The Large Hadron Collider or LHC is the world's largest and most powerful particle collider, the most complex experimental facility ever built and the largest single machine in the world. It was built by the European Organization for Nuclear Research (CERN) between 1998 to 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 27 kilometres (17 mi) in circumference and as deep as 175 metres (574 ft) beneath the France–Switzerland border near Geneva. Its first research took place from March 2010 to early 2013 at an energy of 3.5 to 4 teraelectronvolts (TeV) per beam (7 to 8 TeV total), about four times of the previous world record for a collider and accelerator. Afterwards, the accelerator was taken offline and upgraded over the course of two years. It was restarted in early 2015 for its second research run, reaching 6.5 TeV per beam (13 TeV total, the present world record).

The aim of the LHC is to allow physicists to test the predictions of different theories of particle physics, including measuring the properties of the Higgs boson and searching for the large family of new particles predicted by supersymmetric theories, as well as other unsolved questions of physics.

The collider has four crossing points, around which are positioned seven detectors, each designed for certain kinds of research. The LHC primarily collides proton beams, but it can also use beams of heavy ions. Proton–lead collisions were performed for short periods in 2013 and 2016, lead–lead collisions took place in 2010, 2011, 2013, and 2015, and a short run of xenon–xenon collisions took place in 2017.

The LHC's computing grid is a world record holder. Data from collisions were produced at an unprecedented rate for the time of first collisions, tens of petabytes per year, a major challenge at the time, to be analysed by a grid-based computer network infrastructure connecting 170 computing centres in 42 countries as of 2017 – by 2012 the Worldwide LHC Computing Grid was also the world's largest distributed computing grid, comprising over 170 computing facilities in a worldwide network across 36 countries.


Background

The term hadron refers to composite particles composed of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic force). The best-known hadrons are the baryons such as protons and neutrons; hadrons also include mesons such as the pion and kaon, which were discovered during cosmic ray experiments in the late 1940s and early 1950s.

A collider is a type of a particle accelerator with two directed beams of particles. In particle physics, colliders are used as a research tool: they accelerate particles to relatively high kinetic energies and let them impact other particles. Analysis of the byproducts of these collisions gives scientists good evidence of the structure of the subatomic world and the laws of nature governing it. Many of these byproducts are produced only by high-energy collisions, and they decay after very short periods of time. Thus many of them are hard or nearly impossible to study in other ways.