The Large Hadron Collider (LHC)

The Large Hadron Collider (LHC) at CERN in Switzerland is now back in operation. By smashing particles together at close to the speed of light, it promises to deliver dramatic new...
01 December 2009


A lot has been said about the Large Hadron Collider (LHC) over the past year. It's been called a lot of things including a $9bn atom-smasher, the world's most sophisticated scientific experiment, our key to unravelling the secrets of the Universe, a stupendous collaborative achievement, a stupendous waste of money and even a doomsday device.

But what is the LHC really?

To answer this question let's indulge in a little thought experiment. Consider an early human sitting somewhere in east Africa about 150,000 years ago. He (or she) doesn't know much about the world around him, at least not by modern standards. He is familiar with the predictable rising and setting of the Sun, the phases of the Moon, the turning of the seasons, the movement of the stars. He knows where to find food, which tribes control which territories and the times of day when it's best to stay hidden. He probably hasn't thought much about the fundamental structure of matter - but let's say he has an inquiring mind. He's started to think about what things are made of. What approach might he take?

He might indulge in his own thought experiment, as Leucippus and Democritus did in the 5th century BC when they devised their theory of uncuttable atoms. But he's really more of a practical chap so he picks up two rocks and smashes them together.

What does he learn from doing this? Probably not very much. Most likely he creates a lot of dust and some smaller bits of rock, some of which get in his eyes, thus dissuading him from pursuing his career in experimental science any further. Nevertheless he can at least draw the conclusion that rocks are not fundamental - they are made of smaller parts.

So after 150,000 years of progress in scientific thought and engineering how do modern scientists try to answer the question of the fundamental nature of matter? Well, they do exactly the same thing - they smash things together and see what happens. So while the LHC is certainly an incredibly sophisticated scientific instrument, what it is actually designed to do is really rather crude. The LHC is the world's biggest club, designed for hitting matter harder than it has ever been hit before.

Why so hard?

Our current best theory describing the fundamental particles and forces of nature is known as the Standard Model. This theory describes almost every measurement that has been made in particle physics to incredible precision and is one of the crowning achievements of 20th century physics. The Standard Model describes all matter in terms of 12 fundamental particles (which are listed in Figure 1).

The Fundamental Particles Table.

Figure 1: The fundamental particles of matter.

The matter particles come in three generations, with each subsequent generation being heavier than the last. Almost everything in our world is made of just the 1st generation particles. The electron, the particle responsible for electricity, orbits the nucleus of atoms. The nucleus itself is made up of smaller particles, protons and neutrons, which in turn are made up of the 1st generation quarks - the up quark and the down quark. The electron-neutrino is a neutral and almost massless particle that is spat out of nuclei during beta decay. Neutrinos are also produced in enormous quantities by nuclear reactions inside the Sun - in fact billions of neutrinos are whizzing through you every second but becaus they're not electrically charged you don't notice them.

The 2nd and 3rd generations contain heavier versions of the 1st generation particles but they don't exist for very long in our world because they quickly convert into 1st generation particles via the Weak Nuclear Force.

LHC - Unused Accelerator Section

The Standard Model also describes the fundamental forces in terms of 12 other particles called "gauge bosons". When two particles interact through a fundamental force they exchange these gauge bosons which carry the interaction between the two particles. A nice way to picture it is to think of two ice-skaters throwing a ball back and forth to each other. Every time one skater throws the ball he gets a little push in the opposite direction (thanks to Newton's third law) and every time his partner catches it she gets a little push too. So by repeatedly exchanging the ball the two skaters will drift further and further apart - the ball is acting like a force carrier.

Figure 2 (right) An unused section of LHC accelerator on show outside Restaurant 1 at CERN.

The familiar photon, the particle of light, is the force carrier of the Electromagnetic interaction. The gauge bosons of the Strong Nuclear force are the eight gluons, so called because they 'glue' quarks together inside protons and neutrons. The Weak Nuclear Force is transmitted by the W+, W- and Z bosons which are different from the photon and gluons since they are very massive - about 80 times the mass of a proton.

All of these particles have been produced and detected at previous particle experiments confirming the Standard Model's success at describing the fundamental nature of matter. However, there is one piece in the puzzle still missing - the famous "Higgs Boson", the particle responsible for giving mass to all the others. Finding the Higgs is so important to physicists because the Standard Model cannot survive without it - if we don't find the Higgs there is something seriously wrong with our theory. The Higgs has even been nicknamed the "God Particle" by some because of its crucial role.

Higgs event at the CMS detector

Figure 3 (left) A simulated Higgs event at the CMS detector

So how do we make a Higgs? Well, particle colliders work because of Einstein's famous relation:

E = mc2

The LHC will accelerate protons to 0.999999989 times the speed of light - a point where their kinetic energy exceeds their mass energy by a factor of 7000. When they collide their huge kinetic energy can be converted, through E = mc2, into making new massive particles. However, we know from previous experiments that the Higgs must have a mass at least 100 times larger than the proton's and so we need very high energy collisions to make one. So the very roundabout answer to our original question, "why so hard?", is that to make new fundamental particles we need collisions with a huge amount of kinetic energy.

What else can the LHC do?

You might have got the impression, if you read about these things in the press, that the LHC's only job is to find the Higgs. Thankfully this is very far from the truth - especially since the LHC's American rival, Tevatron, is getting worryingly close to doing it! There is a huge number of other questions that the LHC might give us some answers to:

What is the nature of Dark Matter?

How do we unify Gravity with the other forces?

Why is the Universe made of matter and not antimatter?

Are there extra dimensions of space?

What is beyond the Standard Model?

The last question is one of the most interesting. When we do some calculations with the Standard Model at high energy we get infinite answers and probabilities greater than one. This is bad! It tells us that there is something missing from our theory. Some new particles or forces must appear at higher energy to stop our theory going haywire. There are several "Beyond the Standard Model" theories but the most popular is called Supersymmetry (SUSY for short). SUSY gives every Standard Model particle a super-partner with greater mass, effectively doubling the number of fundamental particles. Experimentalists love SUSY because it gives them a host of new particles to discover, keeping them employed for decades to come. SUSY is also very attractive because the lightest super-partner is predicted to be stable and this provides a very neat candidate for the invisible Dark Matter.

Now for the difficult part...

LHC - CMS DETECTOR end-capMaking new particles at the LHC is relatively straightforward - accelerate beams of protons narrower than a human hair to seven trillion electron volts in opposite directions around a 27 km circumference superconducting particle accelerator chilled to a temperature colder than the vacuum of space and bringing them to collide at four interaction points without at any point allowing the beam to wander astray and vaporise part of your accelerator or multi-million euro detectors...

Figure 4 (right) The endcap of the CMS detector before it was lowered into the detector cavern in 2007.

The difficult part is actually detecting the new particles once you've made them. Trying to discover how the Universe works by smashing protons together is a bit like trying to understand how a pocket-watch works by firing two into each other and taking photographs of the springs, cogs and bits of glass that go flying out from the collision. But unfortunately we can't take a screwdriver and dismantle the proton so this is all that we're left with.

The really tough thing about detecting particles at the LHC is that colliding protons is messy. Protons are haphazard bags of quarks and gluons and so when two protons collide what actually happens is that two bits in each proton interact. If these bits only glance off each other then the two protons remain intact and all is well - although we also won't see any new interesting particles. But if the bits collide head-on then the protons get ripped apart and their internal workings get spewed all over the detector. Quarks also have the unpleasant property in that if you try to pull two apart the gluon bonds between them get stretched until they snap and as they do so new quarks are created. This leads to "jets" of particles being produced as two quarks are knocked apart.

Somewhere, within this noisy splurge will be our new fundamental particle that we are so interested in. Figure 3 (above) shows a computer simulation of the production of a Higgs at the Compact Muon Solenoid detector at the LHC. You can see that finding it in amongst all the other gubbins isn't going to be easy...

Seeing things clearly

So finding new particles at the LHC is like trying to find a needle in a haystack - except the haystack is exploding.

LHC ATLAS DetectorTo solve this problem physicists at CERN have built enormous detectors of which there are four; ATLAS, CMS, LHCb and ALICE. These detectors are all different but ATLAS, CMS and LHCb are built along common lines. In the centre of the detectors, closest to the collision point, are devices designed to record the tracks of particles as they shoot through the detector. Further out are calorimeters, designed to measure the energy of particles. At the outside of the detectors are muon chambers which record the flight path of muons as they leave the experiment. The LHCb experiment has additional clever devices called the RICH (Ring-Imaging Cherenkov) detectors which can be used to tell different kinds of particles apart.

Figure 5 The enormous ATLAS detector - the largest of the four detectors at the LHC. Look at the man on the gangway in the background for scale!

Unfortunately we can't record all this information for every particle - we produce hundreds in each collision and there are around 100 million collisions per second at each interaction point. At this rate we would fill all the hard-disks in the world in no time! To get around this problem the detectors employ "trigger" systems. These triggers do very fast calculations for each collision and decide whether or not the collision looks interesting. If the collision is deemed interesting by the trigger it fires, telling the readout electronics of the experiment to record the information for that collision. The use of this trigger dramatically reduces the amount of data that needs to be recorded and makes the volumes of hard-disk space required manageable.

Even so, the amount of data the LHC will produce is vast - about 15 petabytes per year or 150,000 typical laptop hard disks' worth. To deal with this huge flow of data CERN has developed the worldwide computing GRID - a system linking thousands of computers all over the world so LHC data can be stored and analysed anywhere.

Making discoveries

So finally, at the point when LHC data is being written to disk, physicists can begin their analyses. Discoveries won't come quickly; we have to understand how the accelerator and detectors are performing before we can draw conclusions. It may take years of data collection before the Higgs is discovered and perhaps even longer before we learn what lies beyond the current frontiers of knowledge. But after twenty years of work involving thousands of physicists, engineers and computer scientists the LHC will give us answers to some of the biggest questions in science. Now she's switched on again you had better be watching...


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