What is the LHC?

The LHC may be the biggest particle accelerator in the world but how does it work? We spoke to Ben Allanach from the University of Cambridge to find out...
07 September 2008

Interview with 

Ben Allanach from the University of Cambridge


Chris - What is the LHC all about?

Ben - the LHC is a gigantic experiment. It's the biggest experiment that's ever been staged. We're going to collide protons together and try and find out about the early universe and particles.

Chris - When you say find out about the early universe, why do we need to find out about that? What's different then that's not around today?

Ben - There were particles around we think that have since decayed very quickly. They were produced in the early universe and have decayed away now. We'd like to produce them and study them.

Chris - Specifically, what are these particles?

Ben - There are actually various things we'd like to see. One example is the famous Higgs boson. That's the one that's responsible for fundamental particles getting mass. The theories will tell you that the particles have to be massless and, of course, we know that's not the case. The Higgs boson is the missing piece of the puzzle that would explain why the other particles have mass.

Chris - Well, let's just drill down into what we mean by fundamental particles and what's inside atoms and things. Working outwards-inwards. We had an atom and this has got a nucleus, protons and neutrons and electrons round the outside. Take me from there, further inside.

Quarks in a proton
Quarks in a proton © Arpad Horvath.

Ben - We have this atom so let's go down into the centre of this positive core. That's made of protons and neutrons. They have structure within them as well. If you zoom down you can see three little point-like type things in the protons and neutrons. They're called quarks. They're kind of held together by some strong sticky quantum force that's holding them together.

Chris - We don't actually know what that force is, presumably?

Ben - We know quite a lot about it. It's a strong nuclear force and, in fact, by blowing them apart you can tell quite a lot about what that thing is.

Chris - How do we know those quarks are the simplest, smallest things? How do we know they're not things that make up the quarks themselves?

Ben - We don't. All we know is that they look simple down to 10-15m. That's a billionth, less than a billionth of a millimetre. You can't see any smaller dots at that scale in those but you can't say for sure. If you have an even bigger microscope than the LHC you will see substructure. That's one of the things we want to check.

Chris - Although the LHC is effectively an atom smasher and it's creating an enormous amount of energy, if anything it's behaving as a microscope. You're blasting particles to pieces and this makes the components that make up these particles come out so you can see them?

Ben - That's the idea. It's a weird paradox that to see smaller things you have to build bigger and bigger machines to get to higher energies. Then you can prove things deeply.

Chris - If the LHC doesn't bear fruit does this mean we're going to have to build a bigger one or do you think this is going to basically answer the question, once and for all, what is the fundamental nature of matter?

Simulated Higgs particle event
A simulated event at of the LHC of the european particle physics institute, the CERN. This simulation depicting the decay of a Higgs particle following a collision of two protons in the CMS experiment. © CERN

Ben - It's going to answer the question about the Higgs boson in my opinion. We already know from indirect signals and previous data roughly what mass this Higgs boson has and you can calculate that the LHC's going to have enough energy now to produce them. If the Higgs theory's wrong then there'll be something else there and that would be more exciting, actually. We'll be able to investigate that. There are other possibilities like producing dark particles of dark matter which is a bit more speculative. That would be extremely interesting too.

Chris - When you mention the work of Peter Higgs who was a scientist at Edinburgh University who came up with this notion of particle that everyone wants to see but no one has ever detected, how does that fit into the big picture? What is it? What does it do?

Ben - Particles which we imagine as little dots travelling around are actually ripples on a field that's throughout all the universe. An electron, for instance we might see as a particle. If you look at it really closely it looks like a kind of ripple in the electron sea. We have the same thing for the Higgs boson. The idea is this jelly throughout the Universe. The Universe is still hot it's runny and other particles can zip through it without noticing it. As the Universe gets bigger the jelly kinda condenses. This is the special thing about the Higgs and other particles can feel it enough to be pushed through it. Newton told us that when you have to push something along it has inertia and therefore mass. These Higgs particles and fields drag other particles and give them mass.

Chris - This would be almost like a parachute on the back of a big vehicle or something? It's almost like a drag force?

Ben - Yeah.

Chris - Is it everywhere?

Ben - Yes. All of these field exist throughout all of the universe.

Chris - So when you say you're going to create the Higgs particle in the LHC, if it's there already what are you doing?

Ben - the field, the sea is there but what you want to do is create a little ripple of it which is the particle itself. A localised wave, if you like, that is the actual particle.

Chris - You're not actually making the particle, you're just making it showing itself by disturbing the field that it normally creates?

Ben - That's absolutely right.

Chris - If it does pop up what are you actually going to see? How do you see those ripples?

Ben - This Higgs particle, if you produce it, it decays very quickly within 10-20 seconds: incredibly fast. It decays into other ordinary particles which you see around the collision point. There's all sorts of electronics built around that to track these things coming out. What you have to do is look at their energies and infer back to what happened at the interaction point. Basically, what'll happen is if you produce a Higgs boson they'll come out with half of its mass. Roughly speaking, each particle will have half its mass. You add the energies up of these two things and of course there's all sorts of things happening. Over 1000 billion events , 1000 billion collisions you should see a lot of them coming out with the same kind of energy. You have to extrapolate back to the Higgs.

Chris - What would it mean to the field of particle physics if you don't see the Higgs boson when the LHC gets up to full working capacity?

Ben - It'll mean that a lot of text books have to be re-written. It'll be extremely exciting.

Chris - And expensive, potentially! What would be another explanation? Is there another counterpoint? There's the Higgs theory, is there any other way of thinking about it?

Ben - There are some other contenders but nothing anywhere more successful. I personally believe in something like the Higgs theory. Another example is that there are two quarks that have been very tightly bound together. They can act like a Higgs even though it isn't really a fundamental particle, it still looks like it. Whatever theory you cook up it's got to behave in some way like the Higgs because there are indirect signals from the previous data.

Chris - Is that basically what you'll be working on with the people at the LHC or have you got your own suite of things that you're also interested in?

Ben - My pet theory is actually supersymmetry so this is a theory which goes one step beyond the Higgs and explains why it's so light. You don't expect it to be a billion, billion times heavier just from constant fluctuations unless something happens in the theory to keep it light. Supersymmetry's an example of something that works very well with that. It predicts lots of new particles. It can predict one of the particles as dark matter that's out there in the universe. Astrophysical observations tell us there's some weird stuff out there that we can't see and it's transparent but it has gravitational force. We might be able to produce some of those, hopefully.

Chris - Sounds a bit dodgy to be working on science that's based on science that hasn't even been proven yet but I guess that's cosmology and particle physics all-through, isn't it?

Ben - That's right and that's why we need to do the experiments to check it. This particularly is very speculative. I'd give it about a 50/50 chance.

Chris - About as promising as my next grant application!


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