Large Hadron Collider

A section of the LHC
Large Hadron Collider
(LHC)
LHC experiments
ATLAS A Toroidal LHC Apparatus
CMS Compact Muon Solenoid
LHCb LHC-beauty
ALICE A Large Ion Collider Experiment
TOTEM Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf LHC-forward
MoEDAL Monopole and Exotics Detector At the LHC
LHC preaccelerators
p and Pb Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked) Proton Synchrotron Booster
PS Proton Synchrotron
SPS Super Proton Synchrotron
CERN accelerator complex
List of current particle
accelerators at CERN
Linac2 Accelerates protons
Linac3 Accelerates ions
Linac4 Accelerates negative hydrogen ions
AD Decelerates antiprotons
LHC Collide protons or lead ions
LEIR Accelerates lead ions
PS Accelerates particles
PSB Accelerates protons
SPS Accelerates protons and lead ions
Hadron colliders
Intersecting Storage Rings CERN, 1971–1984
Proton-Antiproton Collider (SPS) CERN, 1981–1991
ISABELLE BNL, cancelled in 1983
Tevatron Fermilab, 1987–2011
Relativistic Heavy Ion Collider BNL, 2000–present
Superconducting Super Collider Cancelled in 1993
Large Hadron Collider CERN, 2009–present
Future Circular Collider Proposed

The Large Hadron Collider (LHC) is the world's largest and most powerful particle collider, most complex experimental facility ever built, and the largest single machine in the world.[1] It was built by the European Organization for Nuclear Research (CERN) between 1998 and 2008 in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories.[2] It lies in a tunnel 27 kilometres (17 mi) in circumference, as deep as 175 metres (574 ft) beneath the France–Switzerland border near Geneva, Switzerland. Its first research run took place from March 2010 to early 2013 at an energy of 3.5 to 4 teraelectronvolts (TeV) per beam (7 to 8 TeV total), about 4 times the previous world record for a collider.[3][4] Afterwards, the accelerator was upgraded for two years. It was restarted in early 2015 for its second research run, reaching 6.5 TeV per beam (13 TeV total, the current world record).[5][6][7][8]

The aim of the LHC is to allow physicists to test the predictions of different theories of particle physics, including measuring the properties of the Higgs boson[9] and searching for the large family of new particles predicted by supersymmetric theories,[10] as well as other unsolved questions of physics.

The collider has four crossing points, around which are positioned seven detectors, each designed for certain kinds of research. The LHC primarily collides proton beams, but it can also use beams of lead nuclei. Proton–lead collisions were performed for short periods in 2013 and 2016, and lead–lead collisions took place in 2010, 2011, 2013, and 2015.

The LHC's computing grid is a world record holder. Data from collisions was produced at an unprecedented rate for the time of first collisions, tens of petabytes per year, a major challenge at the time, to be analysed by a grid-based computer network infrastructure connecting 170 computing centres in 42 countries as of 2017[11][12] – by 2012 the Worldwide LHC Computing Grid was also the world's largest distributed computing grid, comprising over 170 computing facilities in a worldwide network across 36 countries.[13][14][15]

Background

The term hadron refers to composite particles composed of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic 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 with two directed beams of particles. In particle physics, colliders are used as a research tool: they accelerate particles to very high kinetic energies and let them impact other particles. 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

Physicists hope that the Large Hadron Collider will help answer some of the fundamental open questions in physics, concerning 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.

Data is 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. Many theorists expect new physics beyond the Standard Model to emerge at the TeV energy level, as the Standard Model appears to be unsatisfactory. Issues explored by LHC collisions include: [16][17]

Other open questions that may be explored using high-energy particle collisions:

Design

A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two quarks each emit a W or Z boson, which combine to make a neutral Higgs.
Map of the Large Hadron Collider at CERN
The 2-in-1 structure of the LHC dipole magnets

The LHC is the world's largest and highest-energy particle accelerator.[30][31] The collider is contained in a circular tunnel, with a circumference of 27 kilometres (17 mi), at a depth ranging from 50 to 175 metres (164 to 574 ft) underground.

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.[32] It 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 (or beam pipes) that intersect at four points, each containing a beam, which travel in opposite directions around the ring. Some 1,232 dipole magnets keep the beams on their circular path (see image[33]), 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.[34] 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.

Superconducting quadrupole electromagnets are used to direct the beams to four intersection points, where interactions between accelerated protons will take place.

When running at the current energy record of 6.5 TeV per proton,[35] once or twice a day, as the protons are accelerated from 450 GeV to 6.5 TeV, the field of the superconducting dipole magnets will be 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 once around the main ring – a speed of about 11,000 revolutions per second. 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,[36] which was first reached in June 2016.[37]

Prior to 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 LINAC 2 generating 50-MeV protons, which feeds the Proton Synchrotron Booster (PSB). There the protons are accelerated to 1.4 GeV and injected into the Proton Synchrotron (PS), where they are accelerated to 26 GeV. Finally the Super Proton Synchrotron (SPS) is used to further increase their energy 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.[38]

CMS detector for LHC

The LHC physics programme is mainly based on proton–proton collisions. However, shorter running periods, typically one month per year, with heavy-ion collisions are included in the programme. While lighter ions are considered as well, the baseline scheme deals with lead ions[39] (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 reached an energy of 2.3 TeV per nucleon (or 522 TeV per ion),[40] 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.

The LHC protons originate from the small red hydrogen tank.

Detectors

Seven 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.[31] ALICE and LHCb have more specific roles and the last three, TOTEM, MoEDAL and LHCf, are very much smaller and are for very specialized research. The BBC's summary of the main detectors is:[41]

Detector Description
ATLAS One of two general purpose detectors. ATLAS studies the Higgs boson and looks for signs of new physics, including the origins of mass and extra dimensions.
CMS The other general purpose detector, like ATLAS, studies the Higgs boson and look for clues of new physics.
ALICE ALICE is studying a "fluid" form of matter called quark–gluon plasma that existed shortly after the Big Bang.
LHCb Equal amounts of matter and antimatter were created in the Big Bang. LHCb investigates what happened to the "missing" antimatter.

Computing and analysis facilities

Data produced by LHC, as well as LHC-related simulation, was estimated at approximately 15 petabytes per year (max throughput while running not stated)[42] - a major challenge in its own right at the time.

The LHC Computing Grid[43] 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,[11][12] 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.[44] 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.[45] In August 2011, a second application went live (Test4Theory) 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 x 1015) LHC proton-proton collisions had been analysed,[46] 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 (as of 2012), comprising over 170 computing facilities in a worldwide network across 36 countries.[13][14][15]

Operational history

The LHC first went live on 10 September 2008,[47] 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.[48][49][50][51][52]

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).[30][53] 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.[54]

Construction

Operational challenges

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.[38][55] 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).[56]

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.

Cost

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[57] ever built.[58] 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 Jan 2010) for the accelerator and 1.16bn (SFr) (approx. $1.1bn, €0.8bn, or £0.7bn as of Jan 2010) for the CERN contribution to the experiments.[59]

The construction of LHC was approved in 1995 with a budget of SFr 2.6bn, with another SFr 210M towards 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.[60] The superconducting magnets were responsible for SFr 180M of the cost increase. There were also further costs and delays due to engineering difficulties encountered while building the underground cavern for the Compact Muon Solenoid,[61] 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).[62] Due to lower electricity costs during the summer, the LHC normally does not operate over the winter months,[63] 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

Initial lower magnet currents

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.[72][73]

Inaugural tests (2008)

The first beam was circulated through the collider on the morning of 10 September 2008.[41] 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.[47] 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.[74] CERN next successfully sent a beam of protons in an anticlockwise direction, taking slightly longer at one and a half hours due to a problem with the cryogenics, with the full circuit being completed at 14:59.

Quench incident

On 19 September 2008, a magnet quench occurred in about 100 bending magnets in sectors 3 and 4, where an electrical fault led to a loss of approximately six tonnes of liquid helium (the magnets' cryogenic coolant), which was vented into the tunnel. The escaping vapour expanded with explosive force, damaging total 53 superconducting magnets and their mountings, and contaminating the vacuum pipe, which also lost vacuum conditions.[48][49][75]

Shortly after the incident CERN reported that the most likely cause of the problem was a faulty electrical connection between two magnets, and that – due to the time needed to warm up the affected sectors and then cool them back down to operating temperature – it would take at least two months to fix.[76] CERN released an interim technical report[75] and preliminary analysis of the incident on 15 and 16 October 2008 respectively,[77] and a more detailed report on 5 December 2008.[69] The analysis of the incident by CERN confirmed that an electrical fault had indeed been the cause. The faulty electrical connection had led (correctly) to a failsafe power abort of the electrical systems powering the superconducting magnets, but had also caused an electric arc (or discharge) which damaged the integrity of the supercooled helium's enclosure and vacuum insulation, causing the coolant's temperature and pressure to rapidly rise beyond the ability of the safety systems to contain it,[75] and leading to a temperature rise of about 100 degrees Celsius in some of the affected magnets. Energy stored in the superconducting magnets and electrical noise induced in other quench detectors also played a role in the rapid heating. Around two tonnes of liquid helium escaped explosively before detectors triggered an emergency stop, and a further four tonnes leaked at lower pressure in the aftermath.[75] A total of 53 magnets were damaged in the incident and were repaired or replaced during the winter shutdown.[78] This accident was thoroughly discussed in a 22 February 2010 Superconductor Science and Technology article by CERN physicist Lucio Rossi.[79]

In the original timeline of the LHC commissioning, the first "modest" high-energy collisions at a centre-of-mass energy of 900 GeV were expected to take place before the end of September 2008, and the LHC was expected to be operating at 10 TeV by the end of 2008.[80] However, due to the delay caused by the above-mentioned incident, the collider was not operational until November 2009.[81] Despite the delay, LHC was officially inaugurated on 21 October 2008, in the presence of political leaders, science ministers from CERN's 20 Member States, CERN officials, and members of the worldwide scientific community.[82]

Most of 2009 was spent on repairs and reviews from the damage caused by the quench incident, along with two further vacuum leaks identified in July 2009 which pushed the start of operations to November of that year.[71]

Run 1: first operational run (2009–2013)

Seminar on the physics of LHC by John Iliopoulos (2009).[83]

On 20 November 2009, low-energy beams circulated in the tunnel for the first time since the incident, and shortly after, on 30 November, the LHC achieved 1.18 TeV per beam to become the world's highest-energy particle accelerator, beating the Tevatron's previous record of 0.98 TeV per beam held for eight years.[84]

The early part of 2010 saw the continued ramp-up of beam in energies and early physics experiments towards 3.5 TeV per beam and on 30 March 2010, LHC set a new record for high-energy collisions by colliding proton beams at a combined energy level of 7 TeV. The attempt was the third that day, after two unsuccessful attempts in which the protons had to be "dumped" from the collider and new beams had to be injected.[85] This also marked the start of the main research programme.

The first proton run ended on 4 November 2010. A run with lead ions started on 8 November 2010, and ended on 6 December 2010,[86] allowing the ALICE experiment to study matter under extreme conditions similar to those shortly after the Big Bang.[87]

CERN originally planned that the LHC would run through to the end of 2012, with a short break at the end of 2011 to allow for an increase in beam energy from 3.5 to 4 TeV per beam.[4] At the end of 2012 the LHC was planned to get shut down until around 2015 to allow upgrade to a planned beam energy of 7 TeV per beam.[88] In late 2012, in light of the July 2012 discovery of the Higgs boson, the shutdown was postponed for some weeks into early 2013, to allow additional data to be obtained prior to shutdown.

Upgrade (2013–2015)

The LHC was shut down on 13 February 2013 for its 2-year upgrade, which would touch on many aspects of the LHC: enabling collisions at 14 TeV, enhancing its detectors and pre-accelerators (the Proton Synchrotron and Super Proton Synchrotron), as well as replacing its ventilation system and 100 km of cabling impaired by high-energy collisions from its first run.[89] The upgraded collider began its long start-up and testing process in June 2014, with the Proton Synchrotron Booster starting on 2 June 2014, the final interconnection between magnets completing and the Proton Synchrotron circulating particles on 18 June 2014, and the first section of the main LHC supermagnet system reaching operating temperature of 1.9 K (−271.25 °C), a few days later.[90] Due to the slow progress with "training" the superconducting magnets, it was decided to start the second run with a lower energy of 6.5 TeV per beam, corresponding to a current of 11,000 amperes. The first of the main LHC magnets were reported to have been successfully trained by 9 December 2014, while training the other magnet sectors was finished in March 2015.[91]

Run 2: second operational run (2015–2018)

On 5 April 2015, the LHC restarted after a two-year break, during which the electrical connectors between the bending magnets were upgraded to safely handle the current required for 7 TeV per beam (14 TeV).[5][92] However, the bending magnets were only trained to handle up to 6.5 TeV per beam (13 TeV total), which became the operating energy for 2015 to 2017.[93] The energy was first reached on 10 April 2015.[94] The upgrades culminated in colliding protons together with a combined energy of 13 TeV.[95] On 3 June 2015 the LHC started delivering physics data after almost two years offline.[96] In the following months it was used for proton-proton collisions, while in November the machine switched to collisions of lead ions and in December the usual winter shutdown started.

In 2016, the machine operators focused on increasing the luminosity for proton-proton collisions. The design value was first reached 29 June,[37] and further improvements increased the collision rate to 40% above the design value.[97] The total number of collisions in 2016 exceeded the number from Run 1 - at a higher energy per collision. The proton-proton run was followed by four weeks of proton-lead collisions.[98]

Timeline of operations

Date Event
10 Sep 2008 CERN successfully fired the first protons around the entire tunnel circuit in stages.
19 Sep 2008 Magnetic quench occurred in about 100 bending magnets in sectors 3 and 4, causing a loss of approximately 6 tonnes of liquid helium.
30 Sep 2008 First "modest" high-energy collisions planned but postponed due to accident.[34]
16 Oct 2008 CERN released a preliminary analysis of the accident.
21 Oct 2008 Official inauguration.
5 Dec 2008 CERN released detailed analysis.
20 Nov 2009 Low-energy beams circulated in the tunnel for the first time since the accident.[51]
23 Nov 2009 First particle collisions in all four detectors at 450 GeV.
30 Nov 2009 LHC becomes the world's highest-energy particle accelerator achieving 1.18 TeV per beam, beating the Tevatron's previous record of 0.98 TeV per beam held for eight years.[84]
15 Dec 2009 First scientific results, covering 284 collisions in the ALICE detector.[99]
28 Feb 2010 The LHC continues operations ramping energies to run at 3.5 TeV for 18 months to two years, after which it will be shut down to prepare for the 14 TeV collisions (7 TeV per beam).[100]
30 Mar 2010 The two beams collided at 7 TeV (3.5 TeV per beam) in the LHC at 13:06 CEST, marking the start of the LHC research programme.
8 Nov 2010 Start of the first run with lead ions.
6 Dec 2010 End of the run with lead ions. Shutdown until early 2011.
13 Mar 2011 Beginning of the 2011 run with proton beams.[101]
21 Apr 2011 LHC becomes the world's highest-luminosity hadron accelerator achieving a peak luminosity of 4.67·1032 cm−2s−1, beating the Tevatron's previous record of 4·1032 cm−2s−1 held for one year.[102]
24 May 2011 Quark–gluon plasma achieved.[103]
17 Jun 2011 The high-luminosity experiments ATLAS and CMS reach 1 fb−1 of collected data.[104]
14 Oct 2011 LHCb reaches 1 fb−1 of collected data.[105]
23 Oct 2011 The high-luminosity experiments ATLAS and CMS reach 5 fb−1 of collected data.
Nov 2011 Second run with lead ions.
22 Dec 2011 First new composite particle discovery, the χb (3P) bottomonium meson, observed with proton-proton collisions in 2011.[106]
5 Apr 2012 First collisions with stable beams in 2012 after the winter shutdown. The energy is increased to 4 TeV per beam (8 TeV in collisions).[107]
4 Jul 2012 First new elementary particle discovery, a new boson observed that is "consistent with" the theorized Higgs boson. (This has now been confirmed as the Higgs boson itself.[108])
8 Nov 2012 First observation of the very rare decay of the Bs meson into two muons (Bs0 → μ+μ−), a major test of supersymmetry theories,[109] shows results at 3.5 sigma that match the Standard Model rather than many of its super-symmetrical variants.
20 Jan 2013 Start of the first run colliding protons with Lead ions.
11 Feb 2013 End of the first run colliding protons with Lead ions.
14 Feb 2013 Beginning of the first long shutdown, to prepare the collider for a higher energy and luminosity. When reactivated in 2015, the LHC will operate with an energy of 6.5 TeV per proton.[110]
7 Mar 2015 Injection tests for Run 2 send protons towards LHCb & ALICE
5 Apr 2015 Both beams circulated in the collider.[5] Four days later, a new record energy of 6.5 TeV per proton was achieved.[111]

20 May 2015

 Protons collided in the LHC at the record-breaking energy of 13 TeV.[95]
3 Jun 2015 Start of delivering the physics data after almost two years offline for re-commissioning.[96]
4 Nov 2015 End of proton collisions in 2015, start of preparations for ion collisions.
25 Nov 2015 First ion collisions at a record-breaking energy of more than 1 PeV (1015 eV)[112]
13 Dec 2015 End of ion collisions in 2015
23 Apr 2016 Data-taking in 2016 begins
29 June 2016 The LHC achieves a luminosity of 1.0 · 1034 cm−2s−1, its design value.[37] Further improvements over the year increased the luminosity to 40% above the design value.[97]
26 Oct 2016 End of 2016 proton-proton collisions
10 Nov 2016 Beginning of 2016 proton-lead collisions
3 Dec 2016 End of 2016 proton-lead collisions
24 May 2017 Start of 2017 proton-proton collisions

Findings and discoveries

An initial focus of research was to investigate the possible existence of the Higgs boson, a key part of the Standard Model of physics which is predicted by theory but had not yet been observed before due to its high mass and elusive nature. CERN scientists estimated that, if the Standard Model were correct, the LHC would produce several Higgs bosons every minute, allowing physicists to finally confirm or disprove the Higgs bosons existence. In addition, the LHC allowed the search for supersymmetric particles and other hypothetical particles as possible unknown areas of physics.[30] Some extensions of the Standard Model predict additional particles, such as the heavy W' and Z' gauge bosons, which are also estimated to be within reach of the LHC to discover.[113]

First run (data taken 2009–2013)

The first physics results from the LHC, involving 284 collisions which took place in the ALICE detector, were reported on 15 December 2009.[99] The results of the first proton–proton collisions at energies higher than Fermilab's Tevatron proton–antiproton collisions were published by the CMS collaboration in early February 2010, yielding greater-than-predicted charged-hadron production.[114]

After the first year of data collection, the LHC experimental collaborations started to release their preliminary results concerning searches for new physics beyond the Standard Model in proton-proton collisions.[115][116][117][118] No evidence of new particles was detected in the 2010 data. As a result, bounds were set on the allowed parameter space of various extensions of the Standard Model, such as models with large extra dimensions, constrained versions of the Minimal Supersymmetric Standard Model, and others.[119][120][121]

On 24 May 2011, it was reported that quark–gluon plasma (the densest matter thought to exist besides black holes) had been created in the LHC.[103]

Between July and August 2011, results of searches for the Higgs boson and for exotic particles, based on the data collected during the first half of the 2011 run, were presented in conferences in Grenoble[122] and Mumbai.[123] In the latter conference it was reported that, despite hints of a Higgs signal in earlier data, ATLAS and CMS exclude with 95% confidence level (using the CLs method) the existence of a Higgs boson with the properties predicted by the Standard Model over most of the mass region between 145 and 466 GeV.[124] The searches for new particles did not yield signals either, allowing to further constrain the parameter space of various extensions of the Standard Model, including its supersymmetric extensions.[125][126]

On 13 December 2011, CERN reported that the Standard Model Higgs boson, if it exists, is most likely to have a mass constrained to the range 115–130 GeV. Both the CMS and ATLAS detectors have also shown intensity peaks in the 124–125 GeV range, consistent with either background noise or the observation of the Higgs boson.[127]

On 22 December 2011, it was reported that a new composite particle had been observed, the χb (3P) bottomonium state.[106]

On 4 July 2012, both the CMS and ATLAS teams announced the discovery of a boson in the mass region around 125–126 GeV, with a statistical significance at the level of 5 sigma each. This meets the formal level required to announce a new particle. The observed properties were consistent with the Higgs boson, but scientists were cautious as to whether it is formally identified as actually being the Higgs boson, pending further analysis.[128]

On 8 November 2012, the LHCb team reported on an experiment seen as a "golden" test of supersymmetry theories in physics,[109] by measuring the very rare decay of the Bs meson into two muons (Bs0 → μ+μ−). The results, which match those predicted by the non-supersymmetrical Standard Model rather than the predictions of many branches of supersymmetry, show the decays are less common than some forms of supersymmetry predict, though could still match the predictions of other versions of supersymmetry theory. The results as initially drafted are stated to be short of proof but at a relatively high 3.5 sigma level of significance.[129] The result was later confirmed by the CMS collaboration.[130]

In August 2013 the LHCb team revealed an anomaly in the angular distribution of B meson decay products which could not be predicted by the Standard Model; this anomaly had a statistical certainty of 4.5 sigma, just short of the 5 sigma needed to be officially recognized as a discovery. It is unknown what the cause of this anomaly would be, although the Z' boson has been suggested as a possible candidate.[131]

On 19 November 2014, the LHCb experiment announced the discovery of two new heavy subatomic particles,
Ξ′−
b and
Ξ∗−
b. Both of them are baryons that are composed of one bottom, one down, and one strange quark. They are excited states of the bottom Xi baryon.[132][133]

The LHCb collaboration has observed multiple exotic hadrons, possibly pentaquarks or tetraquarks, in the Run 1 data. On 4 April 2014, the collaboration confirmed the existence of the tetraquark candidate Z(4430) with a significance of over 13.9 sigma.[134][135] On 13 July 2015, results consistent with pentaquark states in the decay of bottom Lambda baryons (Λ0
b) were reported.[136][137][138] On 28 June 2016, the collaboration announced four tetraquark-like particles decaying into a J/ψ and a φ meson, only one of which was well established before (X(4274), X(4500) and X(4700) and X(4140)).[139][140]

In December 2016, ATLAS presented a measurement of the W boson mass, researching the precision of analyses done at the Tevatron.[141]

Second run (2015 onward)

At the conference EPS-HEP 2015 in July, the collaborations presented first cross-section measurements of several particles at the higher collision energy.

On 15 December 2015, the ATLAS and CMS experiments both reported a number of preliminary results for Higgs physics, supersymmetry (SUSY) searches and exotics searches using 13 TeV proton collision data. Both experiments saw a moderate excess around 750 GeV in the two-photon invariant mass spectrum,[142][143][144] but the experiments did not confirm the existence of the hypothetical particle in an August 2016 report.[145][146][147]

In July 2017, many analysis based on the large dataset collected in 2016 were shown. The properties of the Higgs boson were studied in more detail and the precision of many other results was improved.[148]

Planned "high-luminosity" upgrade

After some years of running, any particle physics experiment typically begins to suffer from diminishing returns: as the key results reachable by the device begin to be completed, later years of operation discover proportionately less than earlier years. A common response is to upgrade the devices involved, typically in collision energy, luminosity, or improved detectors. In addition to a possible increase to 14 TeV collision energy in 2018, a luminosity upgrade of the LHC, called the High Luminosity LHC, is planned,[149] to be made after 2022.

Safety of particle collisions

The experiments at the Large Hadron Collider sparked fears that the particle collisions might produce doomsday phenomena, involving the production of stable microscopic black holes or the creation of hypothetical particles called strangelets.[150] Two CERN-commissioned safety reviews examined these concerns and concluded that the experiments at the LHC present no danger and that there is no reason for concern,[151][152][153] a conclusion expressly endorsed by the American Physical Society.[154]

The reports also noted that the physical conditions and collision events that exist in the LHC and similar experiments occur naturally and routinely in the universe without hazardous consequences,[152] including ultra-high-energy cosmic rays observed to impact Earth with energies far higher than those in any man-made collider.

The Large Hadron Collider gained a considerable amount of attention from outside the scientific community and its progress is followed by most popular science media. The LHC has also inspired works of fiction including novels, TV series, video games and films.

CERN employee Katherine McAlpine's "Large Hadron Rap"[155] surpassed 7 million YouTube views.[156][157] The band Les Horribles Cernettes was founded by women from CERN. The name was chosen so to have the same initials as the LHC.[158][159]

National Geographic Channel's World's Toughest Fixes, Season 2 (2010), Episode 6 "Atom Smasher" features the replacement of the last superconducting magnet section in the repair of the collider after the 2008 quench incident. The episode includes actual footage from the repair facility to the inside of the collider, and explanations of the function, engineering, and purpose of the LHC.[160]

The Large Hadron Collider was the focus of the 2012 student film Decay, with the movie being filmed on location in CERN's maintenance tunnels.[161]

The feature documentary Particle Fever follows the experimental physicists at CERN who run the experiments, as well as the theoretical physicists who attempt to provide a conceptual framework for the LHC's results. It won the Sheffield International Doc/Fest in 2013.

Fiction

The novel Angels & Demons, by Dan Brown, involves antimatter created at the LHC to be used in a weapon against the Vatican. In response, CERN published a "Fact or Fiction?" page discussing the accuracy of the book's portrayal of the LHC, CERN, and particle physics in general.[162] The movie version of the book has footage filmed on-site at one of the experiments at the LHC; the director, Ron Howard, met with CERN experts in an effort to make the science in the story more accurate.[163]

In the visual novel/manga/anime-series "Steins;Gate", the primary antagonist controls a global research organization called SERN, the name of which is a deliberate misspelling of CERN. In the series SERN designs a particle accelerator and uses the miniature black holes created from experiments to master time travel and take over the world.

The novel FlashForward, by Robert J. Sawyer, involves the search for the Higgs boson at the LHC. CERN published a "Science and Fiction" page interviewing Sawyer and physicists about the book and the TV series based on it.[164]

See also

References

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