Large Hadron Collider

The Large Hadron Collider (LHC) is a particle accelerator at the European nuclear research centre CERN near Geneva. In terms of energy and frequency of particle collisions, the LHC is the most powerful particle accelerator in the world. More than 10,000 scientists and technicians from over 100 countries were involved in its planning and construction, and hundreds of university chairs and research institutes cooperated. The key component is a synchrotron in a 26.7-kilometre-long underground ring tunnel, in which protons or lead nuclei are accelerated in opposite directions to almost the speed of light and brought to collision. The experiments at the LHC are therefore colliding beam experiments.

The research goals at the LHC are the creation and precise investigation of known and as yet unknown elementary particles and states of matter. The starting point is the verification of the current Standard Model of particle physics. Particular attention will therefore be paid to the Higgs boson, the last particle of the Standard Model not yet experimentally detected at the start of operation. In addition, the LHC will be used to search for physics beyond the Standard Model in order to possibly find answers to open questions. Four large and two smaller detectors register the tracks of the particles created in the collisions. Due to the large achievable number of collisions per second (high luminosity), enormous amounts of data are generated. These are pre-sorted with the help of a sophisticated IT infrastructure. Only a small part of the data is forwarded to the participating institutes for analysis by means of a specially established, global computer network.

In the experiments from 2010 onwards, a previously inaccessible energy range was opened up. A major result of the experiments to date (as of March 2019) is an extraordinarily good confirmation of the Standard Model. Several new hadrons were found, a quark-gluon plasma could be produced, and for the first time CP violation was observed for the Bs0 meson during its decay into kaons and pions, as well as its extremely rare decay into two muons. Evidence of CP violation was also obtained for the D0 meson. The experimental detection of the Higgs boson is considered the greatest success to date. This led to the award of the Nobel Prize for Physics 2013 to François Englert and Peter Higgs.

History

CERN's accelerator complex
List of the current particle accelerators at CERN
Linac 2	Accelerates protons
Linac 3	Accelerates ions
Linac 4	Accelerates negative hydrogen ions
AD	Brakes antiprotons
LHC	Collides protons or heavy ions
LEIR	Accelerates lead ions
PSB	Accelerates protons or ions
PS	Accelerates mainly protons
SPS	Accelerates protons, among other things

The direct predecessor of the LHC was the Large Electron-Positron Collider (LEP), which operated until 2000. The ring tunnel in which the LHC is located today was built for the LEP in the 1980s. The possibility of continuing to use the tunnel, which had already been taken into account in the design of the LEP, was a decisive factor in the choice of location for the LHC. Detailed planning for the LHC began while the LEP was still under construction. In the LEP, electrons and positrons, which are among the leptons, were made to collide. In the LHC, on the other hand, protons or atomic nuclei, which belong to the hadrons, collide. This is where the name Large Hadron Collider comes from.

In a ten-year planning and preparation phase, it was clarified which specific questions were to be investigated with the LHC and whether an accelerator based on superconductivity was at all technically feasible. On 16 December 1994, the CERN Council finally gave the go-ahead for construction. Initially, the energy at which the protons collide was to be 10 TeV, later to be increased to 14 TeV. After India and Canada, two non-member states of CERN, declared their willingness to participate in the financing and development of the LHC and thus in its later use, it was decided in December 1996 to dispense with the intermediate step of 10 TeV and to tackle 14 TeV directly. Cooperation agreements with other countries followed. In 1997, the Italian Istituto Nazionale di Fisica Nucleare delivered the first prototype of the dipole magnets, and the first successful test took place the following year. That year, the official Swiss and French authorities also gave their approval for the construction work required for the new caverns for the largest detectors. Tunnel construction began at the end of 2000 and was completed in 2003. Within a good year, 40,000 tons of material were removed from the tunnel.

Between 1998 and 2008, continuous tests were carried out on individual components and subsequently orders were placed for industrial production. In parallel, the detector systems were assembled and the connection to existing accelerators such as the SPS was established. The components came from all over the world; for example, the wire chambers for the muon detector of the ATLAS were manufactured in more than half a dozen countries. The brass supplied by Russia for the CMS detector came from an arms conversion agreement. In 2006, the production of all superconducting main magnets was completed, and in February 2008, the last components of the ATLAS were at their destination.

The preliminary work on data processing led to the launch of the European DataGrid Project in 2001. Two years later, a record was set for data transfer via the Internet. Within one hour, a data volume of one terabyte was sent from CERN to California. After two more years, the number of participants in the LHC Computing Grid had already grown to over 100 data centres in more than 30 countries.

The official start of accelerator operation at the LHC was 10 September 2008, when a proton packet circled the entire ring for the first time. But a technical defect led to a year-long shutdown after only nine days: the weld seam of a superconducting connection could not withstand the load and destroyed a helium tank of the cooling system, the explosion of which in turn displaced one of the 30-ton magnets by half a metre. Six tons of liquid helium were lost in this "quenching", and the affected magnets heated up very quickly by about 100 K. After restarting on 20 November 2009, the first proton-proton collisions took place in the particle detectors three days later, and another six days later the proton beam reached 1.05 TeV, the energy of the Tevatron, the most powerful particle accelerator to date. During the winter of 2009/10, improvements were made to the particle accelerator that allowed 3.5 TeV per beam, a centre-of-mass energy of 7 TeV. On 30 March 2010, collisions with this energy took place for the first time. All those responsible showed great satisfaction, including CERN Director General Rolf-Dieter Heuer:

During the following year and a half, interrupted only by a scheduled maintenance break in winter 2010/11, the detectors were able to investigate proton-proton collisions at 7 TeV centre-of-mass energy. In the process, the originally planned number of collisions was exceeded thanks to continuously improved beam focusing. Operation in proton mode was interrupted on 30 October 2011 in order to insert a short phase of lead nucleus collisions until the next maintenance shutdown in winter 2011/12.

Originally, after about two years of operation, the LHC was to enter a longer refit phase of 15 to 18 months at the end of 2011 to replace the existing connections between the magnets and prepare the accelerator for 7 TeV (centre-of-mass energy 14 TeV). However, in January 2011, it was decided to extend the lifetime by one year, until the end of 2012, before the retrofit phase. Later, this date was postponed to the beginning of 2013. The reason for the decision was the excellent performance of the accelerator in its first year of operation, so that signs of novel particles could be expected after only three years of operation, which was confirmed with the discovery of a new elementary particle, the Higgs boson.

From 5 April 2012 to 17 December 2012, proton-proton collisions were studied again. Thereby, the centre-of-mass energy could be increased to 8 TeV. This was followed again by collisions of lead nuclei and additionally collisions between lead nuclei and protons.

From February 2013 to April 2015, the LHC was in its first extended refit phase, during which the accelerator was prepared for a collision energy of 13 TeV. Some of the superconducting magnets were replaced and more than 10,000 electrical connections and the magnets were better protected against possible faults. The higher collision energy was reached for the first time on May 20, 2015. The proton bunches now contain fewer protons than in 2012, but follow each other at half the distance. By early November 2015, protons were colliding, and lead nuclei were colliding again in late November and early December, also at a higher energy than before. In proton-proton collisions of 2016, the design luminosity and thus the planned collision rate were reached for the first time. In the last four weeks of operation in 2016, collisions of protons with lead nuclei were carried out.

In the winter of 2016/17, the LHC was also overhauled for 17 weeks. One of the superconducting coils was replaced (for which the helium used for cooling had to be drained), and there were also modifications to the Super Proton Synchrotron pre-accelerator. One of the goals for the new operating year was to further increase the luminosity. Data were collected again from May to November 2017. in the process, the collision rate was increased to twice the design value. The last measurement campaign ran from 28 April 2018, again with lead cores since early November, before the accelerator was shut down on 10 December 2018 until spring 2021 for modifications to further increase luminosity. Preparations for Run 3, planned for spring 2022, have been underway since March 2021.

Location and size of the LHC with the smaller ring of the SPS

Design, operation and mode of operation

Accelerator ring

The LHC was built in the existing ring tunnel of the European nuclear research facility CERN, where the Large Electron-Positron Collider was previously installed until its decommissioning in 2000. In addition to the tunnel, two detector chambers of the LEP could continue to be used; only the chambers for the ATLAS and CMS detectors had to be newly built. The tunnel tube has a diameter of about 3.80 metres and a circumference of 26.659 kilometres and lies, with a slight inclination of 1.4 %, at a depth of 50 to 175 metres. The accelerator ring is not exactly circular, but consists of eight circular arcs and eight straight sections. The largest experimental facilities and the pre-accelerators are located in Meyrin in the French-speaking part of Switzerland, and the control room is located in France. Large parts of the accelerator rings and some underground experimental sites are located on French territory.

The LHC tunnel contains two adjacent beam tubes in which two hadron beams circulate in opposite directions. For space reasons, both beam tubes had to be placed in a common tube with the magnets and the cooling devices. To allow collisions of the particles, the beam tubes cross at four points of the ring. In the predecessor, the LEP, this still happened at eight points. An ultra-high vacuum prevails in the beam tubes so that an accelerated particle collides as rarely as possible with a gas molecule in the residual air. For this purpose, 178 turbomolecular pumps and 780 ion getter pumps are installed along the ring. The residual vacuum pressure is 10-14 to 10-13 bar, which is about the measurable atmospheric pressure on the moon. The magnets and the helium supply lines are also surrounded by a vacuum for insulation in order to keep the heat flow as small as possible. The insulating vacuum of the magnets has a volume of about 9,000 cubic metres.

The limiting factor for the achievable energy is the field strength of the magnets that provide the deflection. In order to have to effect less strong changes in direction, fewer straight sections and instead longer, less curved arc sections in the ring would have been better. However, for reasons of cost, a tunnel conversion was not carried out. The high-energy particles are held in orbit in the LHC by 1232 superconducting dipole magnets made of niobium and titanium, which generate a magnetic flux density of up to 8.33 tesla by means of currents of 11,850 amperes. The strength of the magnetic field in the dipoles and the frequency of the electric field in the accelerating cavity resonators are constantly adjusted to the increasing energy of the particles. To keep the particle beams focused and to increase the collision rate when the two beams cross, 392 quadrupole magnets, also superconducting, are used. The magnets are cooled down in two steps to their operating temperature of 1.9 Kelvin (-271.25 °C), close to absolute zero. In the first step, they are precooled to 80 K (-193.2 °C) using 10,080 tonnes of liquid nitrogen, and in the second step they are brought to their final temperature using 100 tonnes of liquid helium. To keep the magnets at their operating temperature, they are constantly surrounded by about 60 tons of liquid helium in a superfluid state. In this state, helium has particularly good thermal conductivity. A total of 140 tonnes of helium are stored at the LHC for cooling purposes. The LHC is therefore the largest cryostat built to date (as of 2018).

In addition to the tidal forces, which change the circumference of the ring by about 1 mm, the water level of Lake Geneva and other external disturbances must be taken into account when operating the accelerator facility.

Proton mode

For the proton mode in the LHC, a centre-of-mass energy of 14 TeV was envisaged; this corresponds to 99.9999991 % of the speed of light. So far, 13 TeV have been achieved. To achieve such energies, the protons are accelerated one after the other through a series of systems. First, negative hydrogen ions are brought to an energy of 160 MeV in a linear accelerator. Then the electrons are removed, and the protons are accelerated to 450 GeV using the rings of the Proton Synchrotron Booster, the Proton Synchrotron and the Super Proton Synchrotron, which already existed before the LHC was built, until they are finally threaded into the main ring of the LHC, where they reach their target energy. The acceleration of the protons takes place according to the synchrotron principle by means of a high-frequency alternating electric field and lasts about 20 minutes.

The protons are bundled into packets in the beam tubes. The length of these bunches is a few centimetres, the diameter about 1 mm. Near the collision zone, the beam is compressed to a width of about 16 µm. Each packet contains over 100 billion protons. In full operation, the LHC is expected to be filled with about 2800 packets orbiting at a frequency of 11 kHz, or 11,000 times per second. In normal operation, a proton packet will remain in the beam tube for up to one day.

When the beams crossed, two proton bunches intersected in the collision zone every 50 nanoseconds until the retrofit in 2013 to 2014. Since 2015, the distance between the collisions has been only 25 nanoseconds. During regular operation, about 20 to 40 protons of each of the two bunches actually collide, which is then up to 800 million collisions per second. The design luminosity of 1034 cm-2s-1 was first reached in June 2016, and during 2017 the collision rate was doubled.

Lead mode

To produce a beam of lead atomic nuclei, isotopically pure lead (^208Pb) is first heated in a microfurnace and the resulting lead vapor is ionized in an electron cyclotron resonance ion source (ECRIS). Among the different charge states produced, the most abundant ^{208Pb29+ ions} are selected and accelerated to 4.2 MeV per nucleon. Then, a carbon foil serves as a "stripper", meaning that as the lead ions pass through the foil, they lose more electrons. Most lose 25 electrons and are now present as Pb54+ ions. These lead ions are accelerated to 72 MeV per nucleon in the Low Energy Ion Ring (LEIR), and subsequently to 5.9 GeV per nucleon in the Proton Synchrotron (PS). During the flight through a second stripper foil, the lead nuclei lose all remaining electrons; they are now fully ionized Pb82+. Finally, these nuclei are accelerated to 117 GeV per nucleon in the Super Proton Synchrotron (SPS) and fed into the LHC, which brings them to 2.76 TeV per nucleon. In total, the collision of the lead nuclei - with 208 nucleons each - thus takes place at a centre-of-mass energy of 1148 TeV (0.2 mJ), which corresponds roughly to the kinetic energy of a fly in flight.

LHC compared to LEP and Tevatron

In the Tevatron, the other large ring accelerator with counter-rotating beams, particles with opposite charges circulated in opposite directions in the two beam pipes. The LHC predecessor LEP worked on the same principle. All particles move along their path through a magnetic field of the same direction. Due to the relativistic Lorentz force, they experience the necessary inward deflection and are thus kept on their ring-shaped path. At the LHC, however, the protons and lead ions moving in opposite directions carry the same charge. In the two beamlines, the magnetic field must therefore point in opposite directions in order to deflect all the particles inwards. According to the concept of John Blewett (1971), this is achieved by a roughly ring-shaped magnetic field which penetrates one beam tube from top to bottom and the other from bottom to top.

While in the LEP electrons and positrons, i.e. the antiparticles to each other, were brought to collision, at the LHC protons or lead nuclei are accelerated and brought to collision, depending on the operating mode. Due to the much larger mass of the hadrons, they lose less energy to synchrotron radiation and can thus reach a much higher energy. The higher centre-of-mass energy compared to previous experiments enables the exploration of new energy ranges. Furthermore, by opting for protons instead of antiprotons in the second beam, as was the case at the Tevatron, a higher luminosity could be achieved. The high particle density at the interaction points leads to the desired high event rates in the particle detectors and makes it possible to collect larger amounts of data in a shorter time.

Security measures

The total energy of the beams circulating in the tunnels is up to 500 megajoules in proton mode; an increase to 600 MJ is planned. This corresponds to the kinetic energy of two ICE trains travelling at 150 km/h and would be sufficient to melt about half a tonne of copper. In the event of an uncontrolled beam loss, the accelerator facility would be severely damaged. Lyn Evans, the project manager of the LHC as of 1994, speaks of an amount of energy equivalent to that contained in 80 kg of TNT. The facility is therefore designed in such a way that within three revolutions, i.e. less than 300 microseconds, an unstable beam is registered and diverted by special magnets into a special side arm of the tunnel. There, there is a special beam stopper made up of a series of graphite plates of different densities that can intercept the beam. The energy stored in the dipole magnets is much higher still, at 11 GJ. The current in the magnet coils is passed through connected resistors when needed and the energy is converted into heat. The damage in the accident that occurred in 2008 during the start of accelerator operation (see History section) stemmed from this energy stored in the magnets.

Both the particle beam on its curved path and the collisions inevitably generate radiation. It is not possible to stay in the tunnel and the caverns of the detectors during the beam times. Maintenance work is accompanied by active and passive radiation protection measures. The soil above the tunnel effectively retains the scattered radiation during operation and the residual radioactivity. The air from the accelerator tunnel is filtered with the aim of always keeping the released radioactivity for the residents below the value of 10 μSv per year.

Detectors

The collision of the protons by crossing the two proton beams takes place in four underground chambers along the accelerator ring. The chambers contain the four large particle detectors ATLAS, CMS, LHCb and ALICE. The detectors TOTEM and LHCf are much smaller and are located in the chambers of the CMS and ATLAS experiments, respectively. They only study particles that graze each other during collisions instead of colliding. In addition, other special experiments with associated detector units are planned, such as MoEDAL for the search for magnetic monopoles as well as relics of microscopic black holes and supersymmetric particles. The FASER detector searches for long-lived hypothetical particles, for example dark photons, and measures neutrino interactions at high energies.

The objective of the four large detector systems can be summarized in the following simplified way:

Detector	Description
ATLAS	Search for the Higgs boson, supersymmetry and for possible substructures of leptons and quarks, study of collisions of heavy ions. About 2700 researchers from over 200 institutes worldwide are participating in the ATLAS experiment.
CMS	Search for the Higgs boson, supersymmetry and for possible substructures of leptons and quarks, study of heavy ion collisions. The CMS group comprises about 3500 people from 200 scientific institutes.
ALICE	Study of the extremely dense and energetic quark-gluon plasma, the state of matter immediately after the Big Bang. Over 1000 employees.
LHCb	Among other things, specialized in the study of decays of hadrons containing a bottom or charm quark, precision measurements on CP violation or rare decays as sensitive tests of the Standard Model. About 800 employees.

The complex internal structure of protons means that collisions often produce many different particles. This leads to high demands on the detector systems, which should detect these particles and their properties as completely as possible. Since the resulting particles are very diverse in their properties, different detector components are needed which are specifically suited to certain problems. The only exception is the neutrinos that are produced, which cannot be detected directly. The determination of the point of origin of the respective collision products is of crucial importance: This does not have to coincide with the collision point of the protons, since a part of the short-lived products decays still during the flight through the detector.

The basic structure of the detectors consists of a series of different detector parts of various designs and operating principles, which surround the collision point as completely as possible according to the onion-shell principle. Strong magnetic fields of superconducting magnets deflect the charged particles. The specific charge and momentum of charged particles can be determined from the orbital curvature. The innermost layer is the so-called track detector, a semiconductor detector with fine spatial resolution. It is surrounded by an electromagnetic and a hadronic calorimeter and a spectrometer for muons.

The lead nuclei are mainly collided in the ALICE detector, which was built specifically to measure these collisions. To a lesser extent, ATLAS and CMS also study such heavy ion collisions. In addition, lead nuclei can be made to collide with protons, which is studied by all four large detectors.

Data Analysis

The amount of data generated during operation by recorded detector signals or computer simulations is estimated at 30 petabytes per year. It would be much larger if sophisticated triggers at the hardware and software level did not discard a large part of the measurement signals before they are processed or permanently stored. The data volume of the CMS detector alone is comparable to that of a 70-megapixel camera taking 40 million pictures per second. Without triggers, such data volumes would be unmanageable with current technology. Thus, at the ATLAS detector, about 75,000 of the data from the 40 million beam crossings per second are selected in the first trigger stage. Of these, less than 1000 pass the second trigger stage, and only these events are fully analyzed. Ultimately, about 200 events per second are permanently stored.

To process this reduced amount of data, the required computing power is still so large that about 170 computer clusters distributed worldwide are used for this purpose. These are connected to form a computer network, the LHC Computing Grid.

For the simulation of particle trajectories in the accelerator ring, the LHC@Home project involves computer owners who make the computing power of their private computers available in accordance with the principle of distributed computing.

Power supply

The main feed-in point for supplying CERN with electrical energy is the 400 kV Prevessin substation, which is connected to the 400 kV Bois-Toillot substation via a short spur line. Another feed point is at 130 kV at the Meyrin station. From these feed points, 66 kV and 18 kV underground cables lead to the larger substations, where a transformation to the operating voltage of the terminal equipment (18 kV, 3.3 kV and 400 V) takes place. In the event of a power failure, emergency power generators with outputs of 275 kVA and 750 kVA are installed in the experiment stations; an uninterruptible power supply is guaranteed for particularly sensitive equipment.

The storage ring requires an electrical power of 120 MW. Together with the cooling system and the experiments, this results in a power requirement of about 170 MW. Due to the higher electricity costs, the LHC is partially switched off in winter, which then reduces the required power to 35 MW. The maximum annual energy consumption of the LHC is estimated at 700-800 GWh. By way of comparison, this is just under 10% of the consumption of the canton of Geneva. Thanks to the use of superconducting magnets, the energy consumption of the LHC is lower than that of previous experiments such as the LEP.

Costs

The immediate cost of the LHC project, excluding the detectors, is about 3 billion euros. When the construction was approved in 1995, a budget of 2.6 billion Swiss francs (corresponding to 1.6 billion euros at the time) was estimated for the construction of the LHC and the underground halls for the detectors. But by 2001, additional costs of 480 million Swiss francs (about 300 million euros) were estimated for the accelerator, of which 180 million Swiss francs (120 million euros) alone were for the superconducting magnets. Further cost increases were caused by technical difficulties in the construction of the underground hall for the Compact Muon Solenoid, partly due to defective parts provided by the partner laboratories Argonne National Laboratory, Fermilab and KEK.

During the first longer retrofit phase (February 2013 to April 2015), costs of around 100 million Swiss francs were incurred for work directly at the LHC.

Fears before commissioning in 2008

In physics beyond the Standard Model, theoretical models exist according to which it is possible that microscopic black holes or strange matter could be created at the LHC. There are isolated warnings that the LHC could destroy the Earth. A group led by biochemist Otto Rössler filed a complaint with the European Court of Human Rights against the commissioning of the LHC. The associated urgent application was rejected by the court in August 2008. The German Federal Constitutional Court refused to accept a constitutional complaint in February 2010 on the grounds of lack of fundamental importance and lack of prospect of success. Expert scientists have repeatedly stated that no dangers emanate from the LHC and other particle accelerators. The main arguments are that, firstly, the theoretically possible microscopic black holes would be immediately destroyed instead of absorbing more and more mass or energy from the environment, as feared, and that, secondly, natural cosmic radiation constantly hits the Earth's atmosphere and other celestial bodies with even higher energy than in the LHC without causing any catastrophes.

The ATLAS detector with 45 m length and 22 m diameter


Simulated detection of particles after proton collision in the CMS detector


Prototype of a dipole magnet


Tunnel of the LHC before installation of the magnets


Tunnel of the LHC in finished state