After nearly two years of maintenance and repair, in the early morning of Wednesday 3 June, the Large Hadron Collider(LHC) at CERN is set to start ready to take new physics data at the unprecedented energy of 13 TeV, almost double the collision energy of its first run. Energies at this level have not been seen since the Big Bang.
The world’s biggest atom smasher is now ready to open the way to new frontiers in physics such as to unlock the mysteries of dark matter (the matter we see in space, such as the stars and galaxies, accounts for only about 4 per cent of the Universe), antimatter and the conditions at the time of the Big Bang.
It is like a time machine that can bring us close to the origins of the universe. Breaking the particle down into its elemental parts causes the release of many smaller short-lived particles that teach us about the structure of matter and how the universe formed.
On 4 July 2012, CERN announced the discovery of a Higgs boson (also known as the ‘God particle’) that provides particles with mass. It is the final particle in the so-called Standard Model, the most widely accepted model of how the universe works, to be experimentally verified.
QUESTIONS AND ANSWERS
Material reproduced with permission from CERN
What is the LHC?
The LHC is a particle accelerator that pushes protons or ions to near the speed of light. It consists of a 27-kilometre ring of superconducting magnets with a number of accelerating structures that boost the energy of the particles along the way.
Why is it called the “Large Hadron Collider”?
“Large” refers to its size, approximately 27km in circumference
“Hadron” because it accelerates protons or ions, which belong to the group of particles called hadrons
“Collider” because the particles form two beams travelling in opposite directions, which are made to collide at four points around the machine
How does the LHC work?
The CERN accelerator complex is a succession of machines with increasingly higher energies. Each machine accelerates a beam of particles to a given energy before injecting the beam into the next machine in the chain. This next machine brings the beam to an even higher energy and so on. The LHC is the last element of this chain, in which the beams reach their highest energies.
Inside the LHC, two particle beams travel at close to the speed of light before they are made to collide. The beams travel in opposite directions in separate beam pipes – two tubes kept at ultrahigh vacuum. They are guided around the accelerator ring by a strong magnetic field maintained by superconducting electromagnets. Below a certain characteristic temperature, some materials enter a superconducting state and offer no resistance to the passage of electrical current. The electromagnets in the LHC are therefore chilled to ‑271.3°C (1.9K) – a temperature colder than outer space – to take advantage of this effect. The accelerator is connected to a vast distribution system of liquid helium, which cools the magnets, as well as to other supply services.
What are the main goals of the LHC?
The Standard Model of particle physics – a theory developed in the early 1970s that describes the fundamental particles and their interactions – has precisely predicted a wide variety of phenomena and so far successfully explained almost all experimental results in particle physics.. But the Standard Model is incomplete. It leaves many questions open, which the LHC will help to answer.
What is the origin of mass? The Standard Model does not explain the origins of mass, nor why some particles are very heavy while others have no mass at all. However, theorists Robert Brout, François Englert and Peter Higgs made a proposal that was to solve this problem. The Brout-Englert-Higgs mechanism gives a mass to particles when they interact with an invisible field, now called the “Higgs field”, which pervades the universe. Particles that interact intensely with the Higgs field are heavy, while those that have feeble interactions are light. In the late 1980s, physicists started the search for the Higgs boson, the particle associated with the Higgs field. In July 2012, CERN announced the discovery of the Higgs boson, which confirmed the Brout-Englert-Higgs mechanism. However, finding it is not the end of the story, and researchers have to study the Higgs boson in detail to measure its properties and pin down its rarer decays.
Will we discover evidence for supersymmetry? The Standard Model does not offer a unified description of all the fundamental forces, as it remains difficult to construct a theory of gravity similar to those for the other forces. Supersymmetry – a theory that hypothesises the existence of more massive partners of the standard particles we know – could facilitate the unification of fundamental forces.
What are dark matter and dark energy? The matter we know and that makes up all stars and galaxies only accounts for 4% of the content of the universe. The search is then still open for particles or phenomena responsible for dark matter (23%) and dark energy (73%).
Why is there far more matter than antimatter in the universe? Matter and antimatter must have been produced in the same amounts at the time of the Big Bang, but from what we have observed so far, our Universe is made only of matter.
How does the quark-gluon plasma give rise to the particles that constitute the matter of our Universe? For part of each year, the LHC provides collisions between lead ions, recreating conditions similar to those just after the Big Bang. When heavy ions collide at high energies they form for an instant the quark-gluon plasma, a “fireball” of hot and dense matter that can be studied by the experiments.
by Claudio Rosmino
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