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The LHC is housed in a 27km circumference tunnel, up to 175m beneath the French-Swiss border near Geneva.
The Large Hadron Collider (LHC), is the world's most powerful particle accelerator. It was designed and built by over 10,000 scientists and engineers from over 100 countries. The LHC accelerates protons to 99.999% the speed of light and collides them, recreating conditions from a billionth of a second after the big bang. Over 1,600 superconducting magnets keep the proton beams on track, most weighing over 27 tonnes.
In 2013 the LHC confirmed the discovery of the The Higgs field, ending a 40 year search for the last missing piece of the standard model of particle physics. As particles pass through the field they interact with the Higgs Boson - an associated particle, which imparts their mass from the energy of the field.
Without the Higgs field, particles would not have the mass needed to hold together in atoms and we could not exist.
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Many theorists suggest that there are extra dimensions, beyond the four familiar dimensions of space and time. We can’t see the extra dimensions because they exist on tiny sub-atomic scales. The LHC allows us to look at things on this scale and could make these extra dimensions visible.
Theorists predict particles that could only exist if extra dimensions are real. They would have the same properties as standard particles, but much greater mass.
The LHC is the only machine able to reach high enough energy levels to reveal these particles. If it does discover them it could explain why gravity is weaker than the other forces of nature (perhaps because it spreads into other dimensions) and why the universe is expanding faster than expected.
Some theorists believe that gravity has a force-carrying particle called a graviton. If this is correct, the LHC should be able to create these particles, but they would quickly disappear into extra dimensions. This would leave an imbalance of momentum and energy in the particle collision, which could be studied to determine the properties of the missing object. The LHC experiments hope to find missing energy in events, which may uncover gravitons, dark matter, or supersymmetric particles.
Extra dimensions may also be revealed through the creation of microscopic black holes. They would almost instantly disintegrate and decay into standard or supersymmetric particles.
The image shows a 3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope.
The distance from the Earth increases from left to right and the dark matter becomes more clumpy over time as it collapses under the pull of gravity.
There is far more gravity the universe than there should be based on the amount of observable mass. Galaxies should be torn apart by their spin, but some strange, invisible matter - dark matter - is exerting extra gravity, holding them together.
The matter we know of only forms around 4% of the universe. Dark matter forms around 26%, and we don’t know what it is. Some theorists believe it contains supersymmetric particles - partners to known particles.
Many theories predict that dark matter particles could be produced at the LHC. They would not be directly detectable, but scientists could infer their existence by studying the energy and momentum missing after a particle collision. Predictions for dark matter often involve theories about supersymmetry and extra dimensions. Discoveries could open doors to new physics beyond the standard model.
Dark energy forms around 70% of the universe and causes it to expand at an accelerating rate. Distributed evenly in space and time - it’s effect is not diluted as the universe expands.
The LHC is the largest cryogenic system in the world and one of the coldest places on Earth. Approximately 120 tonnes of superfluid helium 4 keeps the magnets at their operating temperature of 1.9 K (-271.25 °C) - colder than outer space. The cryogenic system requires 40 MW of electricity – 10 times more than is needed to power a locomotive.
The 7000-tonne ATLAS detector is probing for fundamental particles, through extremely high energy protons collisions. Scientists are hoping for discoveries about extra dimensions, dark matter and the unification of fundamental forces. Following the discovery of the Higgs boson, further data will shed more light on the boson's properties and the origin of mass.
The Compact Muon Solenoid (CMS) detector studies the Standard Model (including the Higgs boson) and searches for extra dimensions and particles that could make up dark matter. Although it has the same scientific goals as the ATLAS experiment, it uses different technical solutions.
The CMS detector is built around a huge solenoid magnet: a cylindrical coil of superconducting cable that generates a field of 4 tesla, about 100,000 times the magnetic field of the Earth. The field is confined by a steel “yoke” that forms the bulk of the detector’s 14,000-tonne weight.
The CMS experiment is one of the largest international scientific collaborations in history, involving 4300 particle physicists, engineers, technicians, students and support staff from 182 institutes in 42 countries (February 2014).
ALICE (A Large Ion Collider Experiment) is a heavy-ion detector on the LHC ring. It is designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms. This plasma is thought to have formed just after the big bang.
All ordinary matter in today’s universe is made up of atoms. Each atom contains a nucleus composed of protons and neutrons, surrounded by a cloud of electrons. Protons and neutrons are in turn made of quarks bound together by other particles called gluons. No quark has ever been observed in isolation: the quarks, as well as the gluons, seem to be bound permanently together and confined inside composite particles, such as protons and neutrons. This is known as confinement.
Collisions in the LHC generate temperatures more than 100,000 times hotter than the centre of the Sun. For part of each year the LHC provides collisions between lead ions, recreating conditions similar to those just after the big bang. Under these extreme conditions, protons and neutrons "melt", freeing the quarks from their bonds with the gluons. This is quark-gluon plasma. The existence of such a phase and its properties are key issues in the theory of quantum chromodynamics (QCD), for understanding the phenomenon of confinement, and for a physics problem called chiral-symmetry restoration. The ALICE collaboration studies the quark-gluon plasma as it expands and cools, observing how it progressively gives rise to the particles that constitute the matter of our universe today.
The 10,000-tonne ALICE detector sits in a vast cavern 56m below ground, receiving beams from the LHC.
The collaboration counts more than 1000 scientists from over 100 physics institutes in 30 countries.
The Large Hadron Collider beauty (LHCb) experiment aims to discover why the universe appears to be composed almost entirely of matter, but no antimatter. The LHCb investigates the slight differences between matter and antimatter by studying a type of particle called the "beauty quark", or "b quark".
Instead of surrounding the entire collision point like ATLAS and CMS, the LHCb mainly detects particles thrown forward by collisions. One subdetector is mounted close to the collision point and others are spread over a length of 20 metres.
During collisions, many different types of quark emerge and quickly decay quickly into other forms. To catch the b quarks, LHCb uses movable tracking detectors close to the path of the beams circling in the LHC.
About 700 scientists from 66 different institutes and universities work with the 5600-tonne detector.
The accelerator complex at CERN is a succession of machines that accelerate particles to increasingly higher energies. The LHC is the last and most powerful in the chain.
The proton source is a simple bottle of hydrogen gas. An electric field strips hydrogen atoms of their electrons to yield protons. Linac 2, the first accelerator in the chain, accelerates the protons to the energy of 50 MeV. The beam is then injected into the Proton Synchrotron Booster (PSB), which accelerates the protons to 1.4 GeV, followed by the Proton Synchrotron (PS), which pushes the beam to 25 GeV. Protons are then sent to the Super Proton Synchrotron (SPS) where they are accelerated to 450 GeV.
The protons are finally transferred to the two beam pipes of the LHC. The beam in one pipe circulates clockwise while the beam in the other pipe circulates anticlockwise. The two beams are brought into collision inside four detectors – ALICE, ATLAS, CMS and LHCb – where the total energy at the collision point is equal to 8 TeV.
The protons start in Linac 2, passing through the conductors, which are alternately charged positive and negative. The conductors behind them push the particles and the conductors ahead of them pull, causing the particles to accelerate. Quadrupole magnets ensure that the protons remain in a tight beam.
By the time they reach the other end, the protons have reached the energy of 50 MeV and gained 5% in mass. They then enter the Proton Synchrotron Booster, the next step in CERN's accelerator chain, which takes them to a higher energy.
The Super Proton Synchrotron (SPS) is the second-largest machine in CERN’s accelerator complex. Measuring nearly 7 kilometres in circumference, it takes particles from the Proton Synchrotron and accelerates them to provide beams for the Large Hadron Collider.
A major highlight for the SPS came in 1983 with the Nobel-prize-winning discovery of W and Z particles.
The SPS operates at up to 450 GeV. It has 1317 conventional (room-temperature) electromagnets, including 744 dipoles to bend the beams round the ring.
Radiofrequency cavities are chambers spaced at intervals along the accelerator, shaped to resonate at specific frequencies, allowing radio waves to interact with passing particle bunches. Each time a beam passes the electric field in an RF cavity, some of the energy from the radio waves is transferred to the particles, nudging them forwards.
It’s important that the particles do not collide with gas molecules, so the beam pipe maintains an ultrahigh vacuum.
The Antiproton Decelerator (AD) provides low-energy antiprotons to study antimatter.
A beam of protons from the Proton Synchrotron (PS) is fired into a block of metal. The energy from the collisions is enough to create a new proton-antiproton pair about once in every million collisions. The antiprotons produced travel at almost the speed of light and have too much energy to be useful for making antiatoms. They also have a range of energies and move randomly in all directions. The job of the AD is to tame these unruly particles into a useful low-energy beam.
A ring of bending and focusing magnets keeps the antiprotons on the same track, while strong electric fields slow them down. Passing the antiprotons through clouds of electrons – a technique known as “cooling” – reduces the sideways motion and the spread in energies. Finally, when the antiprotons have slowed down to about 10% of the speed of light, they are ready to be ejected.
In 2002 the AD made headlines when the ATHENA and ATRAP experiments successfully made large numbers of antiatoms for the first time.
The ACE experiment also uses antiprotons, to assess their suitability for cancer therapy.
The CERN Control Centre combines control rooms for the laboratory’s accelerators, the cryogenic distribution system and the technical infrastructure. It holds 39 operation stations for four different areas – the LHC, the SPS, the PS complex and the technical infrastructure.
Candidate Higgs boson events from collisions between protons in the LHC. The top event in the CMS experiment shows a decay into two photons (dashed yellow lines and green towers). The lower event in the ATLAS experiment shows a decay into four muons (red tracks).
Collision data is analysed by the world’s largest computing grid, connecting over 170 computing centers in 36 countries.
On 4 July 2012, the ATLAS and CMS experiments announced they had each observed a new particle in the mass region around 126 GeV. This particle is consistent with the Higgs boson predicted by the Standard Model. The Higgs boson is the simplest manifestation of the Brout-Englert-Higgs mechanism. Other types of Higgs bosons are predicted by other theories that go beyond the Standard Model.
On 8 October 2013 the Nobel prize in physics was awarded jointly to François Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass..."
In 2015 the colliders energy will almost double, and the protons will travel only 3 meters per second slower than the speed of light - around 11,000 revolutions per second.
The highlight of the first run of the Large Hadron Collider (LHC) was undoubtedly the discovery of the Higgs boson.
Physicists are now asking follow-up questions. Is there any difference between its properties and those predicted in the Standard Model? Is it the only Higgs boson, or are there others? What gives its mass to this Higgs boson? Is it truly an elementary particle, or is it made of some smaller constituents? Is it a portal to some new physics beyond the Standard Model, such as dark matter?
The LHC is now exploring these questions. For example, its higher energy will enable it to probe more deeply for deviations from the Standard Model predictions, and to search for heavier Higgs bosons. It will be possible to measure directly this Higgs boson's coupling to the top quark, and to box in its possible coupling to the muon. These measurements may reveal some substructure inside this Higgs boson, or provide some other evidence for physics beyond the Standard Model. Time will tell!