Large Hadron Collider


The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It was built by the European Organization for Nuclear Research between 1998 and 2008, in collaboration with over 10,000 scientists, and hundreds of universities and laboratories across more than 100 countries. It lies in a tunnel in circumference and as deep as beneath the France–Switzerland border near Geneva.
The first collisions were achieved in 2010 at an energy of 3.5 tera-electronvolts per beam, about four times the previous world record. The discovery of the Higgs boson at the LHC was announced in 2012. Between 2013 and 2015, the LHC was shut down and upgraded; after those upgrades it reached 6.5 TeV per beam. At the end of 2018, it was shut down for maintenance and further upgrades, and reopened over three years later in April 2022.
The collider has four crossing points where the accelerated particles collide. [|Nine detectors], each designed to detect different phenomena, are positioned around the crossing points. The LHC primarily collides proton beams, but it can also accelerate beams of heavy ions, such as in lead–lead collisions and proton–lead collisions.
The LHC's goal is to allow physicists to test the predictions of different theories of particle physics, including measuring the properties of the Higgs boson, searching for the large family of new particles predicted by supersymmetric theories, and studying other unresolved questions in particle physics.

Background

The term hadron refers to subatomic composite particles composed of quarks held together by the strong 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 that brings two opposing particle beams together such that the particles collide. In particle physics, colliders, though harder to construct, are a powerful research tool because they reach a much higher center of mass energy than fixed target setups. 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.

Purpose

Many physicists hope that the Large Hadron Collider will help answer some of the fundamental open questions in physics, which concern the basic laws governing the interactions and forces among elementary particles and the deep structure of space and time, particularly the interrelation between quantum mechanics and general relativity.
These high-energy particle experiments can provide data to support different scientific models. For example, the Standard Model and Higgsless model required high-energy particle experiment data to validate their predictions and allow further theoretical development. The Standard Model was completed by detection of the Higgs boson by the LHC in 2012.
LHC collisions have explored other questions, including:
Other open questions that may be explored using high-energy particle collisions include:
The collider is contained in a circular tunnel, with a circumference of, at a depth ranging from underground. The variation in depth was deliberate, to reduce the amount of tunnel that lies under the Jura Mountains to avoid having to excavate a vertical access shaft there. A tunnel was chosen to avoid having to purchase expensive land on the surface and to take advantage of the shielding against background radiation that the Earth's crust provides.
The wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the Large Electron–Positron Collider. The tunnel crosses the border between Switzerland and France at four points, with most of it in France. Surface buildings hold ancillary equipment such as compressors, ventilation equipment, control electronics and refrigeration plants.
The collider tunnel contains two adjacent parallel beamlines each containing a beam, which travel in opposite directions around the ring. The beams intersect at four points around the ring, which is where the particle collisions take place. Some 1,232 dipole magnets keep the beams on their circular path, while an additional 392 quadrupole magnets are used to keep the beams focused, with stronger quadrupole magnets close to the intersection points in order to maximize the chances of interaction where the two beams cross. Magnets of higher multipole orders are used to correct smaller imperfections in the field geometry. In total, about 10,000 superconducting magnets are installed, with each of the 1232 dipole magnets having a mass of 35 tonnes. About 96 tonnes of superfluid helium-4 is needed to keep the magnets, made of copper-clad niobium-titanium, at their operating temperature of, making the LHC the largest cryogenic facility in the world at liquid helium temperature. LHC uses 470 tonnes of Nb–Ti superconductor.
During LHC operations, the CERN site draws roughly 200 MW of electrical power from the French electrical grid, which, for comparison, is about one-third the energy consumption of the city of Geneva; the LHC accelerator and detectors draw about 120 MW thereof. Each day of its operation generates 140 terabytes of data.
When running an energy of 6.5 TeV per proton, once or twice a day, as the protons are accelerated from 450 GeV to 6.5 TeV, the field of the superconducting dipole magnets is increased from 0.54 to. The protons each have an energy of 6.5 TeV, giving a total collision energy of 13 TeV. At this energy, the protons have a Lorentz factor of about 6,930 and move at about, or about slower than the speed of light. It takes less than for a proton to travel 26.7 km around the main ring. This results in per second for protons whether the particles are at low or high energy in the main ring, since the speed difference between these energies is beyond the fifth decimal.
Rather than having continuous beams, the protons are bunched together, into up to, with in each bunch so that interactions between the two beams take place at discrete intervals, mainly apart, providing a bunch collision rate of 40 MHz. It was operated with fewer bunches in the first years. The design luminosity of the LHC is 1034 cm−2s−1, which was first reached in June 2016. By 2017, twice this value was achieved.
Before being injected into the main accelerator, the particles are prepared by a series of systems that successively increase their energy. The first system is the linear particle accelerator Linac4 generating 160 MeV negative hydrogen ions, which feeds the Proton Synchrotron Booster. There, both electrons are stripped from the hydrogen ions leaving only the nucleus containing one proton. Protons are then accelerated to 2 GeV and injected into the Proton Synchrotron, where they are accelerated to 26 GeV. Finally, the Super Proton Synchrotron is used to increase their energy further to 450 GeV before they are at last injected into the main ring. Here, the proton bunches are accumulated, accelerated to their peak energy, and finally circulated for 5 to while collisions occur at the four intersection points.
The LHC physics programme is mainly based on proton–proton collisions. However, during shorter running periods, typically one month per year, heavy-ion collisions are included in the programme. While lighter ions are considered as well, the baseline scheme deals with lead ions. The lead ions are first accelerated by the linear accelerator LINAC 3, and the Low Energy Ion Ring is used as an ion storage and cooler unit. The ions are then further accelerated by the PS and SPS before being injected into LHC ring, where they reach an energy of 2.3 TeV per nucleon, higher than the energies reached by the Relativistic Heavy Ion Collider. The aim of the heavy-ion programme is to investigate quark–gluon plasma, which existed in the early universe.

Detectors

Nine detectors have been built in large caverns excavated at the LHC's intersection points. Two of them, the ATLAS experiment and the Compact Muon Solenoid, are large general-purpose particle detectors. ALICE and LHCb have more specialized roles, while the other five—TOTEM, MoEDAL, LHCf, SND and FASER—are much smaller and are for very specialized research. The ATLAS and CMS experiments discovered the Higgs boson, which is strong evidence that the Standard Model has the correct mechanism of giving mass to elementary particles.