The Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, is the world's largest and most powerful particle accelerator. Buried 100 metres underground in a circular tunnel 27 kilometres in circumference, it is designed to accelerate beams of protons or heavy ions to speeds approaching the speed of light. By colliding these particles head-on at enormous energies, the LHC recreates conditions that existed just after the Big Bang. This allows physicists to study the fundamental building blocks of matter and the forces that govern them. The LHC has been operational since 2008 and has already led to groundbreaking discoveries, including the Higgs boson in 2012.
Its primary goal is to test the predictions of the Standard Model of particle physics and to search for new physics beyond it. The LHC works by using a series of superconducting magnets to steer and focus particle beams. These magnets are cooled to minus 271. 3 degrees Celsius, colder than outer space, using liquid helium. The protons are first accelerated in a linear accelerator and then injected into the Proton Synchrotron and the Super Proton Synchrotron before entering the main LHC ring. In the main ring, the protons travel in opposite directions in two separate beam pipes.
They are accelerated to an energy of 6. 5 teraelectronvolts (TeV) per beam, resulting in a collision energy of 13 TeV. At four points around the ring, the beams cross and collide, producing a shower of subatomic particles that are detected by massive instruments. The four main detectors at the LHC are ATLAS, CMS, ALICE, and LHCb. ATLAS and CMS are general-purpose detectors designed to study a wide range of physics, including the Higgs boson and possible new particles. ALICE is specialised for studying heavy-ion collisions, which produce a state of matter called quark-gluon plasma that existed in the early universe.
The protons are first accelerated in a linear accelerator and then injected into the Proton Synchrotron and the Super Proton Synchrotron before entering the main LHC ring.
LHCb focuses on studying the differences between matter and antimatter by examining particles containing bottom quarks. Each detector is a marvel of engineering, weighing thousands of tonnes and containing millions of electronic channels. They record the trajectories, energies, and identities of particles produced in collisions, generating petabytes of data every year. When two protons collide at the LHC, their energy is converted into mass according to Einstein's equation E=mc², creating a variety of particles. These particles are unstable and decay almost instantly into more stable particles. The detectors track these decay products to reconstruct what happened in the collision.
Physicists analyse the data to look for deviations from the Standard Model, which could indicate new particles or forces. One of the key achievements of the LHC was the discovery of the Higgs boson, the particle associated with the Higgs field that gives mass to other particles. This discovery confirmed a crucial part of the Standard Model and earned the Nobel Prize in Physics in 2013. Beyond the Higgs boson, the LHC is searching for dark matter, extra dimensions, and supersymmetric particles. Dark matter is thought to make up about 27% of the universe, but it has never been directly detected.
If dark matter particles are produced in LHC collisions, they would escape the detectors without leaving a trace, but their presence could be inferred from missing energy and momentum. Supersymmetry is a theory that predicts a partner particle for every known particle, which could solve several problems in physics. So far, no evidence for supersymmetry has been found, but the LHC continues to push the energy frontier. The LHC also studies the properties of the top quark, the heaviest known elementary particle, and the behaviour of the strong nuclear force.
The LHC operates in runs separated by long shutdowns for upgrades. Run 1 (2010-2013) achieved a collision energy of 8 TeV and discovered the Higgs boson. Run 2 (2015-2018) reached 13 TeV and collected a vast amount of data. Run 3 began in 2022 and is expected to continue until 2026, with a goal of collecting even more data. After Run 3, the LHC will undergo a major upgrade to become the High-Luminosity LHC (HL-LHC), which will increase the number of collisions by a factor of ten. This will allow physicists to study rare processes and improve the precision of measurements.
The HL-LHC is expected to start operating around 2029 and will continue into the 2030s. The LHC represents an extraordinary achievement of human ingenuity and international collaboration. Thousands of scientists, engineers, and technicians from over 100 countries have contributed to its design, construction, and operation. The data produced by the LHC is analysed by a global network of computing centres, using the Worldwide LHC Computing Grid. The knowledge gained from the LHC not only deepens our understanding of the universe but also drives technological innovation. For example, the World Wide Web was invented at CERN to facilitate data sharing among particle physicists. As the LHC continues to explore the frontiers of physics, it promises to reveal more secrets about the nature of matter, energy, space, and time.