The Large Hadron Collider at CERN

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Topical Science The Large Hadron Collider at CERN About the LHC The Large Hadron Collider (LHC) is an example of ‘big’ science, on a par with space exploration in terms of cost and the number of scientists and engineers involved. It is housed in a huge underground circular tunnel, almost 27 kilometres in circumference, at the site of the European Organisation for Nuclear Research (CERN). This organisation was established in the 1950s, through the collaboration of 12 European countries. It is located near Geneva, Switzerland, close to the border with France. While almost all of the installations above ground are in Switzerland, the underground tunnel is mainly buried deep beneath French soil.

The aim of CERN is to provide European scientists with a facility where they can carry out fundamental research into the nature of matter and the origin of the Universe.

The Entrance to CERN

The picture on the left, above, shows flags of participating nations flying at the entrance to the CERN facility, near Geneva. The spherical building on the right houses an exhibition, sponsored by Rolex, called ‘The Universe of Particles’. This is free to the general public and is self guided. It has interactive videos and touch screen technology, describing the Big Bang and explaining the Standard Model of particle physics.

KEY DATES 1971

1981

2008

The first proton collider was commissioned at CERN.

The Super Proton Synchrotron (SPS) was commissioned, followed by the LEP (Large Electron-

Two proton beams were circulated in the LHC for the first time.

Positron collider.)

TOPICAL SCIENCE

THE LARGE HADRON COLLIDER

FEBRUARY 2019

There is little activity in the control room in CERN right now, because the LHC is on a scheduled shut down. But the engineers are busy carrying out maintenance and preparing for the next run in two years’ time. In the meantime, an enormous amount of DATA has been accumulated during the run that ended in December 2018, which will keep scientists busy analyzing it for the next two years, until the LHC is started up again. Scientists can study this data from their own universities or research institutes, without having to travel to the CERN campus.

The Control Room at CERN

The data is sent to researchers through a network of computers, similar to the Internet, known as ‘The Grid.’

Guided Tours at CERN Since CERN is funded by taxpayers’ money, through the governments of the participating countries, the organization takes seriously its obligation to the general public. It offers free, guided tours of its facility for individuals and groups, including groups from schools and universities.

The tours start at the Reception building, which houses the visitor centre and an exhibition called ‘Micro-cosmos’, where visitors can spend time, at their leisure, after completing the guided tours.

The official tour brings visitors to the Control Room, which is viewed through a large glass window. There they can see the scientists working at a bank of computers. The visitors are taken via a simulated lift, to a mock-up of a portion of the underground tunnel, which really looks exactly like the real thing. They are ferried, by bus, to buildings housing the ATLAS and CMS detectors and see full-scale replicas of these detectors. The tours also get to visit the real cryogenics section, where the powerful magnets are tested before being deployed underground.

A model of part of the CMS detector.

The actual cryogenics section, where the superconducting magnets are tested

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TOPICAL SCIENCE

THE LARGE HADRON COLLIDER

FEBRUARY 2019

What happens inside the Large Hadron Collider?

At the moment, the LHC is in a so-called ‘Long Shutdown’, for scheduled maintenance. But when it is in operation, it is capable of accelerating charged particles to speeds close to the speed of light. It is the largest particle accelerator in the world. Particle accelerators use strong electric fields to accelerate charged particles (such as protons or electrons) to very high speeds and therefore very high kinetic energies. Each time a stream of charged particles travels around the ring, it gets a further ‘kick’ from the field and so travels even faster. When two streams of particles are made to travel in opposite directions around the circular tunnel, they can be made to collide at very high energies. The LHC uses powerful superconducting magnets to guide the beam and keep it on track and to focus the beam, thus increasing the ‘Luminosity’, which increases the probability of collisions between such miniscule particles.

The first proton collider at CERN was commissioned in 1971 and the Super Proton Synchrotron (SPS) in 1981. The SPS was responsible for the discovery of what physicists call the massive W and Z particles.

This was followed by the Large Electron-Positron Collider (LEP), which was used to test the ‘Standard Model’ of particle physics. By 1996, the LEP was capable of accelerating an electron beam to an energy of 90 GeV. In particle physics, energies are expressed in electron volts (eV). An electron volt is the energy an electron gains when subjected to a potential difference of one volt. It is an extremely tiny amount of energy, so in high-energy physics, the measurements are made in giga electron volts. (The prefix ‘giga’ in the international system of units means 10^9, or a thousand million, so 90 GeV means ninety thousand million electron volts.). To develop the research further, physicists needed to go to even higher energies, in the TeV range. (1 TeV = 1,000 GeV), so the Large Hadron Collider was built.

The LHC has more powerful magnets, which can accelerate particles to even higher energies, than the super proton synchrotron. But the SPS is still used to give protons an initial acceleration, so that they are already travelling at very high speed, before entering the LHC. Right: A mock-up of a linear section of the underground tunnel at CERN

Left: A model of one of the superconducting magnets, cut away to show the various layers inside.

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TOPICAL SCIENCE

THE LARGE HADRON COLLIDER

The Large Hadron Collider: The Story so far.

FEBRUARY 2019

The significance of the discoveries at CERN. The detection of the Higgs boson at CERN in 2012 was of major significance, as it served to reinforce the Standard Model of particle physics. If the Higgs particle had not been found, the theory might have had to be modified, or perhaps abandoned altogether.

On September 10th, 2008, amid a blaze of media publicity, two beams of protons were successfully circulated in the main rings of the LHC for the first time. Unfortunately, the operations had to be suspended nine days later because of a technical fault. The fault arose because the superconducting magnets need to be maintained at a temperature close to absolute zero in order to function properly. This extremely low temperature (about 271 degrees below the freezing point of water on the Celsius scale) is maintained using liquid helium and the helium had sprung a leak. Before any maintenance work could be done, the temperature of the apparatus had to be slowly raised to normal temperatures. Then, after the repairs had been completed, the whole apparatus had to be evacuated again (the proton beams have to operate in a near total vacuum) and the temperature lowered once more.

When the Large Hadron Collider resumes experiments in two years’ time, at even higher energies and greater luminosity, who knows what new discoveries may be made?

We all tend to take the World Wide Web for granted now. Tim Berners Lee and Robert Caillou invented it, while they were working at CERN. It is possible that The Grid, also developed at CERN, could revolutionise communications systems in the future, in ways we cannot now imagine.

For more information, visit the CERN website: https://home.cern/

It was not until late November 2009 that the experiment was re-started. Finally, on November 23rd, 2009, the first proton-proton collisions were recorded. However, these were ‘only’ at 450 GeV per beam. The LHC was shut down, for the winter of 2009-2010. On re-opening, the energies were ramped up. Then, on 30th March 2010, the first collisions at 3.5 TeV occurred. At the time, they were the highest energy man-made particle collisions ever recorded. The LHC did not run at full energy (7 TeV per beam) until 2014.

By making protons beams collide at such high energy, physicists can re-create, on a miniscule scale, conditions that were present at the ‘Big Bang’, the event which is believed to have given rise to the beginning of space and time and the creation of our universe. By doing this, scientists hope to reach a better understanding of the cosmos.

About the Author.

Topical Science is written and published by Margaret Franklin, a retired chemistry lecturer, who spent most of her career at Athlone institute of Technology, in the centre of Ireland. Prior to that, she taught Science at an international school in Switzerland & became familiar with the work of CERN in the 1970s.

Inside the LHC, two beams of protons are sent through separate tunnels, in opposite directions. There are four crossing points where the beams can intersect and it is there that the collisions occur. Detectors are set up at these intersections, to track and measure the energies of the particles that are produced in the collisions.

One may wonder, what is the point of smashing charged particles together at such high energies? Imagine sending two high-speed trains hurtling towards each other on the same track, until they crash, and then trying to find out how they work by examining the wreckage!

It sounds a bit like that. But it seems this is the only way that scientists have found, so far, to examine subatomic particles.

The aim of this newsletter is to explain, in plain language, for the general public, the science behind topical news items.

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