The Large Hadron Collider (LHC)

The Large Hadron Collider (LHC) is the world's largest and most powerful particle collider, the largest, most complex experimental facility ever built, and the largest single machine in the world. 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. 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 30 March 2010 to 13 February 2013 at an initial energy of 3.5 tera electronvolts (TeV) per beam (7 TeV total), almost 4 times more than the previous world record for a collider, rising to 4 TeV per beam (8 TeV total) from 2012. On 13 February 2013 the LHC's first run officially ended, and it was shut down for planned upgrades. 'Test' collisions restarted in the upgraded collider on 5 April 2015, reaching 6.5 TeV per beam on 20 May 2015 (13 TeV total, the current world record for particle collisions). Its second research run commenced on schedule, on 3 June 2015.

The LHC's aim is to allow physicists to test the predictions of different theories of particle physics, high-energy physics and in particular, to prove or disprove the existence of the theorized Higgs boson and the large family of new particles predicted by super symmetric theories, and other unsolved questions of physics, advancing human understanding of physical laws. It contains seven detectors, each designed for certain kinds of research. The proton-proton collision is the primary operation method, but the LHC has also collided protons with lead nuclei for two months in 2013 and used lead–lead collisions for about one month each in 2010, 2011, and 2013 for other investigations.

The LHC's computing grid was (and currently is) a world record holder. Data from collisions was anticipated to be produced at an unprecedented rate for the time, of tens of petabytes per year, a major challenge at the time, to be analysed by a grid-based computer network infrastructure connecting 140 computing centres in 35 countries – 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.

Purpose

Physicists hope that the LHC 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, where current theories and knowledge are unclear or break down altogether. 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 models 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 possibly to be explored by LHC collisions include:

•  Are the masses of elementary particles actually generated by the Higgs mechanism via electroweak symmetry breaking?] It is expected that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, thereby allowing physicists to consider whether the Standard Model or its Higgs less alternatives are more likely to be correct.
• Is supersymmetry, an extension of the Standard Model and Poincaré symmetry, realized in nature, implying that all known particles have supersymmetric partners?
• Are there extra dimensions, as predicted by various models based on string theory, and can we detect them?
• What is the nature of the dark matter that appears to account for 27% of the mass-energy of the universe?

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

• It is already known that electromagnetism and the weak nuclear force are different manifestations of a single force called the electroweak force. The LHC may clarify whether the electroweak force and the strong nuclear force are similarly just different manifestations of one universal unified force, as predicted by various Grand Unification Theories.
• Why is the fourth fundamental force (gravity) so many orders of magnitude weaker than the other three fundamental forces? See also Hierarchy problem.
• Are there additional sources of quark flavour mixing, beyond those already present within the Standard Model?
• Why are there apparent violations of the symmetry between matter and antimatter? See also CP violation.
• What are the nature and properties of quark–gluon plasma, thought to have existed in the early universe and in certain compact and strange astronomical objects today? This will be investigated by heavy ion collisions, mainly in ALICE, but also in CMS and ATLAS. Findings published in 2012 confirmed the phenomenon of jet quenching in heavy-ion collisions, and was first observed in 2010

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.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

Timeline of Operations

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 was predicted by theory but had not yet been observed or proven to exist due to its elusive nature and exceedingly brief lifespan. 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 to search for supersymmetric particles and other hypothetical particles as possible unknown areas of physics. Some extensions of the Standard Model predict additional particles, such as the heavy W' and Z' gauge bosons, which were also estimated to be within reach of the LHC to discover.

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. 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.

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. 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.

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.

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 and Mumbai. 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. 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

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.

On 22 December 2011, it was reported that a new particle had been observed, the χb (3P) bottomonium state. 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 5sigma. This meets the formal level required to announce a new particle which is 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

On 8 November 2012, the LHCb team reported on an experiment seen as a "golden" test of supersymmetry theories in physics,by measuring the very rare decay of the Bsmeson into two muons (B_s⁰ → 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. The result was later confirmed by the CMS collaboration.

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.

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.

On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons

Second run (2015 onwards)

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