The LHC in general
Large, powerful, unique
- What does LHC stand for?
- When was it designed?
- How much does it cost?
- Why large?
- Why collider?
- Why hadrons?
- Why is the LHC built underground?
- What is the collision energy at the LHC and what is so special about it?
- What are the main goals of the LHC?
LHC stands for Large Hadron Collider. Large due to its size
(approximately 27km in circumference), Hadron because it accelerates protons or ions, which are hadrons, and Collider because these particles form two beams travelling in opposite directions, which collide at four points around the machine’s circumference.
Hadrons (from the Greek ‘adros’ meaning ‘bulky’) are particles composed of quarks. The protons and neutrons that atomic nuclei are made of belong to this family. On the other hand, leptons are particles that are not made of quarks. Electrons and muons are examples of leptons (from the Greek ‘leptos’ meaning ‘thin’).
Back in the early 1980s, while the Large Electron-Positron (LEP) collider was being designed and built, groups at CERN were already busy looking at the long-term future. After many years of work on the technical aspects and physics requirements of such a machine, their dreams came to fruition in December 1994 when CERN’s governing body, the CERN Council, voted to approve the construction of the LHC. The green light for the project was given under the condition that the new accelerator be built within a constant budget and on the understanding that any non-Member State contributions would be used to speed up and improve the project. Initially, the budgetary constraints implied that the LHC was to be conceived as a 2-stage project. However, following contributions from Japan, the USA, India and other non-Member States, Council voted in 1995 to allow the project to proceed in a single phase. Between 1996 and 1998, four experiments—ALICE, ATLAS, CMS and LHCb—received official approval and construction work commenced on the four sites. Since then, two smaller experiments have joined the quest: TOTEM, installed next to CMS, and LHCf, next to ATLAS (see also experiments and LHC milestones).
The cost for the machine alone is about 4.6 billion CHF (about 3 billion Euro). The total project cost breaks down roughly as follows:
} 4.6 billion CHF total cost of the accelerator
} 1.1 billion CHF total CERN contribution to the experiments (about 20% of the detector costs, supported by large collaborations of institutes worldwide)
} 0.26 billion CHF total contribution to computing (manpower and materials and both CERN’s and external contributions).
The experimental collaborations are individual entities, funded independently from CERN. CERN is a member of each experiment, and contributes to the budget of CMS and LHCb at the 20% level, 16% for ALICE and 13% for ATLAS. TOTEM is a much smaller experiment, with a total material cost of about 6.5 million CHF. The CERN share of this is, however, 30%.
NB: 1 billion = 1 thousand million.
The size of an accelerator is related to the maximum energy obtainable. In the case of a collider or storage ring, this is a function of the radius of the machine and the strength of the dipole magnetic field that keeps particles on their orbits. The LHC re-uses the 27km circumference tunnel that was built for the previous big accelerator, LEP. The LHC uses some of the most powerful dipoles and radio-frequency cavities in existence. The size of the tunnel, magnets, cavities and other essential elements of the machine, represent the main constraints that determine the design energy of 7 TeV per proton beam.
A collider (that is a machine where counter-circulating beams collide) has a big advantage over other kind of accelerator where a beam collides with a stationary target. When two beams collide, the energy of the collision is the sum of the energies of the two beams. A beam of the same energy that hits a fixed target would produce a collision of much less energy.
The energy available (for example, to make new particles) in both cases is the centre-of-mass energy. In the first case it is simply the sum of the energies of the two colliding particles (E=Ebeam1+ Ebeam2), whereas in the second, it is proportional to the square root of the energy of the particle hitting the target (E ∝ √Ebeam).
The LHC will accelerate two beams of particles of the same kind, either protons or lead ions, which are hadrons. An accelerator can only accelerate certain kinds of particle: firstly they need to be charged (as the beams are manipulated by electromagnetic devices that can only influence charged particles), and secondly, except in special cases, they need not to decay. This limits the number of particles that can practically be accelerated to electrons, protons, and ions, plus all their antiparticles.
In a circular accelerator such as the LHC, heavy particles such as protons (protons are around 2000 times more massive than electrons) have a much lower energy loss per turn through synchrotron radiation than light particles such as electrons. Therefore, in circular accelerators, to obtain the highest-energy collisions it is more effective to accelerate massive particles.
Synchrotron radiation is the name given to the radiation that occurs when charged particles are accelerated in a curved path or orbit. This kind of radiation represents an energy loss for particles, which in turn means that more energy must be provided by the accelerator to keep the beam energy constant.
The LHC re-uses the tunnel that was built for CERN’s previous big accelerator, LEP, dismantled in 2000. The underground tunnel was the best solution to house a 27km circumference machine because it is cheaper to excavate a tunnel rather than acquire the land to build at the surface and the impact on the landscape is reduced to a minimum. In addition, the Earth’s crust provides good shielding for radiation.
The tunnel was built at a mean depth of 100m, due to geological considerations (again translating into cost) and at a slight gradient of 1.4%. Its depth varies between 175m (under the Jura) and 50m (towards Lake Geneva).
The tunnel has a slope for reasons of cost. At the time when it was built for hosting LEP, the construction of the vertical shafts was very costly. Therefore, the length of the tunnel that lies under the Jura was minimized. Other constraints involved in the positioning of the tunnel were:
} it was essential to have a depth of at least 5m below the top of the ‘molasse’ (green sandstone) stratum
} the tunnel had to pass in the vicinity of the pilot tunnel, constructed to test excavation techniques
} it had to link to the SPS. This meant that there was only one degree of freedom (tilt). The angle was obtained by minimising the depth of the shafts.
Each proton beam flying around the LHC will have an energy of 7 TeV, so when two protons collide the collision energy will be 14 TeV. Lead ions have many protons, and together they give an even greater energy: the lead-ion beams will have a collision energy of 1150 TeV. Both collision energies have never been reached before in a lab.
Energy concentration is what makes particle collisions so special. When you clap your hands you probably do a ‘collision’ at an energy higher than protons at the LHC, but much less concentrated! Now think of what you would do if you were to put a needle in one of your hands. You would certainly slow your hands down as you clapped!
In absolute terms, these energies, if compared to the energies we deal with everyday, are not impressive. In fact, 1 TeV is about the energy of motion of a flying mosquito. What makes the LHC so extraordinary is that it squeezes energy into a space about a million million times smaller than a mosquito.
Our current understanding of the Universe is incomplete. The Standard Model of particles and forces summarizes our present knowledge of particle physics. The Standard Model has been tested by various experiments and it has proven particularly successful in anticipating the existence of previously undiscovered particles. However, it leaves many unsolved questions, which the LHC will help to answer.
} The Standard Model does not explain the origin of mass, nor why some particles are very heavy while others have no mass at all. The answer may be the so-called Higgs mechanism. According to the theory of the Higgs mechanism, the whole of space is filled with a ‘Higgs field’, and by interacting with this field, particles acquire their masses. Particles that interact intensely with the Higgs field are heavy, while those that have feable interactions are light. The Higgs field has at least one new particle associated with it, the Higgs boson. If such a particle exists, experiments at the LHC will be able to detect it.
} 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. If supersymmetry is right, then the lightest supersymmetric particles should be found at the LHC.
} By using powerful telescopes, both on the ground and in orbit, we have found that all the visible matter accounts for only 4% of the Universe. The search is open for particles or phenomena responsible for dark matter (23%) and dark energy (73%). A very popular idea is that dark matter be made of neutral – yet undiscovered – supersymmetric particles.
} The LHC will also help us to investigate the mystery of antimatter. 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. Why? The LHC could help to provide an answer.
It was once thought that antimatter was a perfect ‘reflection’ of matter—that if you replaced matter with antimatter and looked at the result as if 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. The strongest limits on the amount of antimatter in our Universe come from the analysis of the ‘diffuse cosmic gamma-rays’ and the inhomogeneities of the cosmic microwave background (CMB). Assuming that after the Big Bang, the Universe separated somehow into different domains where either matter or antimatter was dominant, it is evident that at the boundaries there should be annihilations, producing cosmic (gamma) rays. Taking into account annihilation cross-sections, distance, and cosmic redshifts, this leads to a prediction of the amount of diffuse gamma radiation that should arrive on Earth. The free parameter in the model is the size of the domains. Comparing with the observed gamma ray flux, this leads to an exclusion of any domain size below 1000 MParsec (3.7 Giga light years), which is not so far away from the entire Universe. Another limit comes from analyzing the inhomogeneities in the CMB - antimatter domains (at any size) would cause heating of domain boundaries and show up in the CMB as density fluctuations. The observed value of ~10-5 sets strong boundaries to the amount of antimatter in the early Universe.
} In addition to the studies of proton–proton collisions, heavy-ion collisions at the LHC will provide a window onto the state of matter that would have existed in the early Universe, called ‘quark-gluon plasma’. When heavy ions collide at high energies they form for an instant a “fireball” of hot, dense matter that can be studied by the experiments.
According to the current theories, the Universe, born from the Big Bang, went through a stage during which matter existed as a sort of extremely hot, dense soup – called quark-gluon plasma (QGP) – composed of the elementary building blocks of matter. As the Universe cooled, the quarks became trapped into composite particles such as protons and neutrons. This phenomenon is called the confinement of quarks. The LHC is able to reproduce the QGP by accelerating and colliding together two beams of heavy ions. In the collisions, the temperature will exceed 100 000 times that of the centre of the Sun. In these conditions, the quarks are freed again and the detectors can observe and study the primordial soup, thus probing the basic properties of the particles and how they aggregate to form ordinary matter.