Science Interviews


Sun, 7th Sep 2008

What is the LHC?

Ben Allanach from the University of Cambridge

Listen Now    Download as mp3 from the show The Large Hadron Collider

Chris - What is the LHC all about?

Ben - the LHC is a gigantic experiment. Itís the biggest experiment thatís ever been staged. Weíre 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? Whatís different then thatís 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. Weíd like to produce them and study them.

Chris - Specifically, what are these particles?

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

Chris - Well, letís just drill down into what we mean by fundamental particles and whatís 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 letís go down into the centre of this positive core. Thatís 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. Theyíre called quarks. Theyíre kind of held together by some strong sticky quantum force thatís holding them together.

Chris - We donít actually know what that force is, presumably?

Ben - We know quite a lot about it. Itís 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 theyíre not things that make up the quarks themselves?

Ben - We donít. All we know is that they look simple down to 10-15m. Thatís a billionth, less than a billionth of a millimetre. You canít see any smaller dots at that scale in those but you canít say for sure. If you have an even bigger microscope than the LHC you will see substructure. Thatís one of the things we want to check.

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

Ben - Thatís the idea. Itís 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 doesnít bear fruit does this mean weíre 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 - Itís 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 LHCís going to have enough energy now to produce them. If the Higgs theoryís wrong then thereíll be something else there and that would be more exciting, actually. Weíll 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 thatís 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 itís 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? Itís 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 youíre going to create the Higgs particle in the LHC, if itís 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 - Youíre not actually making the particle, youíre just making it showing itself by disturbing the field that it normally creates?

Ben - Thatís 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. Thereís 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, whatíll happen is if you produce a Higgs boson theyíll 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 thereís 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 donít see the Higgs boson when the LHC gets up to full working capacity?

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

Chris - And expensive, potentially! What would be another explanation? Is there another counterpoint? Thereís 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 isnít really a fundamental particle, it still looks like it. Whatever theory you cook up itís got to behave in some way like the Higgs because there are indirect signals from the previous data.

Chris - Is that basically what youíll be working on with the people at the LHC or have you got your own suite of things that youíre 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 itís so light. You donít expect it to be a billion, billion times heavier just from constant fluctuations unless something happens in the theory to keep it light. Supersymmetryís 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 thatís out there in the universe. Astrophysical observations tell us thereís some weird stuff out there that we canít see and itís 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 thatís based on science that hasnít even been proven yet but I guess thatís cosmology and particle physics all-through, isnít it?

Ben - Thatís right and thatís why we need to do the experiments to check it. This particularly is very speculative. Iíd give it about a 50/50 chance.

Chris - About as promising as my next grant application!


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