A Brief History of CERN

30th July 2013

Simulated image of the Higgs boson decaying.

A simulated image of the Higgs boson decaying into four muons. Image credit: CERN/CC-A.

1. Life before CERN

Before construction began on the European Organization for Nuclear Research (CERN) in 1954[1a], the atom was known to be composed of electrons (an elementary particle, and a type of lepton) and a nucleus containing neutrons and protons (which are hadrons, particles now known to be made of smaller particles called quarks and gluons), and all of these particles were thought to have an antimatter partner.

Fusion and fission reactions had taken place, and new particles such as muons (another elementary particle, and type of lepton) and pions and kaons (which are also hadrons) had been discovered in cosmic rays, using particle detectors like cloud chambers and bubble chambers. 'Cosmic ray' is a general term for different types of high-energy particles that originate from space, including photons, which are 'particles' of light.

1.1 The cloud chamber and bubble chamber

British physicist Charles Thomson Rees Wilson invented the cloud chamber in 1911[2]. He received the 1927 Nobel Prize in Physics for his invention and help in its development.

A cloud chamber is composed of a jar of gaseous alcohol that's cooled at the bottom with dry ice. When a highly energetic charged particle moves through this gas, it removes electrons from the atoms it passes, leaving them positively charged. The neutral atoms in the gas then condense around them, and along the path of the charged particle.

If a magnetic field is placed across the chamber, then it will deflect the path of charged particles. Austrian physicists Marietta Blau and Hertha Wambacher showed how the energy and charge of the particle can be determined from the shape of the track, which could be photographed[3].

The cloud chamber was made somewhat obsolete in 1952, when American physicist Donald Glaser invented the bubble chamber[4], for which he was awarded the 1960 Nobel Prize in Physics.

Bubble chambers work in the same way as cloud chambers, but here energetic charged particles travel through a superheated liquid - a liquid heated by changing pressure - such as liquid hydrogen, instead of a cold gas, and the liquid begins to boil around the ionised atoms that form along the path of the charged particle.

Diagram of a bubble chamber, where particles travel through a liquid, inside of a magnetic field. A camera is placed above the liquid.

A simple bubble chamber. Image credit: Stannered/aarchiba/Public domain.

Photograph from a bubble chamber, showing the effects of the decay of a kaon particle.

An image from a bubble chamber, showing the decay of a positive kaon. The decay products move in a spiral due to the magnetic field. Image credit: CERN/CC-A.

In order to for a particle to be detected, it must be energetic enough. Cosmic rays are extremely energetic before they enter the atmosphere - reaching energies of up to 100 billion, billion electron volts (1020 eV), which is 16 Joules - but they cannot travel through the atmosphere[5]. This means that they could only enter chambers that were taken to the top of mountains, or above the atmosphere in balloons, and they could not be controlled.

At sea-level, the particles emitted by radioactive material produce tracks, but these only reach energies of up to about 150 million eV (150 MeV) or 40 billionths of a Joule (2.4 × 10-11 J)[6]. Physicists wanted to fire particles at the nuclei of atoms in order to see what they are made of, and what keeps them together, as well as to see short-lived particles that could only be created in high-energy collisions. If this were to happen, then energies of more than 150 MeV would have to be reached. Kinetic energy is proportional to velocity, and so a higher energy is reached at higher speeds.

More systematic and precise experiments could only be conducted if it were possible to produce particles with higher energies on demand, at sea level, and this was achieved with the invention of particle accelerators.

1.2 Linear accelerators

A battery is the simplest particle accelerator. This works because the small voltage between its terminals (produced from having a negative end and a positive end) produces a proportional electric field. A charged particle, an electron in this case, is accelerated in this field, and can then travel down a wire.

This is where the unit of the electron volt comes from; it is the unit of energy gained by one electron, accelerated by a voltage of 1 volt. This is a useful unit to use when the number would be extremely small if measured in Joules.

Norwegian physicist Rolf Widerøe built the first linear particle accelerator in 1928[7]. This increases the velocity of charged particles by subjecting them to a series of alternating voltages. Like in a single battery, the particle is accelerated across the gap between differing voltages, but here they then meet another gap and travel across this. More and more gaps can be added, making the particle travel faster and faster. The longer the accelerator, the faster the particle can travel. Particles were fired at a fixed target and the aftermath could be recorded in cloud or bubble chambers.

Diagram of a linear accelerator.

A linear accelerator. Image credit: Sgbeer/CC-SA.

1.3 Cyclotrons and betatrons

American physicist Ernest Lawrence and his graduate student Milton Stanley Livingston invented another type of particle accelerator, the cyclotron, in 1932[8]. A cyclotron places charged particles in an alternating electric field, which causes them to accelerate between 'D' shaped electrodes, known as 'dees'. A uniform magnetic field is then placed perpendicular to this.

The magnetic field causes the charged particles to move in a spiral. This is because the magnetic field produces a force that's always perpendicular to the particle's velocity, causing it to continually change direction, which makes it move in a circle. The radius of the circle increases as the particles get faster, due to the electric field, creating a spiral.

Higher velocities are reached the higher the magnetic field and the larger the radius of the cyclotron. Lawrence and Livingston's first cyclotron was about 30 cm in diameter with a field of about half a Tesla. It accelerated protons to just over 1 MeV.

Cyclotrons only work up to about 20 MeV because they don't consider relativistic effects. These cause particles to become more difficult to accelerate as they approach the speed of light. This is more of a problem the lower the rest mass of the particle. Electrons have a particularly small rest mass, and so cannot be accelerated by cyclotrons.

These problems were resolved with the invention of the betatron, by American physicist Donald Kerst in 1940[9]. The betatron is like the cyclotron, but accelerates electrons with a varying magnetic field. The field is stronger at larger radii, and so the electrons are accelerated with a stronger force the closer they get to the speed of light. As with the cyclotron, increased speeds can be achieved with larger magnetic fields and larger radii.

Diagram of a cyclotron.

A simple cyclotron viewed from above and from the side, the 'D' shaped electrodes are called 'dees', magnets are placed below and above these. Image credit: Ernest O. Lawrence/Public domain.

Diagram showing a charged particle spiralling in a magnetic field.

A charged particle in a magnetic field. Image credit: Jfmelero/CC-SA.

1.4 Synchrocyclotrons and synchrotrons

American physicist Edwin McMillan developed another type of improved cyclotron, the synchrocyclotron, in 1945[10]. The synchrocyclotron is the same as a cyclotron but only has one dee, and compensates for relativistic effects by changing the frequency of the electric field, instead of keeping it constant.

The synchrocyclotron was soon surpassed by the synchrotron, which was theorised by Russian physicist Vladimir Veksler[11], and first constructed by McMillan that year[12].

In a synchrotron, particles are accelerated by cavities, which provide an alternating electric field. The particles move in a circle because of a magnetic field that increases in strength as the particle gets faster. The magnets are placed in the path of the accelerated particles, rather than across the whole device, and so synchrotrons can be built with larger radii.

When charged particles are forced to travel in a circle by a magnetic field, they emit photons with an energy that's related to the strength of the field and the speed of the particle. The photons emitted by synchrotrons can be used in medicine[13].

Diagram showing synchrotron radiation, when a charged particle spirals in a magnetic field and emits photons.

Synchrotron radiation. Image credit: NASA's Imagine the Universe/Public domain.

Synchrotron radiation can cause problems for accelerators, however, because charged particles loose energy when they emit photons. The energy they lose is equal to the energy of the photon. The power lost is inversely proportional to the radius of the circle they move in, and so synchrotron accelerators are generally more effective the larger they are.

2. CERN, the European Organization for Nuclear Research

CERN was first envisioned by French engineer Raoul Dautry, French physicists Pierre Auger and Lew Kowarski, Italian physicist Edoardo Amaldi, and Danish physicist Niels Bohr, in 1949[14a].

They wished to create a laboratory to study atomic physics with particle accelerators that would be so large, and expensive, that they could not be built by a single county alone. The first official proposal was put forward by French physicist Louis de Broglie in December of that year[14b].

In June of 1950, American physicist Isidor Rabi asked UNESCO (the United Nations Educational, Scientific and Cultural Organization) for assistance in the creation of the laboratory, in order to encourage the collaboration of scientists from across Europe[14c]. Rabi was born in Galicia, which is situated on the border between Poland and Ukraine, and had won the 1944 Nobel Prize in Physics for his work on the magnetic moment and nuclear spin of atoms.

By December of the following year, members of UNESCO adopted the first resolution to establish a European Council for Nuclear Research (or Conseil Européen pour la Recherche Nucleaire in French, which is where the acronym CERN came from)[14d].

This council came into effect in February 1952, and 11 counties agreed to participate. These were: Belgium, Denmark, France, the Federal Republic of Germany, Greece, Italy, the Netherlands, Norway, Sweden, Switzerland, and Yugoslavia. Geneva was selected as a site for the laboratory, and the United Kingdom joined the council the following year[15]. Construction began in May 1954, and by September of that year, the Council officially became an Organisation[1b].

CERN's first accelerator - the Synchrocyclotron (SC) - was turned on in 1957 and remained in operation for 33 years[14e].

2.1 The highest energy accelerator in the world

A second accelerator, the Proton Synchrotron (PS), was turned on in 1959, and is still in operation, feeding particles to newer, higher powered accelerators. Reaching energies as high as 28 billion electron Volts (28 GeV), the PS became the highest energy accelerator in the world, beating the SCs 600 MeV, and Russia's synchrotron, the Synchrophasotron, which had reached 10 GeV[14f]. The following year, the PS's record was surpassed by the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory in New York, which reached energies of 33 GeV[16].

2.2 The discovery of antimatter nuclei

In 1965, antimatter nuclei were simultaneously created from anti-neutrons and anti-protons, in experiments using the PS at CERN and the AGS in New York[17][14g]. Antimatter had first been proposed in the 1920s[18], and the first antimatter particle, the antielectron (known as the positron), was discovered in the 1930s[19]. The antiproton[20] and antineutron[21] were discovered in the 1950s. Physicists at CERN would go on to create entire atoms out of antimatter[22a].

2.3 The first computerised detections

In 1968, French physicist Georges Charpak developed the multi-wire proportional chamber, known as the wire chamber, while working at CERN[14h]. This soon replaced bubble chambers because it could detect particles more quickly, and could be linked to a computer so that data did not need to be physically examined in the same way that photographs from bubble chambers were. Charpak was awarded the 1992 Nobel Prize in Physics for his invention.

Charpak's invention involved using a proportional counter. Proportional counters were invented by British physicist Samuel Curran in 1948[23], and combine a Geiger Müller tube - the sensor used in Geiger counters, which were developed in the 1920s[24] - with an ionisation chamber - which measures the charge of ions created by high-energy charged particles. These were developed in the 1800s[25].

Diagram showing a Geiger-Müller tube and counter.

A Geiger-Müller tube and counter. Image credit: Dirk Hünniger/Nevdka/CC-SA.

Diagram of a wire chamber.

A simple wire chamber. Image credit: modified by Helen Klus, original image by Michael Schmid/CC-SA.

In proportional counters, high-energy charged particles travel through a gas. This strips electrons from the gases' atoms, leaving free electrons and positive ions. The free electrons are attracted to the positively charged electrode, the anode, and the positively charged ions are attracted to the negative electrode, the cathode. The movement of these particles creates an avalanche effect, where they ionise other particles until there are enough to create a measureable electric field. Each charged particle creates a new field, and its position can be determined from the time taken for the negative and positive particles to reach their corresponding electrodes.

Charpak's multi-wire proportional chamber was composed of many thin wires placed next to each other. He showed that each wire behaves as a proportional counter. This means that the wire chamber can detect hundreds of particles a second, which led to more accurate measurements of the particle's position.

2.4 The first hadron collider

By the 1950s, physicists had realised that they would be able to create higher energy collisions if they fired two moving targets at each other, instead of firing accelerated particles at a fixed target.

The first collider accelerator, ADA (Anello Di Accumulazione), was developed by Austrian physicist Bruno Touschek for the National Institute of Nuclear Physics in Italy, and was turned on in 1961[26]. This collided electrons and positrons (both of which are leptons, not hadrons). Positrons have the same energy as electrons but they are positively charged. This means that they travel in the opposite direction to elections when placed in a particle accelerator, and so electrons and positrons can be placed in the same tube.

The beams of particles must be lined up very precisely in order for them to collide. This is easier to do in lepton collisions, since leptons are elementary particles. Collisions between hadrons are less precise because they are made of quarks, and the total energy is shared between them, with some having more energy than others. Hadron collisions may be more difficult, but hadrons can collide at a wide range of energies, and so are more useful than leptons when physicists are trying to create new particles.

Physicists at CERN suggested that protons (which are hadrons) could be made to collide by using the PS to feed two rings - known as the Intersecting Storage Rings (ISR) - where beams of protons could be fired in different directions. This project was approved in 1965, construction on the ISR began the following year, and it became operational in January of 1971, becoming the first hadron collider in the world[14i].

Diagram showing hadrons are split into mesons – made of a quark and an antiquark – and baryons, made of three quarks or three antiquarks. Kaons and pions are mesons, and neutrons and protons are baryons.

Different types of hadrons. Image credit: Helen Klus/CC-NC-SA.

The following month, the Super Proton Synchrotron (SPS) - a synchrotron collider - was commissioned. This was to be CERN's largest accelerator yet, built about 40 metres below the ground with a circumference of 7 km. The SPS crossed the border into France becoming the first accelerator to cross an international border. It became operational in 1976, operating at energies of hundreds of GeV[14j].

2.5 The discovery of neutral currents

All of the detectors discussed so far, detect charged particles, and so they cannot be used to detect neutral particles, like neutrons or neutrinos. Neutrinos are elementary particles, and a type of lepton, they were first theorised in the 1930s[27] and detected in the 1950s[28].

Electroweak theory, which was developed in the 1960s, showed that electromagnetism and the weak nuclear force are two manifestations of a single force, the electroweak force[29][30]. It was predicted that the weak nuclear force is carried by particles known as the negatively and positively charged W bosons, and the Z boson, which has a neutral charge. Weak interactions involving the exchange of Z bosons were thought to create a neutral current, and this could occur if neutrinos interacted with electrons.

Physicists at CERN created a bubble chamber that could detect neutral particles, known as Gargamelle, which was operational from 1970 until 1979. It was attached to the PS until 1976 when it was moved to the SPS[31].

Gargamelle worked by measuring the effects of charged particles that were affected by collisions with neutrinos. It used heavy-liquid freon instead of liquid hydrogen, which is lighter, in order to increase the number of collisions.

Gargamelle first showed evidence of neutral currents via the detection of neutrinos in 1973[32]. The discovery of W and Z bosons was not possible, however, until they could be produced in more powerful collisions.

Diagram showing that matter is composed of leptons and quarks. There are six quarks and six leptons. There are also four gauge bosons plus the Higgs boson.

The standard model. Image credit: MissMJ/CC-A.

2.6 The discovery of the W and Z bosons

The SPS was converted into a proton-antiproton collider in 1979[33]. The first proton-antiproton collisions occurred in 1981, and W and Z bosons were discovered in these collisions in 1983[34]. Italian physicist Carlo Rubbia and Dutch physicist Simon van der Meer were awarded the 1984 Nobel Prize in Physics for their role in this discovery.

Now that W and Z bosons had been discovered using a hadron collider, more precise collisions using a lepton collider were needed in order to determine their mass. The Large Electron-Positron Collider (LEP) was first proposed in order to measure the mass of the Z boson produced in collisions between electrons and positrons[35a].

The LEP would be the largest lepton collider in the world, located about 100 metres below the ground, with a circumference of 27 km. It had been approved in 1981, and became operational in 1989, at energies of about 100 GeV. Millions of Z bosons were produced, and it was shown that they produced only three generations of particles of matter[36]. W bosons required more energy, and these were not produced until the 1990s when the LEP was improved, with more cavities added so that the collisions doubled in energy[35b].

2.7 The development of the World Wide Web

By the end of 1990, British computer scientist Tim Berners-Lee had developed URL, http, html, and the first browser and server software, while working at CERN. The world's first website and server - info.cern.ch - went live in December that year[37].

Before this, the internet - which was developed in the late 1960s and introduced to the public in 1969 - was mainly used by scientists to send information to each other in plane text. It was not possible to send the vast amounts of complex data that scientists at CERN dealt with[38].

Diagram showing that the internet is composed of many things besides the World Wide Web, such as email, telnet, P2P, and Chat.

The World Wide Web is a part of the internet. Image credit: Helen Klus/Public domain.

Photograph of the computer that the first website and server were run on, next to the original proposal.

The computer that the first website and server were run on, next to the original proposal. Both are currently located in the Globe at CERN's visitor centre. The writing on the computer says: 'This machine is a server. DO NOT POWER [?] DOWN!!'. Image credit: Helen Klus/CC-NC-SA.

The world's first web page - http://info.cern.ch/hypertext/WWW/TheProject.html - gave information on how others could create their own websites and search the web for information. In 1993, CERN made the World Wide Web free for anyone who wanted to use it[39].

2.8 Creation of the first antimatter atoms

The Low Energy Anti-Proton Ring (LEAR) was constructed at CERN in 1982 in order to store antimatter, which would be used to create antihydrogen atoms in the PS[40a]. Nine atoms of antihydrogen were created in collisions between antiprotons and xenon atoms in 1995[22b].

LEAR was shut down in 1996 and replaced with the lower energy Antiproton Decelerator (AD), which was approved in 1997 and became operational in 2000[40b]. In 2011, the ALPHA experiment used the AD to trap 300 antihydrogen atoms and study them in detail for over 16 minutes[41].

2.9 The discovery of direct CP violation

Matter and antimatter annihilates on contact, and the universe we observe is almost entirely made of matter. This implies that more matter than antimatter was created in the big bang, and is an example of CP symmetry being broken[42]. CP symmetry states that the laws of physics should be the same if all the matter particles in the universe were swapped with their antimatter partners and vice versa.

In 1964, it was shown that interactions involving the weak nuclear force break CP symmetry[43]. This was shown in experiments involving neutral kaon particles. Kaons spontaneously transform into antikaons, and back again, and each can be thought of as being composed of two particles, known as K1 and K2.

K1 is short lived, and soon decays into two particles called pions. K2 lasts longer, and eventually decays into three pions.

Indirect CP violation occurs when a K2 particle spontaneously changes into a K1 particle, and decays into two pions instead of three. This means that the total number of pions, from the K1 and K2 particles that make up each kaon or antikaon, is four instead of five.

Direct CP violation occurs when the K2 particle produces only two pions without first decaying into a K1 particle. Evidence for this was found by CERN in 1988[44], but this result was not verified until 1999, when evidence came from CERN's SPS[45] and the KTeV experiment at the Fermi National Accelerator Laboratory (Fermilab)[46], which was founded in Illinois in 1967.

2.10 The creation of quark-gluon plasma

Quark-gluon plasma is a state of matter that's thought to have occurred in the very early universe, when quarks and gluons existed as single particles rather than within atoms, as they do now. This state can be recreated by increasing the temperature or density of hadrons.

Physicists at CERN first tried to create this plasma in 1986 by colliding heavy nuclei - nuclei containing many neutrons and protons - in the SPS. They hoped that this would make the quarks and gluons, which make up protons and neutrons, separate. Oxygen and sulfur nuclei were used first, heavier lead nuclei were used in experiments first conducted in 1994, and by 2000, they were able to prove that they had created a quark-gluon plasma[47][48].

3. The Large Hadron Collider and the Higgs boson

Even before the LEP became operational in 1989, physicists considered how they could create higher energy collisions if it were converted into a hadron collider. Experiments that could be conducted in such a machine were first considered in 1984, although the construction of the Large Hadron Collider (LHC) was not approved until 1994[49a].

The LHC was initially planned to be developed in two stages, but donations from non-European counties such as Japan, India, Russia, the United States, and Canada meant that it was completed in one stage, becoming operational in 2009.

The PS and SPS are used to accelerate particles before they are injected into the LHC. The first attempt to circulate a beam of protons was conducted in 2008, but it failed due to a faulty magnet connection, which took a year to repair[49b].

In November 2009, two beams successfully circulated the LHC, and by March of 2010, physicists were able to make two beams collide with an energy of 7 trillion eV (7 TeV), beating Fermilab's Tevatron, which reached about 2 trillion eV (2 TeV). This made the LHC the highest energy accelerator in the world. The LHC produced so much data that it took years to analyse[49c].

The LHC was designed to run a number of experiments, all with their own detectors. These detectors are much more complicated than bubble or wire chambers, and are much larger, the largest, ATLAS (A Toroidal LHC Apparatus), is the size of a seven-story building[50a].

Detectors are placed around points where particles can be made to collide, and each detects the effects of the collision in different ways, applicable to different experiments. The largest four detectors are: ATLAS, the CMS (Compact Muon Solenoid), ALICE (A Large Ion Collider Experiment), and the LHCb (Large Hadron Collider beauty).

Diagram showing the current CERN accelerator complex.

The current CERN accelerator complex. Image credit: Forthommel/CERN/CC-SA.

Map showing the location of the Large Hadron Collider and Super Proton Synchrotron.

Map showing the location of the Large Hadron Collider and Super Proton Synchrotron. Image credit: Zykure/CC-SA.

3.1 The ATLAS and CMS detectors

ATLAS and the CMS are both general purpose detectors designed with the same scientific goals: to search for extra dimensions, dark matter, and particles known as Higgs bosons. They both do this in different ways, and can be run simultaneously, since only a small percentage of particles in a particle beam interact with each detector. This means that a detection in one can be confirmed by the other[50b][51].

ATLAS and the CMS both have the same four basic parts. A tracker is placed around the edge of the tunnel containing the beam. It uses a magnetic field to deflect all the charged particles that pass through it, and measures their direction and momentum. This is surrounded by an electromagnetic calorimeter, which stops photons and electrons and measures their energy, a hadron calorimeter, which does the same thing for hadrons, and finally a muon detector, which detects the direction and momentum of muons (which are elementary particles, and leptons).

Diagram of the CMS, showing the tracker, calorimeter, coil, muon detector, and iron return yoke.

Diagram of the CMS. Image credit: modified by Helen Klus, original image by CERN/CC-A.

The differences between ATLAS and the CMS come from the fact that there are two ways to detect muons. The first is to use the same magnet as the tracker, and the second is to place another magnet beyond the calorimeters. The CMS does the former and ATLAS does the latter.

In 2012, it was shown that the Higgs boson had been created in the LHC, and detected by both CMS and ATLAS[52]. This was confirmed earlier this year[53].

Photograph of the CMS tracker's outer barrel.

The CMS tracker's outer barrel. Image credit: CERN/CC-A.

Photograph of the CMS tracker's inner barrel.

The first half of the CMS inner tracker barrel. Image credit: CERN/CC-A.

3.2 The ALICE detector

The ALICE experiment was designed to study the quark-gluon plasma, which existed in the early universe, and had been recreated in the SPS in the 1990s. Unlike ATLAS and the CMS, which measure the effects of collisions between protons, ALICE was designed to study the collisions of iron nuclei. It has a tracking system and muon detectors but, unlike ATLAS and the CMS, its main detector is a time projection chamber. This is a particle detector similar to a wire chamber[54].

3.3 The LHCb detector

The LHCb detector was designed to detect antimatter, particularly the antibeauty quark (which is another name for the antibottom quark), so that physicists can study CP violation. Instead of surrounding the entire collision point, it uses a series of sub-detectors to detect particles thrown forwards in the collision[55].

3.4 Other experiments

The three other experiments at the LHC are the MoEDAL (Monopole and Exotics Detector At the LHC) experiment, the TOTEM (TOTal Elastic and diffractive cross section Measurement) experiment, and the LHCf (Large Hadron Collider forward) experiment. MoEDAL was designed to detect magnetic monopoles[56]. TOTEM was designed to measure the size of protons with unprecedented precision[57], and the LHCf was designed to measure the energy of neutral pions and explain the origin of the highest energy cosmic rays[58].

4. The future of CERN

CERN currently has 20 member states, though Israel, the Republic of Serbia, and Romania are in the process of becoming members, and co-operation agreements, scientific contacts, and observers exist from 59 counties around the world[59].

Earlier this year, the LHC was shut down so that it could be upgraded, and it's expected to become operational again in 2015, when it should be able to achieve energies of 14 TeV[60]. This will allow physicists to explore states of matter that haven't existed since just after the big bang, to test theories of quantum gravity, and, hopefully, to detect particles even more elusive than the Higgs boson, such as those responsible for dark matter.

Possibilities for future accelerators include the Very Large Hadron Collider (VLHC), which could have a circumference of over 200 km, and reach energies of about 30 TeV, although this is not being seriously considered at the moment[61].

Another possibility is an accelerator designed to collide muons and antimuons. Muons are about 200 times as massive as electrons but they are unstable, and soon decay into electrons and neutrinos, so there are still many technical difficulties with this at the moment[62a].

A more likely option is building a more powerful linear accelerator. CERN is currently considering a proposal for a Compact Linear Collider (CLIC) to accelerate electrons and positrons to energies of up to 3 TeV, which would make it the highest energy lepton collider in the world[62b].

5. Visiting CERN

Tourists are welcome at CERN. People who just turn up at its visitor centre in Meyrin can experience the Microcosm, a museum showing both the history of CERN, and how everything works, and the Globe. The Globe houses a number of historic items, including the computer that the first website and server were run on, and shows a short film Universe of Particles twice an hour.

If you book in advance, you can also go on a guided tour where you get to watch a film about the history of CERN and then visit the control room of ATLAS, a detector on the LHC where the discovery of the Higgs Boson was announced.

Unfortunately, visitors are not able to go underground at the moment, and can instead watch a short 3D film showing the Collider being built, and in use. All of these things are free to the general public.

UPDATE: On 3rd June 2015, the LHC was switched back on. It can now reach energies of up to 13 TeV. In July 2015, data from the LHC led to the discovery of a new class of particles, known as pentaquarks.

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