Everything you need to know about the Large Hadron Collider, CERN and the Higgs boson

Need to impress a sexy physics fan or wow your boss with your massive brain? Our idiot's guide to the LHC will turn you into an expert in five easy steps

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The Large Hadron Collider is back up and running, but what the heck is the big ring thing all about? We present our bluffer's guide to the fun, frolics, facts and physics that powers the most expensive thingamabob ever built. What's the LHC, what's it doing, and why? All will be revealed -- with no maths required. When you're done, make sure you check out our, er, in-depth interview with one of the scientists who works there.

What is CERN?

CERN does particle physics, which means studying the fundamental particles -- the basic constituents of matter. The goal is to find out what the universe is made of and how it works.

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they're made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions. From these results, scientists learn about a particle's properties, such as mass and charge.

What is the 'hadron' in the Large Hadron Collider?

There are 17 known fundamental particles -- six quarks, six leptons and five bosons (not counting the theoretical Higgs boson) -- and their corresponding anti-particles. They are called 'fundamental' or 'elementary' particles because they have no smaller constituent parts.

The quarks combine in various combinations to form other particles, such as protons and neutrons. Collectively, all the particles that are made up of quarks are called 'hadrons'. The LHC collides protons, which are hadrons -- that's why it's called the Large Hadron Collider.

Bosons -- not to be confused with rum-swilling, cat-swinging bosuns -- are the force-carrying particles. Examples of bosons are the photon and the hypothetical Higgs boson, which is being sought by the Atlas and CMS experiments at the LHC.

All this knowledge about the fundamental particles and how they interact is called the 'standard model' -- but there's more to discover, and that's the goal of the LHC.

What is the Higgs boson?

We don't know what causes the fundamental particles to have masses. There's a theory that mass comes from the particles interacting with an (as yet unobserved) particle called the Higgs boson. The more they interact, the heavier they become, whereas particles that never interact are left with no mass at all.

The Higgs boson is named after physicist Peter Higgs. Its nickname is 'the God particle'. Higgs himself calls it "the particle named after me".

Click 'Continue' below to find out more about the Large Hadron Collider, or watch our Bluffer's Guide below.

The LHC in a nutshell

The LHC is a particle accelerator. It does just what it says on the tin: it collides hadrons together -- in this case, protons, which are a type of hadron particle -- at very high speeds.

The protons are created when hydrogen atoms, which consist of one electron orbiting a single proton, are stripped of their electron. The protons are divided into two streams, made up of bunches of about 100 billion protons, and sped up by a series of smaller accelerators before being injected into the main LHC ring. Superconducting electromagnets guide the two streams in opposite directions around the ring and an electric field boosts their energy until they are travelling at 99.99 per cent of the speed of light.

After about 20 minutes, and 13.5 million trips around the LHC, two streams are brought together in an enormously energetic collision at one of the four main detector sites along the LHC: Atlas, Alice, CMS or LHCb.

The protons are so small that most of them fly right past each other -- there are only 20 collisions each time two bunches of 100 billion protons are brought together. But there are so many in the beam, that's still 600 million collisions per second.

At the point of collision, each proton has 7 tera-electronvolts (TeV) of energy. One TeV is about the energy of a flying mosquito, but a proton is a trillion times smaller.

When the protons smash together, it creates a shower of energetic particles that shoot off in all directions. Detectors measure the energies, directions and velocities of these particles, and the resulting data is fed to a supercomputer for analysis.

The questions being asked at the LHC

What is mass? Why do particles weigh the amount they do? Why do some particles have no mass at all? There are no established answers to these questions, but one explanation is the Higgs boson. First hypothesised in 1964, it has yet to be observed.

The Atlas and CMS experiments will be searching for signs of the Higgs boson.

What is dark matter/dark energy? Everything we see in the universe is made up of ordinary matter, but that forms only 4 per cent of the universe. Dark matter and dark energy are believed to make up the remaining 96 per cent, but they're incredibly difficult to detect and study, other than through the gravitational forces they exert.

The Atlas and CMS experiments will look for supersymmetric particles to test a likely hypothesis for the make-up of dark matter.

Why is there no more antimatter? Antimatter is like a twin version of matter, but with opposite electric charge. At the birth of the universe, equal amounts of matter and antimatter should have been produced in the Big Bang. But when matter and antimatter particles meet, they annihilate each other, transforming into energy. Somehow, a tiny fraction of matter must have survived to form the universe we live in today, with hardly any antimatter left. Why does nature appear to have this bias for matter over antimatter?

The LHCb experiment will be looking for differences between matter and antimatter to help answer this question. Previous experiments have already observed a tiny behavioural difference, but what has been seen so far is not nearly enough to account for the apparent matter-antimatter imbalance in the universe.

How did quarks form particles just after the Big Bang? Protons and neutrons (which make up the atomic nucleus) are made of quarks, which are bound together by other particles called gluons. The bond is so strong that quarks are never seen in isolation. Collisions with lead ions in the Alice detector will generate temperatures more than 100,000 times hotter than the Sun's core, possibly freeing the quarks from their bonds with the gluons. This should create a state of matter called quark-gluon plasma, which probably existed just after the Big Bang.

The Alice experiment will recreate conditions similar to those just after the Big Bang to study the quark-gluon plasma as it expands and cools, observing how it progressively gives rise to the particles that constitute the matter of our Universe today.

Are there more than three dimensions? Einstein showed that the three dimensions of space are related to time. Subsequent theories propose that further hidden dimensions of space may exist -- for example, string theory implies that there are additional spatial dimensions yet to be observed. These may become detectable at very high energies, so data from all the detectors will be carefully analysed to look for signs of extra dimensions.

Click 'Continue' below to find out more about the experiments.

The experiments: Atlas, Alice, CMS and LHCb

There are four main experiments (and two small ones) at the LHC, each with its own detector. The detectors are installed in four huge underground caverns located around the ring of the LHC.

Atlas (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid): Both Atlas and CMS are general-purpose detectors. They will investigate the myriad particles produced by the collisions in the accelerator. They will search for the Higgs boson, extra dimensions, and particles that could make up dark matter. Having two independently designed detectors is vital for cross-confirmation of any new discoveries. They are huge: CMS is double the mass of Atlas at 12,500 tonnes; Atlas is eight times the volume of CMS and as big as St Paul's cathedral.

Alice (A Large Ion Collider Experiment): Alice will investigate the quark-gluon plasma, looking at free quarks for the first time and how quarks combine to form particles such as protons and neutrons (which make up the atomic nucleus).

LHCb (Large Hadron Collider beauty): Although absent from the Universe today, 'beauty' (also called 'bottom' or 'b') quarks were common in the aftermath of the Big Bang, and will be generated in their billions by the LHC, along with their antimatter counterparts, anti-beauty quarks. They decay rapidly into a range of other particles, and the LHCb will compare these decays to find out why nature prefers matter over antimatter.

Click 'Continue' below to read some fun facts about the project

Fun facts about the LHC

The largest machine in the world: The LHC is 17 miles (27km) in circumference and runs about 100 metres under the surface, spanning the border of France and Switzerland. It's the world's largest particle accelerator. It took 20 years of planning and $10bn to build.

The most powerful supercomputer in the world: The LHC will generate 40,000GB of data each day -- this would fill 20 million CDs a year. To handle this data, tens of thousands of computers located around the world are being connected over the Internet in a distributed computing network called the Grid -- a virtual supercomputer.

The emptiest space in the Solar System: To avoid colliding with gas molecules inside the accelerator, the beams of particles travel in an ultra-high vacuum -- a cavity as empty as interplanetary space. The internal pressure of the LHC is 10-13 atmospheres, ten times less than the pressure on the Moon.

Hotter than the Sun: When two beams of protons collide, they will generate temperatures more than 100,000 times hotter than the core of the Sun, concentrated within a minuscule space.

Colder than outer space: The LHC's 9,300 magnets will be pre-cooled to -193.2°C (80K) using 10,080 tonnes of liquid nitrogen, before they are filled with nearly 60 tonnes of liquid helium to bring them down to -271.3°C (1.9K) -- colder than outer space.

Click 'Continue' below to find out more about CERN.

What does 'CERN' stand for?

When CERN was founded in the 1950s, the name stood for 'Conseil Européen pour la Recherche Nucléaire' or European Council for Nuclear Research. That was the name of the provisional organising body, and was quickly changed, but the nickname CERN stuck. The lab at CERN is called the European Laboratory for Particle Physics.

A brief history of CERN

The provisional body -- the origin of the name CERN -- was set up in 1952. After being ratified by the 12 founding member countries, CERN officially came into being in 1954.

CERN's first accelerator was the Synchrocyclotron (SC), built in 1957. As part of various experiments, it stayed in use until 1990 -- 33 years.

The Super Proton Synchrotron (SPS) was the first of CERN's giant rings. Started in 1976, it was 7km in circumference. Among other experiments, it was a collider that smashed together protons and anti-protons -- in this role in 1983 it was used to discover the W and Z particles, which provide the weak force.

In 1989, the Large Electron-Positron (LEP) collider was commissioned. It is the largest electron-positron accelerator ever built, at 27km in circumference. The excavation of the LEP tunnel was Europe's largest civil-engineering project prior to the Channel Tunnel. LEP was closed down in 2000 to make way for the construction of the LHC in the same tunnel.

In 1990, CERN scientist Tim Berners-Lee invented the World Wide Web. He defined the Web's basic concepts and he wrote the first Web browser and server software. The very first Web site was info.cern.ch. (Note the Web is not the same thing as the Internet. The Internet is the network of computers and the protocols (standards) that they use to communicate. The Web is a way to access information over the Internet, using Web browsers, Web pages and hyperlinks.)

In 1995, CERN made the first antimatter atoms. They were anti-hydrogen atoms, there were nine of them, and they only lasted about forty billionths of a second before annihilating with ordinary matter. Since then, they've created thousands more.

The countries and staff at CERN

CERN is run by 20 European member states, but many non-European countries are also involved. Scientists come from around the world to use CERN's facilities.

The current member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the UK.

CERN employs around 2,500 people. Around 8,000 visiting scientists, half of the world's particle physicists, come to CERN for their research. They represent 580 universities and 85 nationalities. We recently interviewed Stanford's Dr Paul Jackson about his work there, whether there's an alarm that goes 'AWOOGA!' and what it smells like.

(All statistics sourced from CERN. Additional material from the print edition of the Guardian, 30 June 2008.)

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