# What is the Large Hadron Collider? (LHC)

The Large Hadron Collider (LHC) is the highest-energy particle collider and was built by the European Organisation for Nuclear Research (CERN) in collaboration with hundreds of universities and laboratories, as well as more than 100 countries. It lies in a tunnel 17 miles in circumference, near Geneva.

# What is the Large Hadron Collider (LHC) and how does it work?

Hadron means subatomic composite particles composed of quarks bound by a strong electromagnetic force. Hadrons can be baryons such as protons and neutrons and may include mesons such as the pion and kaon.

A collider is a particle accelerator with two beams of particles, which accelerate to high kinetic energies. Analysis of the particle collisions gives evidence of the structure of the subatomic nature. The bi-products decay after a short period of time, therefore they are nearly impossible to study via other methods of experimentation.

### Purpose of the Large Hadron Collider

The LHC answers fundamental questions, covering basic laws governing the interactions and forces among elementary objects, space and time and the relationship between quantum mechanics and general relativity. High-energy particle experiments also validate the very nature of the Standard Model and the Higgsless model.

### Questions being answered by the LHC

Is the mass of elementary particles being generated by the Higgs mechanism via electroweak symmetry breaking? Experiments demonstrate or disprove the existence of the elusive Higgs boson, allowing physicists to consider whether the Standard Model or its Higgsless alternative are accurate theories.

Is supersymmetry realised in nature, implying all particles have supersymmetric partners?

Are there extra dimensions, as posed in string theory?

What is dark matter, accounting for 27% of the mass-energy within the universe?

### Extended open questions explored by the LHC

Electromagnetism and the weak nuclear force are different variations of the electroweak force. The LHC may clarify whether the electroweak force and the strong nuclear force are simply different manifestations of one universal force.

Why is gravity much weaker than other fundamental forces?

Why are there violations of the symmetry between matter and antimatter?

## Large Hadron Collider design and construction

The LHC sits within a 27km circular tunnel, at a depth of between 50 to 175 metres. An underground tunnel conveniently shields against background radiation from the earth. Constructed between 1983 and 1988, the LHC crosses the border between Switzerland and France at four points. Ancillary equipment such as compressors, ventilation equipment and refrigeration plants are all present on the surface.

Superconducting quadrupole electromagnets are used to direct the beams to four intersection points. The collider tunnel contains two adjacent parallel beamlines, each containing a beam, which travel in opposite directions. The beams intersect at four points around the ring, which is where the collisions occur. 1,232 dipole magnets maintain the beams’ path, while an additional 392 quadrupole magnets focus them. Further quadrupole magnets enhance the probability of interaction.

### LHC – Superfluid Helium 4

Superfluid helium-4 keeps the magnets, made of copper-clad niobium-titanium, at −271.25 °C. Each day of operation it generates 140 terabytes of data. When running at 6.5 TeV per proton, acceleration from 450 GeV to 6.5 TeV, the field of the superconducting 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 and velocity of 3.1 m/s slower than the speed of light, equating to approx 90 microseconds for a proton to travel the perimeter of 26.7 km (11,245 revolutions per second).

### Large hadron collider proton acceleration

Pre-main acceleration, particles are prepared by increasing their energy. Initially, the linear particle accelerator LINAC 2 feeds the Proton Synchrotron Booster. Protons will then be accelerated to 1.4 GeV and passed into the Proton Synchrotron, accelerated to 26 GeV and finally, the Super Proton Synchrotron increases their energy to 450 GeV. Here, the proton bunches are accumulated, accelerated (over a period of 20 minutes) to their peak energy.

The LHC is concerned with proton–proton collisions. However, shorter running periods, typically one month per year, heavy-ion collisions are included in the programme. The scheme deals with lead ions which are accelerated by the linear accelerator LINAC 3. The ions are then further accelerated where they reach an energy of 2.3 TeV per nucleon. The objective is to investigate quark–gluon plasma, which existed in the early universe.

## Black Holes and Micro-black Holes generation at the Large Hadron Collider

It is suggested that specific extensions of the Standard Model would lead to the existence of extra spatial dimensions, potentially realising micro black holes. These would decay by Hawking radiation very quickly and are therefore not a threat in reality. This radiation is emitted by black holes due to quantum effects. As Hawking radiation leads to a loss of mass, black holes that lose more matter than they gain would shrink and cease to exist. Smaller micro black holes (MBHs), highly possible to construct at the LHC.

The LSAG are confident that even if micro black holes were produced by the LHC, they would be unable to accrete matter. They would also have been produced by cosmic rays and have stopped in neutron stars and white dwarfs, and the stability of these astronomical bodies means that they cannot be dangerous.

## Discoveries at the Large Hadron Collider

The core focus of research from the beginning was to question the possible existence of the Higgs boson, part of the Standard Model. The LHC also searched for supersymmetric particles, aiming to uncover other possible unknown areas of physics. The Standard Model predicts particles, such as the W’ and Z’ gauge bosons, which were a target for the research in Geneva.

### LHC discoveries from 2009–2013

Key early observations at  the LHC, involving 284 collisions (ALICE detector), were found in 2009. The first proton–proton collisions at energies higher than Fermilab’s Tevatron proton–antiproton collisions were announced in 2010.  In May 2011, the generation of quark–gluon plasma had been fulfilled in the LHC. Between July and August 2011, results of searches for the Higgs boson and other particles. In Mumbai, it was reported that ATLAS and CMS exclude with 95% confidence the existence of a Higgs boson with the properties predicted by the Standard Model between 145 and 466 GeV. On 13 December 2011, CERN reported that the Standard Model Higgs boson to have a mass constrained to 115–130 GeV. On 22 December 2011, it was reported that a new composite particle had been observed, the χb (3P) bottomonium state.

On 4 July 2012, both the CMS and ATLAS teams announced a boson, with a statistical significance at the level of 5 sigma each. The observed properties were consistent with the Higgs boson. Further analysis and final confirmation that the observed particle was the predicted Higgs Boson in 2013.

Another key observation, based on a classic question around supersymmetry, resulted in data that match those predicted by the non-supersymmetrical Standard Model rather than the predictions of many branches of supersymmetry. This shows the decays are less common than some forms of supersymmetry predict, 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. 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.

The LHCb collaboration has observed multiple exotic hadrons. On 4 April 2014, the collaboration confirmed the existence of the tetraquark candidate Z(4430). On 13 July 2015, results consistent with pentaquark states in the decay of bottom Lambda baryons (Λ0b) were also reported. In 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)).

### LHC discoveries from 2015-2019

In July 2015, the collaborations presented first cross-section measurements of several particles at the higher collision energy. In December, ATLAS and CMS reported 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. In July 2017, many analyses based on the large dataset collected in 2016 were shown. The properties of the Higgs boson were studied in much more detail.

The UK efforts as part of the collaborative update to the LHC is supported by scientists, engineers and technicians. A planned £26 million upgrade of the Large Hadron Collider. Key stakeholders include the Science and Technology Facilities Council (STFC), CERN, Cockcroft Institute, John Adams Institute eight UK universities including; University of Dundee, University of Huddersfield, Lancaster University, University of Liverpool, The University of Manchester, University of Oxford, Royal Holloway, University of London (RHUL) and University of Southampton.

The upgrade will focus on several optimisations. In addition to a possible increase to 14 TeV collision energy, a crucial  luminosity upgrade, called the High Luminosity Large Hadron Collider, will boost the accelerator’s potential for new discoveries in physics, starting in 2027.  This increases the luminosity of the machine by a factor of 10, up to 1035 cm−2s−1, providing a better chance to see rare processes and improving statistically marginal measurements.

Professor Mark Thomson, particle physicist and Executive Chair of STFC, said, ”This is a significant undertaking, yet one with fantastic benefits for the UK. The aim is for this project to involve UK industry at every stage, with specialist companies being invited to bid for contracts to manufacture high-tech components.”

### LHC – the search for dark matter

95% of the universe is theoretically predicted to be dark matter (27%) and dark energy (68%) but physicists have not observed either. The increased luminosity of HL-LHC will facilitate possible evidence that could solve this conundrum.

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