CERN Accelerating science

All about accelerators

The Large Hadron Collider (LHC) is a particle accelerator based at CERN, the European Organization for Nuclear Research, the world's largest particle physics laboratory. It is designed to be the most powerful instrument ever built to investigate the properties of fundamental particles.

The LHC is located in a 26.658 km long tunnel, about 100m underground. It will accelerate two separate beams of protons travelling in opposite directions at up to an energy of 7 TeV. At this energy the beams which are only few microns or so in diameter at the interaction point will pack a punch comparable to a high-speed train!

At four specific points along the LHC ring, the two beams are made to cross paths, to produce head-on collisions (hence the C of LHC). The proton collision energy will then be of 14 TeV. But the LHC will not be limited to the study of proton-proton collisions as it can also collide heavy ions, such as lead, with a collision energy of 1148 TeV. Since the constituents of ions are protons and neutrons, and belong to a family of particles called hadrons (hence the H of LHC)


CERN accelerator complex

The CERN accelerator complex with the path of the protons from the hydrogen source to the LHC

Before being injected into the LHC, proton beams will be prepared by CERN's existing "accelerator complex". This is a succession of machines with increasingly higher energies, injecting the beam each time into the next one, which takes over to bring the beam to an even higher energy.

To bend the 7 TeV protons around the ring, the LHC dipoles must be able to produce magnetic fields of 8.36 Tesla, a value which is made possible by the use of "superconductivity". This is the ability of certain materials, usually at very low temperatures, to conduct electric current without resistance and power losses, and therefore produce high magnetic fields. The LHC operates at about 300 degrees below room temperature (even colder than outer space!) and uses the most advanced superconducting magnet and accelerator technologies ever employed. 1,296 superconducting dipoles and more than 2,500 other magnets guide and collide the LHC beams. They range from small, normally conducting bending magnets to large, superconducting focusing quadrupoles. The LHC is the largest superconducting installation in the world.

Six experiments, with huge detectors, have been built to study what happens when the LHC's beams collide. They are designed to handle huge amounts of data, millions of gigabytes per second, and condense this into a more manageable, yet still daunting, stream of a few gigabytes per second. This data must then be stored and analysed by the thousands of particle physicists who have helped construct the detectors.

How an accelerator works

An accelerator usually consists of a vacuum chamber surrounded by a long sequence of vacuum pumps, magnets, radio-frequency cavities, high voltage instruments and electronic circuits. Each of these pieces has its specific function.

The vacuum chamber is a metal pipe where air is permanently pumped out (by the vacuum pumps) to avoid that the accelerated particles collide with normal matter (like air molecules) and annihilate or get deflected off course.

Inside the pipe, particles are accelerated by electric fields. These are provided by Radio-Frequency (RF) cavities. Each time charged particles traverse an RF cavity, the electric field inside the cavity gives them a "kick", i.e. some of the energy of the radio wave is transferred to them and they are accelerated. To make a more effective use of a limited number of RF cavities, accelerator designers can force the particle beam to go through them many times, by curving the beam trajectory into a closed loop. That is why most accelerators are roughly circular.

The curving of the beam's path, to make sure the particles stay within their circular track, is usually achieved by the magnetic field of dipole magnets (which have a North and a South pole, like the well-known horseshoe magnet). They are also called "bending magnets". This is because the magnetic force exerted on moving charged particles is always perpendicular to their velocity - perfect for curving the trajectory! The higher the energy of a particle, the stronger the field that is needed to bend the particle's path. This means that, as the maximum magnetic field is limited (to some 2 Tesla for conventional magnets, some 10 Tesla for superconducting ones), the more powerful a machine is, the larger it needs to be.

In addition to just curving the beam, it is also necessary to focus it. Just like a shot from a shotgun, a particle beam spreads out as it travels. Focussing the beam allows its width and height to be constrained so that it stays inside the vacuum chamber. This is achieved by quadrupole magnets (which have four poles), which act on the beam of charged particles exactly the same way a lens would act on a beam of light. They are also called "focussing magnets".

These are some of the basic ingredients needed to make an accelerator. If you look around CERN's existing accelerators, like the PS or the SPS, you will see that there are many more objects such as:

  • other magnets (to perform "fine tuning" on the trajectory or of the focusing)
  •  injection/ejection elements( to put the beam into the accelerator or to take it out)
  •  measurement devices (to give the operators information on the behaviour of the beam
  •  safety elements (to ensure smooth operation of the accelerator).

All of these elements are controlled from a control centre, which is very much like the control centres used for space missions!

LHC operations

Having to accelerate two particles beams at the same time, the LHC is in fact "two machines in one". It will consist of two " superconducting magnetic channels" or "rings" housed in the same yoke and cryostat, a unique configuration that not only saves space but also gives a 25 % cost saving over separate rings!

The two rings will be filled with protons delivered from the SPS and its pre-accelerators at 0.45 TeV, and will accelerate them up to almost the speed of light, at an energy of 7 TeV

What we call a "proton beam" is in fact a succession of squeezed groups of protons called "bunches". The two LHC beams will consist each of 2808 bunches of 1.15×1011 particles each.

Once the 7 TeV energy is reached, the beams will counter-rotate for several hours, at a rate of 11,245 revolutions per second. Since the LHC crosses the Franco-Swiss border at two points along its circular path, that's 22,490 border crossings a second, luckily there are no border checks for protons!

At each turn, the beams will be forced to collide in determined places, where the experiments are located.

After about 10 hours the beams will become so degraded that the machine will have to be emptied and refilled.


Higgs and collisions from 2013
The image shows collisions at the LHC leading to the discovery of the Higgs boson.


Humans use the information of bounced-around light waves to perceive the world (other animals, like dolphins and bats, emit and detect sound waves). In fact, any kind of reflected wave can be used to get information about the surroundings. The problem with using waves to detect the physical world is that the quality of the image is limited by the wavelength you use.

Our eyes are adjusted to visible light, which has a wavelength between 0.4 and 0.8 micrometres (0.000001 metres, or 10-6m). This means that it can't be used to investigate details smaller then that, which usually is not warring since we don't need to look at things that are less then 0.000001 metres wide!

But CERN physicists need to investigate the constituents of matter at the subatomic level, where the typical distances are of the order of the femtometre (0.000000000000001 m , or 10-15m) or smaller!

Early in the 20th century, it was discovered that particles of matter can also be considered as waves, the wavelength of which becomes smaller as the energy of the particle becomes higher. Therefore, details smaller than a micron could, for instance, be examined using electrons, provided their energy is large enough. This is the principle of the electron microscope, which is used, among other things, in biology and metallography to look at details of cells or alloys. However, even the best scanning electron microscope can only show a fuzzy picture of an atom.

Since ALL particles have wave properties, physicists can use particles with the shortest possible wavelengths as their probes. To be able to investigate details a billion times smaller, we need to use particles that have energies a billion times higher. This means that the smaller the details you want to look at, the larger the machine you will have to build!

Accelerators are the ultimate microscope!

Why do we need the LHC?

Scientists have found that everything in the Universe is made from a small number of basic building blocks called elementary particles, governed by a few fundamental forces.


Some of these particles, such as the electron, are stable and form normal matter. Others, such as the muon, have a fleeting existence before decaying to the stable ones. Still others, such as the Higgs boson, are believed to have existed for a few instants after the Big Bang, but they are absent in today's universe.

The enormous concentration of energy that can be reached in collisions between particles such as electrons or protons in an accelerator can recreate the conditions of the early universe, and generate particles like the Higgs boson for a fraction of a second, before such particles decay into more ordinary ones. It is the tell-tale traces left by these more ordinary particles that the physicists capture in huge detectors, placed around collision points on the accelerator. By a process of computational detective work, they can then deduce the proprieties on the new particles they created.

Therefore, studying particle collisions is like "looking back in time", recreating the environment present at the origin of our Universe.

The theories and discoveries of thousands of physicists over the past century have created a remarkable picture of the fundamental structure of matter, which is called the Standard Model of Particles and Forces. The Standard Model is by now a well-tested physics theory, used to explain and exactly predict a vast variety of phenomena. High-precision experiments have repeatedly verified subtle predicted effects. Nevertheless, physicists know that it can't be the end of the story, as it leaves many unsolved questions.


Among them, the reason why elementary particles have mass, and why their masses are different, is the most perplexing one. It is remarkable that such a familiar concept is so poorly understood! The answer may lie within the Standard Model, in an idea called the Higgs mechanism. According to this, the whole of space is filled with a 'Higgs field', and by interacting with this field, particles acquire their masses. Particles which interact strongly with the Higgs field are heavy, whilst those which interact weakly are light. The Higgs field has at least one new particle associated with it, the Higgs boson. If such a particle exists, the LHC should be able to detect it.

Another puzzle concerns the existence of four different forces. When the Universe was young and much hotter than today, perhaps these forces all behaved as one. Particle physicists hope to find a single theoretical framework to prove this, and have already had some success. Two forces, the electromagnetic force and the weak force were 'unified' into a single theory in the 1970s. This theory was experimentally verified in a Nobel prize winning experiment at CERN a few years later. The weakest and the strongest forces, however, gravity and the strong force, remain apart. A very popular idea suggested by the unification of the forces is called supersymmetry or SUSY for short. SUSY predicts that for each known particle there is a 'supersymmetric' partner. If SUSY is right, then supersymmetric particles should be found at the LHC.

Antimatter poses another riddle the LHC will help us to solve. It was once thought that antimatter was a perfect 'reflection' of matter - that if you replaced matter with antimatter and looked at the result in a mirror, you would not be able to tell the difference. We now know that the reflection is imperfect, and this could have led to the matter-antimatter imbalance in our Universe today. The LHC is a very good 'antimatter-mirror', allowing us to put the Standard Model through one of its most grueling tests yet.

These are just a few of the questions the LHC should answer, but history has shown that the greatest advances in science are often unexpected. Although we have a good idea of what we hope to find at the LHC, nature may well have surprises in store.

One thing seems certain, the LHC will change our view of the Universe.


(based on the "Antimatter: Mirror of the Universe" Briefing Room)

If you could convert all of the energy contained in 1 kg of sugar, or 1 kg of water, or 1 kg of anything at all, you could drive a car for about 100,000 years without stopping! Why? Albert Einstein, in 1905, wrote down the famous equation E=mc2, which he deduced from his special theory of relativity. It says that mass is a very concentrated form of energy.

Energy is like the 'money' of nature; it comes in two different currencies, and with an enormous exchange rate - the square of the speed of light. 1 kg corresponds to 25,000,000,000 kWh of energy; 1 g would be enough to supply energy to a medium-sized town for a whole day!

Energy and matter can be converted into each other. Accelerators enable doing so!

In a coin factory, hot metal is pressed into coins. They only come in specific sizes and values, as 1c, 2c, 5c, 10c, 50c and 1 Euro. Similarly, nature does not allow energy to be converted into just any kind of matter. Nature has provided us with 'moulds', corresponding to a precisely defined amount of energy, as well as having some other particular properties.

These moulds are analogous to particles, the most important ones in our daily lives being the proton, the neutron and the electron. They have very precisely defined properties, such as their mass, their electric charge or the way they interact with other particles.

But the extreme energy available in the accelerator collisions can produce also rare, exotic particles, which do not exist anymore in our universe today. These are a bit like rare ancient coins, much heavier than today's coins, and which give us a unique insight into the past. They are very 'fragile', decaying into more mundane particles in a fraction of a second.

Some of these rare particles, with names such as the Higgs boson, give scientists an insight into a past far more distant than ancient coins. By investigating such particles, scientists try to unravel the mysteries of the very first instants of the Universe, some 13 billion years ago, just after the Big Bang.