Large Hadron Collider
Wikipedia
The
Large Hadron Collider (
LHC) is the world's largest and highest-energy
particle collider.
[1][2] It was built by the
European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and hundreds of universities and laboratories, as well as more than 100 countries.
[3] 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.
The first collisions were achieved in 2010 at an energy of 3.5
teraelectronvolts (TeV) per beam, about four times the previous world record.
[4][5] 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 (13 TeV total collision energy).
[6][7][8][9] At the end of 2018, it was shut down for three years for further upgrades.
The collider has four crossing points where the accelerated particles collide.
Seven 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:
lead–lead collisions and
proton–lead collisions are typically performed for one month a year.
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,
[10] searching for the large family of new particles predicted by
supersymmetric theories,
[11] and other
unresolved questions in particle physics.
Background[edit]
The term
hadron refers to
subatomic composite particles composed of
quarks held together by the
strong force (analogous to the way that
atoms and
molecules are held together by the
electromagnetic force).
[12] 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.
[13]
A
collider is a type of a
particle accelerator which 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.
[1] 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.
[14]
Purpose[edit]
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 the
elementary objects, the deep structure of space and time, and in particular the interrelation between
quantum mechanics and
general relativity.
[15]
Data are also needed from
high-energy particle experiments to suggest which versions of current scientific models are more likely to be correct – in particular to choose between the
Standard Model and
Higgsless model and to validate their predictions and allow further theoretical development.
Issues explored by LHC collisions include:
[16][17]
Other open questions that may be explored using high-energy particle collisions:
Design[edit]
The collider is contained in a circular tunnel, with a circumference of 26.7 kilometres (16.6 mi), at a depth ranging from 50 to 175 metres (164 to 574 ft) 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, which would also have an impact on the landscape and to take advantage of the shielding against background radiation that the earth's crust provides.
[29]
Map of the Large Hadron Collider at CERN
The 3.8-metre (12 ft) wide concrete-lined tunnel, constructed between 1983 and 1988, was formerly used to house the
Large Electron–Positron Collider.
[30] 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.
Superconducting
quadrupole electromagnets are used to direct the beams to four intersection points, where interactions between accelerated protons will take place.
The collider tunnel contains two adjacent parallel
beamlines (or
beam pipes) 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 (see image
[31]), 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 the dipole magnets having a mass of over 27 tonnes.
[32] Approximately 96 tonnes of
superfluid helium-4 is needed to keep the magnets, made of copper-clad
niobium-titanium, at their
operating temperature of 1.9 K (−271.25 °C), making the LHC the largest
cryogenic facility in the world at liquid helium temperature. LHC uses 470 tonnes of Nb–Ti superconductor.
[33]
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.
[34] Each day of its operation generates 140
terabytes of data.
[35]
When running an energy of 6.5 TeV per proton,
[36] 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 7.7
teslas (T). 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 0.999999990
c, or about 3.1 m/s (11 km/h) slower than the
speed of light (
c). It takes less than 90
microseconds (μs) for a proton to travel 26.7 km around the main ring. This results in 11,245 revolutions 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.
[37]
Rather than having continuous beams, the protons are bunched together, into up to 2,808 bunches, with 115 billion protons in each bunch so that interactions between the two beams take place at discrete intervals, mainly 25
nanoseconds (ns) 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,
[38] which was first reached in June 2016.
[39] By 2017, twice this value was achieved.
[40]
The LHC protons originate from the small red hydrogen tank.
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 (H− ions), which feeds the
Proton Synchrotron Booster (PSB). 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 (PS), where they are accelerated to 26 GeV. Finally, the
Super Proton Synchrotron (SPS) is used to increase their energy further to 450 GeV before they are at last injected (over a period of several minutes) into the main ring. Here, the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak energy, and finally circulated for 5 to 24 hours while collisions occur at the four intersection points.
[41]
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
[42] (see
A Large Ion Collider Experiment). The lead ions are first accelerated by the linear accelerator
LINAC 3, and the
Low Energy Ion Ring (LEIR) 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 (or 522 TeV per ion),
[43] 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.
[44]
Detectors[edit]
See also:
List of Large Hadron Collider experiments
Nine detectors have been constructed at the LHC, located underground in large caverns excavated at the LHC's intersection points. Two of them, the
ATLAS experiment and the
Compact Muon Solenoid (CMS), are large general-purpose
particle detectors.
[2] ALICE and
LHCb have more specialized roles and 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.
[45]
CMS detector for LHC
Computing and analysis facilities[edit]
Main article:
Worldwide LHC Computing Grid
Data produced by LHC, as well as LHC-related simulation, were estimated at approximately 15
petabytes per year (max throughput while running is not stated)
[46]—a major challenge in its own right at the time.
The
LHC Computing Grid[47] was constructed as part of the LHC design, to handle the massive amounts of data expected for its collisions. It is an international collaborative project that consists of a grid-based
computer network infrastructure initially connecting 140 computing centres in 35 countries (over 170 in 36 countries as of 2012). It was designed by
CERN to handle the significant volume of data produced by LHC experiments,
[48][49] incorporating both private fibre optic cable links and existing high-speed portions of the public
Internet to enable data transfer from CERN to academic institutions around the world.
[50] The
Open Science Grid is used as the primary infrastructure in the United States, and also as part of an interoperable federation with the LHC Computing Grid.
The
distributed computing project
LHC@home was started to support the construction and calibration of the LHC. The project uses the
BOINC platform, enabling anybody with an Internet connection and a computer running
Mac OS X,
Windows or
Linux to use their computer's idle time to simulate how particles will travel in the beam pipes. With this information, the scientists are able to determine how the magnets should be calibrated to gain the most stable "orbit" of the beams in the ring.
[51] In August 2011, a second application (Test4Theory) went live which performs simulations against which to compare actual test data, to determine confidence levels of the results.
By 2012, data from over 6 quadrillion (6×1015) LHC proton–proton collisions had been analysed,
[52] LHC collision data was being produced at approximately 25
petabytes per year, and the LHC Computing Grid had become the world's largest
computing grid in 2012, comprising over 170 computing facilities in a
worldwide network across 36 countries.
[53][54][55]
Operational history[edit]
The LHC first went operational on 10 September 2008,
[56] but initial testing was delayed for 14 months from 19 September 2008 to 20 November 2009, following a
magnet quench incident that caused extensive damage to over 50
superconducting magnets, their mountings, and the
vacuum pipe.
[57][58][59][60][61]
During its first run (2010–2013), the LHC collided two opposing
particle beams of either
protons at up to 4
teraelectronvolts (4 TeV or 0.64
microjoules), or
lead nuclei (574 TeV per nucleus, or 2.76 TeV per
nucleon).
[62][63] Its first run discoveries included the
long-sought Higgs boson, several composite particles (
hadrons) like the χb (3P)
bottomonium state, the first creation of a
quark–gluon plasma, and the first observations of the very rare decay of the
Bs meson into two
muons (Bs0 → μ+μ−), which challenged the validity of existing models of
supersymmetry.
[64]
Construction[edit]
Operational challenges[edit]
The size of the LHC constitutes an exceptional engineering challenge with unique operational issues on account of the amount of energy stored in the magnets and the beams.
[41][65] While operating, the total
energy stored in the magnets is 10 GJ (2,400 kilograms of TNT) and the total energy carried by the two beams reaches 724 MJ (173 kilograms of TNT).
[66]
Loss of only one ten-millionth part (10−7) of the beam is sufficient to
quench a
superconducting magnet, while each of the two
beam dumps must absorb 362 MJ (87 kilograms of TNT). These energies are carried by very little matter: under nominal operating conditions (2,808 bunches per beam, 1.15×1011 protons per bunch), the beam pipes contain 1.0×10−9 gram of hydrogen, which, in
standard conditions for temperature and pressure, would fill the volume of one grain of fine sand.
See also:
List of megaprojects
With a budget of €7.5 billion (approx. $9bn or £6.19bn as of June 2010), the LHC is one of the most expensive scientific instruments
[1] ever built.
[67] The total cost of the project is expected to be of the order of 4.6bn
Swiss francs (SFr) (approx. $4.4bn, €3.1bn, or £2.8bn as of January 2010) for the accelerator and 1.16bn (SFr) (approx. $1.1bn, €0.8bn, or £0.7bn as of January 2010) for the CERN contribution to the experiments.
[68]
The construction of LHC was approved in 1995 with a budget of SFr 2.6bn, with another SFr 210M toward the experiments. However, cost overruns, estimated in a major review in 2001 at around SFr 480M for the accelerator, and SFr 50M for the experiments, along with a reduction in CERN's budget, pushed the completion date from 2005 to April 2007.
[69] The superconducting magnets were responsible for SFr 180M of the cost increase. There were also further costs and delays owing to engineering difficulties encountered while building the cavern for the
Compact Muon Solenoid,
[70] and also due to magnet supports which were insufficiently strongly designed and failed their initial testing (2007) and damage from a
magnet quench and
liquid helium escape (inaugural testing, 2008)
(see: Construction accidents and delays).
[71] Because electricity costs are lower during the summer, the LHC normally does not operate over the winter months,
[72] although exceptions over the 2009/10 and 2012/2013 winters were made to make up for the 2008 start-up delays and to improve precision of measurements of the new particle discovered in 2012, respectively.
Construction accidents and delays[edit]
- On 25 October 2005, José Pereira Lages, a technician, was killed in the LHC when a switchgear that was being transported fell on top of him.[73]
- On 27 March 2007, a cryogenic magnet support designed and provided by Fermilab and KEK broke during an initial pressure test involving one of the LHC's inner triplet (focusing quadrupole) magnet assemblies. No one was injured. Fermilab director Pier Oddone stated "In this case we are dumbfounded that we missed some very simple balance of forces". The fault had been present in the original design, and remained during four engineering reviews over the following years.[74] Analysis revealed that its design, made as thin as possible for better insulation, was not strong enough to withstand the forces generated during pressure testing. Details are available in a statement from Fermilab, with which CERN is in agreement.[75][76] Repairing the broken magnet and reinforcing the eight identical assemblies used by LHC delayed the start-up date, then planned for November 2007.
- On 19 September 2008, during initial testing, a faulty electrical connection led to a magnet quench (the sudden loss of a superconducting magnet's superconducting ability owing to warming or electric field effects). Six tonnes of supercooled liquid helium—used to cool the magnets—escaped, with sufficient force to break 10-ton magnets nearby from their mountings, and caused considerable damage and contamination of the vacuum tube. Repairs and safety checks caused a delay of around 14 months.[77][78][79]
- Two vacuum leaks were found in July 2009, and the start of operations was further postponed to mid-November 2009.[80]
Exclusion of Russia[edit]
With the 2022 invasion of
Ukraine by
Russia, the participation of Russians with CERN was called into question. Approximately 8% of the workforce are of Russian nationality. In June 2022 CERN said the governing council "intends to terminate" CERN's cooperation agreements with Belarus and Russia when they expire, respectively in June and December 2024. CERN said it would monitor developments in Ukraine and remains prepared to take additional steps as warranted.
[81][82] CERN further said that it would reduce the Ukrainian contribution to CERN for 2022 to the amount already remitted to the Organization, thereby waiving the second instalment of the contribution.
[83]
Initial lower magnet currents[edit]
Main article:
Superconducting magnet § Magnet "training"
In both of its runs (2010 to 2012 and 2015), the LHC was initially run at energies below its planned operating energy, and ramped up to just 2 x 4 TeV energy on its first run and 2 x 6.5 TeV on its second run, below the design energy of 2 x 7 TeV. This is because massive superconducting magnets require considerable
magnet training to handle the high currents involved without
losing their superconducting ability, and the high currents are necessary to allow a high proton energy. The "training" process involves repeatedly running the magnets with lower currents to provoke any quenches or minute movements that may result. It also takes time to cool down magnets to their operating temperature of around 1.9
K (close to
absolute zero). Over time the magnet "beds in" and ceases to quench at these lesser currents and can handle the full design current without quenching; CERN media describe the magnets as "shaking out" the unavoidable tiny manufacturing imperfections in their crystals and positions that had initially impaired their ability to handle their planned currents. The magnets, over time and with training, gradually become able to handle their full planned currents without quenching.
[84][85]
Inaugural tests (2008)[edit]
The first beam was circulated through the
collider on the morning of 10 September 2008.
[86] CERN successfully fired the protons around the tunnel in stages, three kilometres at a time. The particles were fired in a clockwise direction into the accelerator and successfully steered around it at 10:28 local time.
[56] The LHC successfully completed its major test: after a series of trial runs, two white dots flashed on a computer screen showing the protons travelled the full length of the collider. It took less than one hour to guide the stream of particles around its inaugural circuit.
[87] CERN next successfully sent a beam of protons in an anticlockwise direction, taking slightly longer at one and a half hours owing to a problem with the
cryogenics, with the full circuit being completed at 14:59.
