Large Hadron Collider

• 1. Introduction

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  On the morning of the 10th of September 2008, the Large Hadron Collider received its first beam of protons. The particles were fired clockwise into the ring in stages, three kilometres at a time. At 10:28 local time, two white dots flashed on a computer screen. They showed that the protons had travelled the full length of the machine. The whole circuit took less than one hour. The collider is the world's largest and highest-energy particle accelerator, run by CERN, the European Organization for Nuclear Research, near Geneva. How do you build such a machine underground? What has it found about the smallest pieces of matter? And what happened when something went badly wrong?

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• 2. A Borrowed Tunnel

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  Between 1983 and 1988, a concrete-lined tunnel 3.8 metres wide was constructed beneath the border between France and Switzerland. It was built for an earlier machine, the Large Electron-Positron Collider. The Large Hadron Collider later took over the same tunnel. The ring has a circumference of 26.7 kilometres, and it lies between 50 and 175 metres underground. That variation in depth was deliberate. It reduced the amount of tunnel lying under the Jura Mountains, so engineers did not need to excavate a vertical access shaft there. A tunnel also spared CERN from buying expensive land on the surface, and the Earth's crust shields the experiments from background radiation.

  Most of the ring lies in France, and the tunnel crosses the border four times. Surface buildings hold the supporting equipment: compressors, ventilation equipment, control electronics and refrigeration plants. Between 1998 and 2008, CERN built the collider with more than 10,000 scientists, together with hundreds of universities and laboratories in more than 100 countries.

  Inside the tunnel run two adjacent parallel beam pipes. Each pipe holds one beam, and the two beams travel in opposite directions around the ring. They cross at the intersection points, which is where the collisions happen. Keeping both beams on their circular path falls to 1,232 dipole magnets.

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• 3. Cold Magnets

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  Each dipole magnet has a mass of 35 tonnes, and all of them operate at 1.9 K, close to absolute zero. Besides the dipoles, 392 quadrupole magnets keep the beams focused. The strongest quadrupoles sit near the intersection points, where the two beams cross and the chance of an interaction matters most. Magnets of higher multipole orders correct smaller imperfections in the field. In total, about 10,000 superconducting magnets are installed.

  Holding them at that temperature takes about 96 tonnes of superfluid helium-4. The magnets are made of copper-clad niobium-titanium, and the collider uses 470 tonnes of that superconductor. This makes the LHC the largest cryogenic facility in the world at liquid helium temperature. Once or twice a day, as protons are accelerated from 450 GeV to 6.5 TeV, the field of the dipoles is raised from 0.54 to 7.7 teslas.

  While operating, the magnets together store 10 GJ of energy, and the two beams carry 724 MJ. Losing only one ten-millionth of a beam is enough to quench a superconducting magnet. A quench is the sudden loss of a magnet's superconducting ability, caused by warming or electric field effects. Each of the two beam dumps must absorb 362 MJ. Yet very little matter carries all that energy. Under normal operating conditions, the beam pipes hold about a billionth of a gram of hydrogen, which at standard temperature and pressure would fill the volume of one grain of fine sand.

  In both of its first two runs, the LHC started below its planned operating energy. It ramped up to two beams of 4 TeV in the first run and two beams of 6.5 TeV in the second, short of the design figure of two beams of 7 TeV. The reason is magnet training. Operators repeatedly run the magnets at lower currents to provoke quenches and minute movements. Over time the magnets bed in. CERN media describe them as shaking out tiny manufacturing imperfections in their crystals and positions.

  During operations, the CERN site draws roughly 200 MW from the French electrical grid, about one-third of the consumption of the city of Geneva. The accelerator and detectors account for about 120 MW of that. Because electricity costs are lower in summer, the LHC normally does not operate during the winter months.

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• 4. Preparing The Beams

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  A proton's journey begins in Linac4, a linear accelerator that generates negative hydrogen ions with an energy of 160 MeV. These ions feed the Proton Synchrotron Booster, where both electrons are stripped away, leaving a nucleus that is a single proton. The protons are accelerated to 2 GeV and injected into the Proton Synchrotron, which raises them to 26 GeV. The Super Proton Synchrotron then lifts them to 450 GeV. Only then are they injected into the main ring, over a period of several minutes. There the bunches accumulate and are accelerated over 20 minutes to peak energy. Collisions continue while the beams circulate for 5 to 24 hours.

  The protons travel in as many as 2,808 bunches, each holding 115 billion protons. Because of the bunching, the two beams meet at discrete intervals, mainly 25 nanoseconds apart. That gives a bunch collision rate of 40 MHz. In the first years, the machine ran with fewer bunches.

  At 6.5 TeV, a proton has a Lorentz factor of about 6,930, a measure of how close it runs to the speed of light. It moves only about 3.1 metres per second slower than light itself. A lap of the 26.7 kilometre ring takes less than 90 microseconds, which works out to 11,245 revolutions per second. The figure is the same at low or high energy, since the speed difference between the two appears only beyond the fifth decimal place.

  For roughly one month each year, the LHC swaps protons for heavy ions, with lead as the baseline. The linear accelerator LINAC 3 accelerates the lead first, and the Low Energy Ion Ring stores and cools it. The Proton Synchrotron and Super Proton Synchrotron lift it further before injection. In the main ring the ions reach 2.3 TeV per nucleon, or 522 TeV per ion, higher than the energies reached by the Relativistic Heavy Ion Collider. The aim is to investigate quark-gluon plasma, which existed in the early universe.

  The design luminosity, a measure of how many collisions the beams can produce, is 10 to the power of 34 per square centimetre per second. The LHC first reached it in June 2016, and by 2017 it had achieved twice that value.

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• 5. Detectors And Data

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  Nine detectors sit in large caverns excavated at the LHC's intersection points. ATLAS and the Compact Muon Solenoid, known as CMS, are large general-purpose particle detectors. ALICE and LHCb have more specialized roles. The other five, TOTEM, MoEDAL, LHCf, SND and FASER, are much smaller and built for very specialized research. Together with the collider, they generate 140 terabytes of data each day of operation, and the data from the LHC plus related simulation were estimated at 200 petabytes per year.

  The LHC Computing Grid was built as part of the original design to handle that flood. It began as a network connecting 140 computing centres in 35 countries. By 2012 it had grown to over 170 computing facilities in more than 40 countries, making it the world's largest computing grid. CERN designed it to send data to academic institutions around the world, using private fibre optic links as well as existing high-speed portions of the public Internet. Its federations stretch across Europe, Asia Pacific and the Americas.

  Volunteers helped too. LHC@home, which runs on the BOINC platform, lets anyone with an Internet connection use a computer's idle time. The computers run Mac OS X, Windows or Linux and simulate how particles travel in the beam pipes. From those simulations, scientists work out how to calibrate the magnets for the most stable orbit of the beams. By 2012, data from over 6 quadrillion proton-proton collisions had been analysed. In August 2011, a second application called Test4Theory went live, running simulations to compare with actual test data and so set confidence levels for the results.

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• 6. Delays And Damage

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  In 1995, the construction of the LHC was approved with a budget of SFr 2.6bn, plus another SFr 210M for the experiments. A major review in 2001 estimated overruns of around SFr 480M for the accelerator and SFr 50M for the experiments. CERN's own budget was also reduced. Together these pushed the completion date from 2005 to April 2007. The superconducting magnets accounted for SFr 180M of the increase, and engineering difficulties with the cavern for the CMS detector added more. With a budget of €7.5 billion, the LHC is one of the most expensive scientific instruments ever built.

  On the 25th of October 2005, a technician named José Pereira Lages was killed in the LHC when a switchgear being transported fell on top of him. On the 27th of March 2007, a cryogenic magnet support designed and provided by Fermilab and KEK broke during a pressure test of one of the inner triplet focusing magnet assemblies. No one was injured. Fermilab director Pier Oddone said, "In this case we are dumbfounded that we missed some very simple balance of forces". The fault had been in the original design and survived four engineering reviews. The support had been made as thin as possible for better insulation, and it was too weak for the forces of pressure testing. Repairing the magnet and reinforcing eight identical assemblies delayed the start-up planned for November 2007.

  On the afternoon of the 10th of September 2008, CERN also sent a beam of protons anticlockwise. A cryogenics problem slowed it to one and a half hours, and the full circuit was completed at 14:59. Nine days later, on the 19th of September 2008, a magnet quench struck about 100 bending magnets in sectors 3 and 4. A faulty electrical connection between two magnets had triggered a failsafe power abort, as designed. But it also caused an electric arc that damaged the enclosure and vacuum insulation of the supercooled helium. Temperature and pressure rose beyond what the safety systems could contain, and some magnets heated by about 100 degrees Celsius.

  Around two tonnes of liquid helium escaped explosively before detectors triggered an emergency stop, and a further four tonnes leaked at lower pressure afterwards. The force broke 10-ton magnets from their mountings and contaminated the vacuum pipe. In all, 53 superconducting magnets were damaged, and they were repaired or replaced during the winter shutdown. CERN first estimated at least two months of repairs, needed to warm up the affected sectors and cool them back down. It released an interim technical report on the 15th of October 2008, a preliminary analysis on the 16th, and a detailed report on the 5th of December. Despite the damage, the LHC was officially inaugurated on the 21st of October 2008, in front of political leaders and science ministers from CERN's 20 Member States. Two further vacuum leaks, found in July 2009, pushed the restart to mid-November 2009. The total delay from the accident came to around 14 months.

  CERN physicist Lucio Rossi later gave a thorough account of the accident in a Superconductor Science and Technology article dated the 22nd of February 2010.

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• 7. Discoveries And Questions

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  On the 30th of November 2009, the LHC reached 1.18 TeV per beam and became the world's highest-energy particle accelerator. It overtook the Tevatron, whose record of 0.98 TeV per beam had stood for eight years. The first physics results followed on the 15th of December 2009, covering 284 collisions in the ALICE detector. Then on the 30th of March 2010, the two beams collided at a combined 7 TeV, on the third attempt of the day. Two earlier attempts had failed, forcing operators to dump the protons and inject fresh beams. That collision marked the start of the main research programme.

  Dark matter, a hypothetical form of matter that appears to account for 27% of the mass-energy of the universe, is one of the open questions the collisions probe. Others include whether extra dimensions exist, as various models based on string theory predict. Physicists also ask why gravity is so many orders of magnitude weaker than the other three forces. They want to know why matter and antimatter behave with apparent asymmetry. Some hope the LHC will help connect quantum mechanics with general relativity.

  The Higgs boson was the first target. The Standard Model predicted it, but its high mass and elusive nature had kept it unobserved. CERN scientists estimated that, if the model was correct, the LHC would produce several Higgs bosons every minute. In summer 2011, results presented at conferences in Grenoble and Mumbai let ATLAS and CMS exclude, at 95% confidence, a Standard Model Higgs over most of the mass region between 145 and 466 GeV. On the 13th of December 2011, CERN reported that the particle, if it existed, was most likely between 115 and 130 GeV. Both detectors showed intensity peaks at 124-125 GeV, consistent with either background noise or the real thing.

  On the 4th of July 2012, the CMS and ATLAS teams each announced a boson with a mass around 125-126 GeV, at a statistical significance of 5 sigma. That is the formal level required to announce a new particle. Scientists stayed cautious about calling it the Higgs boson until further analysis. On the 14th of March 2013, CERN announced confirmation that it was indeed the predicted Higgs boson. Its detection completed the Standard Model. Because of the discovery, the shutdown planned for late 2012 was postponed by some weeks into early 2013, to collect more data.

  Not every result favoured the Standard Model's rivals or its challengers equally. On the 8th of November 2012, the LHCb team measured the very rare decay of the Bs meson into two muons, seen as a golden test of supersymmetry. The result, at 3.5 sigma, matched the Standard Model rather than many branches of supersymmetry, though other versions could still fit. CMS later confirmed it. In August 2013, LHCb revealed an anomaly in the angular distribution of B meson decay products, with a statistical certainty of 4.5 sigma. That is just short of the 5 sigma needed for a discovery, and the Z' boson has been suggested as a possible cause.

  On the 24th of May 2011, it was reported that quark-gluon plasma, the densest matter thought to exist besides black holes, had been created in the LHC. On the 22nd of December 2011, a new composite particle was reported, the chi-b (3P) bottomonium state. On the 19th of November 2014, LHCb announced two new heavy baryons, each made of one bottom, one down and one strange quark. Results consistent with pentaquark states followed on the 13th of July 2015. By March 2021, the LHC experiments had discovered 59 new hadrons in the data from the first two runs, and by February 2026 the count was 79. On the 5th of July 2022, LHCb reported a new type of pentaquark and the first ever pair of tetraquarks. On the 18th of September 2024, ATLAS reported the first observation of quantum entanglement between quarks, the highest-energy observation of entanglement so far.

  Long Shutdown 1 began on the 13th of February 2013, and it replaced the ventilation system and 100 kilometres of cabling impaired by the first run's high-energy collisions. The LHC restarted on the 5th of April 2015, and it started delivering physics data on the 3rd of June 2015 at 13 TeV collision energy. In 2016, the total number of collisions exceeded that of the whole first run, at a higher energy per collision. Run 2 ended on the 3rd of December 2018, and Long Shutdown 2 began on the 10th of December 2018. The LHC became operational again on the 22nd of April 2022 with a new maximum of 6.8 TeV per beam, or 13.6 TeV in collisions. Run 3 was expected to continue until 2026.

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• 8. Fears, Fame And Futures

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  Two safety reviews commissioned by CERN examined fears that the LHC's collisions might produce stable microscopic black holes or hypothetical particles called strangelets. Both concluded that the experiments present no danger, a conclusion endorsed by the American Physical Society. The reports pointed out that such conditions occur naturally and routinely in the universe. They cited the Oh-My-God particle, an ultra-high-energy cosmic ray with 320 million TeV of energy, which struck Earth with a collision energy tens of times higher than the most energetic collisions produced in the LHC.

  About 8% of CERN's workforce is of Russian nationality, which put participation in question after the 2022 Russian invasion of Ukraine. In June 2022, CERN said its governing council "intends to terminate" cooperation agreements with Belarus and Russia when they expire, in June and December 2024 respectively. CERN also reduced the Ukrainian contribution for 2022 to the amount already remitted.

  In popular culture, the novel Angels & Demons by Dan Brown imagines antimatter created at the LHC and used as a weapon against the Vatican. CERN responded by publishing a "Fact or Fiction?" page. The film version includes footage filmed on-site at one of the experiments, and director Ron Howard met CERN experts to make the science more accurate. CERN employee Katherine McAlpine's "Large Hadron Rap" had passed 8 million YouTube views by 2022. The band Les Horribles Cernettes was founded by women from CERN, and the name was chosen to share the LHC's initials. In the anime Steins;Gate, CERN appears as the secret organization SERN, which uses the LHC to create Kerr black holes for time travel.

  A luminosity upgrade called the High Luminosity Large Hadron Collider started in June 2018. It aims to raise luminosity by a factor of 10, up to 10 to the power of 35 per square centimetre per second, from 2030. After some years, any particle physics experiment suffers diminishing returns, because the key results within its reach are completed. Upgrading energy, luminosity or detectors is the usual response. CERN also has preliminary designs for a Future Circular Collider, with costs ranging from around €9 billion to €21 billion. It would use the LHC ring as a pre-accelerator, just as the LHC uses the Super Proton Synchrotron. As of 2023, no fixed plan exists, and it is unknown whether construction will be funded.

  The tentative LHC schedule runs well beyond all of this, with its final entry a lead-ion run in December 2041.

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