but its first major application was to allow researchers at CERN to share global computing power to manage and process the huge quantities of data that the LHC produces. By linking desk top computers in a global network, managed by so-called middleware, the Grid brings supercomputing power to desk tops.

Figure 4: The supercomputer at CERN
(Credit: CERN)

Through the organisation GridPP (Particle Physics Grid) the UK has a significant role in developing Grid computing with several GridPP computer ‘farms’ located around the UK in University departments. So far the UK contributes approximately 40,000 PC’s to the worldwide LHC computing grid.

The results from the LHC are not completely predictable as the experiments are testing ideas that are at the frontiers of our knowledge and understanding. Researchers expect to confirm predictions made on the basis of what we know from previous experiments and theories. However, part of the excitement of the LHC project is that it may uncover new facts about matter and the origins of the Universe.

Last updated: 03 March 2016


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