07/12/2010

The Large Hadron Collider

lhc_hall_1.jpgThe LHC is a machine of discovery. It will take us into a new energy regime where, through the famous relationship E = mc2, new particles of greater mass, inaccessible to existing machines will be produced. It is not known what the LHC will discover, but there are a number of fundamental questions in physics on which the machine will certainly shed light. The four detectors of the LHC are each designed to answer these questions.


The origin of mass

Mass or weight (mass under the influence of a gravitational field) is such a familiar concept that we take its existence for granted. The structure of matter at the most fundamental level has been elucidated over the last 35 years by experiments at CERN and at other laboratories around the world. These experiments have revealed an astonishingly simple picture. All matter is made up of a small number of elementary particles (six quarks and six leptons) held together by forces mediated by a small number of “force” particles. The most familiar of these is the photon, the particle of light. The photon has no mass but this does not mean that it is useless. It brings the energy from the sun that allows life on earth, it allows us to see, and where would we be today without TV or mobile phones? On the other end of the spectrum, the particles that mediate the weak nuclear force (the W and Z bosons discovered at CERN in 1983), which are responsible for the burning of the sun, are very heavy, weighing respectively 80.4 GeV and 91.2 GeV, a little less than an atom of silver. The fundamental mechanism of how particles acquire mass and why there is such a large difference between them is not understood. The most promising theory predicts the existence of a particle called the Higgs boson (there may be more than one) which is responsible for the process that gives mass to the other particles. If the Higgs exists, it is very heavy. It must be heavier than 113 GeV or it would have been seen already at the CERN large electron-positron collider (LEP), the previous CERN flagship accelerator. On the other hand, fundamental arguments require that it is less than about 850 GeV . The LHC is designed to cover the whole energy range; if the Higgs exists, the LHC will find it. The two largest detectors, ATLAS (an acronym for A Toroidal LHC ApparatuS) and CMS (the Compact Muon Solenoid detector) are general-purpose detectors capable of observing the unexpected, but they are especially sensitive to all possible manifestations of the Higgs boson.

Matter-antimatter asymmetry

All matter particles have antimatter cousins which can be created in our accelerators. These are particles of the same mass but opposite electric charge. When matter and antimatter meet, they annihilate one another, converting into radiation. If there were perfect symmetry between matter and antimatter, then during the early big bang, these annihilations would have taken place leaving a universe with only photons, no place for us! The LHCb detector (“b” for the Beauty Experiment) is designed to study this very subtle asymmetry.

Dark matter and dark energy

The first person to postulate the existence of a vast unseen form of matter was Swiss astrophysicist Fritz Zwicky in 1933. In the late 1960’s and early 1970’s, solid experimental evidence began to emerge from the work of Vera Rubin, a young American astronomer. Rubin and colleagues measured the rotational velocities of galaxies as a function of distance from the galactic centre using spectroscopic techniques. They found that, instead of dropping off with increasing distance, most of the stars are orbiting at roughly the same speed. The only way that this can be explained is that the density of matter was constant far beyond the visible galaxy. Another equally mysterious effect was discovered in 1998, when the LHC was well into construction. Measurements of the recession speeds of distant supernova have produced evidence that the expansion of the universe is accelerating. To explain this, a new kind of energy, “dark energy” has been postulated. It is now thought that this invisible dark matter and dark energy make up for about 96% of the total mass; only 4% of the universe is observable.
These two phenomena are examples of the convergence of particle physics and cosmology. The LHC will provide a laboratory environment where it may be possible to elucidate their cause.

The quark-gluon plasma

By colliding beams of lead ions, the LHC will be able to produce a state of matter that only existed a few millionths of a second after the big bang. The properties of the so-called quark-gluon plasma can be studied in detail in the specially built ALICE detector (an acronym for A Large Ion Collider Experiment).

A brief history of CERN colliders

From its foundation in the 1950s until the late 1960s, particle physics research at CERN was done in a way similar to that Rutherford used at the beginning of the 20th century when he discovered the atomic nucleus by bombarding a thin foil target with energetic alpha particles (the nucleus of the helium atom) from radioactive decay. In CERN’s early accelerators, beams of protons (the hydrogen nucleus) replaced the alpha particles as projectile. They could be accelerated to much higher energy and could be made to collide with the nucleons in any selected target material. Now, when a high-energy proton collides with a stationary proton or neutron in a target, new particles can be created by the conversion of energy into mass according to the famous Einstein relationship. However not all of the energy of the incoming projectile is available due to the conservation laws of energy and momentum. As a consequence, the available energy for new particle production only increases very slowly, as the square root of the energy of the incoming proton. For example, in the 450 gigaelectron volt (GeV) Super Proton Synchrotron (SPS) at CERN operating in this “fixed target” mode, only about 30 GeV is available for making new particles. On the other hand, if the two particles can be made to collide head-on, each with 450 GeV, the full 900 GeV is available. This is equivalent to the real life observation that the damage is much worse if two cars collide head-on with a given velocity than if a car struck a stationary vehicle with the same velocity. In the second case, much of the energy is dissipated in pushing the stationary car forward.

These colliding beam machines (storage rings), with two beams of particles circulating in opposite directions and colliding at a point on the circumference where particle detectors could be placed were the dream of accelerator builders in the late 1950s. In the early 1960s the first machines started to appear at Stanford in the US, Frascati in Italy and Novosibirsk in Russia. Instead of protons, these machines collided leptons (electrons or positrons). One great advantage in using leptons is that, when bent on a circular orbit, they emit light (synchrotron radiation). The dynamics is such that the emission of this radiation has a natural damping effect on the transverse dimensions, concentrating the particles into a very intense beam, essential if there is to be a reasonable probability of two particles colliding instead of the beams just passing through each other like two clouds. It is also desirable that the beams can circulate for many hours while data can be collected. During this time the particles are subjected to perturbations due to imperfections in the guide field or the electromagnetic field of the other beam that can drive them unstable. Synchrotron radiation also plays an important role in combating these external perturbations due to its natural damping effect. However, the emission of synchrotron radiation makes the particles lose energy, which has to be replaced by the acceleration system. Essentially, the beams have to be permanently accelerated in order to keep them at constant energy. The energy lost each revolution increases dramatically (with the fourth power) as the energy of the machine increases, eventually making it impossible for the accelerating system to replace it. In spite of its usefulness, synchrotron radiation naturally limits the maximum achievable energy of the machine. The way around this is to revert to particles that emit much less radiation.

Proton storage rings

In the late 1960s, a very bold step was taken at CERN with the construction of the first proton storage rings, called the Intersecting Storage Rings (ISR), which started operation in 1969. The advantage of protons is that they do not emit synchrotron radiation of any consequence since the energy loss per revolution varies as the inverse fourth power of the mass of the particle, and protons are 2000 times heavier than electrons. The disadvantage is that they have to operate without the benefit of the strong damping provided by synchrotron radiation. Indeed, many accelerator physicists doubted that proton storage rings would work at all.
In the end, the ISR was a big success and an essential step on the road to the LHC. The machine eventually reached 31 GeV per beam, compared with the few GeV available from the lepton beams at that time. The accelerator physicists learned how to build proton storage rings that overcame the lack of synchrotron radiation damping. The experimentalists learned how to build detectors that worked in the difficult environment of a proton-proton collider.

Another disadvantage of using hadrons (protons and antiprotons) is that, unlike leptons, they are composite objects. Each proton contains three more fundamental particles (quarks) held together by gluons. Each quark carries, on average, about one fifth of the hadrons’ energy. The rest is stored in the other quarks and the gluon field. When two quarks collide, the exact collision energy is not known a priori. It must be measured in the detectors by calorimetry, a technique that measures the energies of all the created particles. In addition, there is a very large background of unwanted events due to “soft” collisions of the gluon fields. In fact, many physicists were initially skeptical about our ability to dig out rare events from this large background.

Construction of the LEP

For these reasons, it was decided that the next machine for CERN would be LEP, the Large Electron Positron collider. In order to minimize the effect of synchrotron radiation it was necessary to build a very large, 27-km circumference ring. Even so, the maximum energy of LEP was limited to around 100 GeV, at which point it was radiating away a substantial fraction of its energy each revolution. Although LEP produced an enormous amount of precision data, it came to the end of its useful life when it hit the synchrotron radiation barrier. It was shut down in 2001 to make way for the LHC. The way to higher energies was once more to revert to protons as projectiles. The LEP tunnel is the major piece of real estate inherited by the LHC.

The final step on the road to the LHC was taken during the long period of LEP construction. During this time, Carlo Rubbia proposed that the Super Proton Synchrotron (SPS), built in the 1970’s as a “fixed target” machine, could be turned into a hadron collider using the newly discovered technique of accumulating and cooling antiprotons produced in CERN’s oldest machine, the CERN Proton Synchrotron (PS). Since protons and antiprotons have the same mass but opposite charge, they could be accelerated in opposite directions in the single vacuum chamber of the SPS. Collisions at 273 GeV per beam produced the first W and Z bosons, the mediators of the weak nuclear force responsible for radioactive decay. The Nobel Prize was awarded to Rubbia and van der Meer (who developed the cooling method of antiprotons) in 1984.

The Proton-Antiproton collider (PPBAR) also provided the essential remaining information needed for the design of the LHC and its detectors. For the LHC machine it elucidated the main factors that would limit the performance of the LHC, and the two detectors UA1 and UA2 served as prototypes for the much larger LHC detectors. Indeed, the nucleus of the teams designing ATLAS and CMS comes from these earlier collaborations.

The path of the protons

An essential feature of the LHC in terms of cost reduction is that the CERN infrastructure, with 50 years of investment, is used to produce the beams that eventually collide in the LHC. Without this, the cost of the project would have doubled. No single machine can accelerate the beam all the way up to 7 TeV. It needs a cascade of accelerators, all working in tandem.

Protons are the positively charged nuclei of the hydrogen atom. They are created in an ion source called a duoplasmatron from which they are extracted with an energy of 50 kilo-electron volts (KeV). The next step in their journey to the LHC is through a 35 meter long linear accelerator (Linac) where their energy is increased to 50 mega-electron volts (MeV). Originally, the beam was injected directly into CERN’s oldest machine, the 100 meter radius Proton Synchrotron (PS), built in 1959, but in 1972 a booster synchrotron (PSB) was inserted between the Linac and PS to improve its performance. The PSB is one quarter of the circumference of the PS and contains four superposed rings to allow filling of the whole PS circumference in one pulse.

The beam is accelerated to 1.4 giga-electron volts (GeV) in the PSB and then transferred to the PS where it is further accelerated to 26 GeV. It is in the PS that the particles are grouped into a train of bunches, each containing one hundred billion protons. Each bunch is about 1.2 meters long and they are separated by 7 meters. This separation is maintained all the way to collision in the LHC.

The Super Proton Synchrotron

At 26 GeV, the bunches are transferred into the next machine in the chain, the 1100-meter-radius Super Proton Synchrotron (SPS) built in 1976, where they are further accelerated up to 450 GeV and injected into the LHC, first in one ring and then into the other. When the two rings are filled, the magnetic field of the LHC is slowly ramped up and the beams are simultaneously accelerated by the radio frequency system which keeps them in the center of the vacuum chamber as the magnetic field rises. After about 20 minutes the beams reach the nominal collision energy of 7 TeV. They are then steered into collision in each of the four detectors.
At nominal intensity, there will be millions of collisions per second. However, in view of the enormous number of protons in each bunch, the intensity only decays very slowly. Typically the beams will remain in collision for about 10 hours after which any remaining beam is safely dumped onto an absorber block and the machine is brought back to its injection energy where the whole filling and acceleration cycle is repeated.
The injection of lead ions for the heavy ion program is slightly more complicated. It needs a special source and a Linac capable of accelerating lead ions. In order to get sufficient intensity, it also needs an accumulator ring where several Linac pulses can be accumulated before transferring the beam to the PS. The rest of the path through the PS and SPS to the LHC is similar to that of protons.

The design of the LHC

The fact that the LHC was to be constructed at CERN making the maximum possible use of existing infrastructure to reduce cost imposed a number of strong constraints on the technical choices to be made.
The first of these was the 27-km circumference of the LEP tunnel. The maximum energy attainable in a circular machine depends on the product of the bending radius in the dipole magnets and the maximum field strength attainable. Since the bending radius is constrained by the geometry of the tunnel, the magnetic field should be as high as possible. The field required to achieve the design energy of 7 TeV, is 8.3 tesla, about 60% higher than that achieved in previous machines. This pushed the design of superconducting magnets and their associated cooling systems to a new frontier. The next constraint was the small (3.8 m) tunnel diameter. It must not be forgotten that the LHC is (just like the ISR) not one but two machines. A superconducting magnet occupies a considerable amount of space. To keep it cold, it must be inserted into an evacuated vacuum vessel called a cryostat and well insulated from external sources of heat. Due to the small transverse size of the tunnel, it would have been impossible to fit two independent rings, like in the ISR, into the space. Instead, a novel and elegant design with the two rings separated by only 19 cm inside a common yoke and cryostat was developed. This was not only necessary on technical grounds but also saved a considerable amount of money, some 20% of the total project cost.
Finally, the re-use of the existing injector chain governed the maximum energy at which beams could be injected into the LHC.

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L'auteur :

Dr Lyndon Evans, is a Welsh scientist and the project leader of the CERN's Large Hadron Collider, based in Switzerland.

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