Dr Matthew Mccullough, CERN & Will Kalderon, Oxford University
The Large Hadron Collider - or LHC - in Geneva is the worldís most powerful particle accelerator. Itís now in the process of being switched back on after a two-year upgrade that will allow it to smash together the positive particles in atoms, called protons, at higher energies than ever before. Scientists are hoping that, when they do this, theyíll be able to make dark matter, giving them a chance to study it directly for the first time. Graihagh Jackson went to meet LHC physicist Matthew McCullough to hear how...
Matthew - Underneath our feet is a very large tunnel that contains one of the biggest experiments thatís ever been constructed. Within this tunnel is contained the Large Hadron Collider otherwise known as the LHC. In the Large Hadron Collider, protons are smashed together at extremely high energies, very close to the speed of light. We do that here at CERN in order to try and better understand the universe.
Graihagh - So, protons are actually whizzing underneath our feet right now as we speak.
Matthew - Absolutely and whenever the protons hit head-on, these extraordinary events Ė we call them Ė where large numbers of particles are produced and they are detected and measured by these extremely large detectors. One of them is CMS and the other one is the atlas detector.
Graihagh - So, why are we doing this? Why are we creating these sort of events as you call them?
Matthew - Well, the goal is to try and understand other aspects of fundamental physics. So, we have this theory called the standard model. This theory predicts all sorts of things that we observe in nature. One of those things is the Higgs Boson but there are the puzzles beyond the standard model that we donít understand. One of those is dark matter. So, one hope for the LHC is that by smashing together these protons, we might be able to produce dark matter particles and then try and understand something about the nature of dark matter.
Graihagh - How does smashing together two protons equal dark matter? Proton plus proton equals dark matter. It doesnít add up in my head but perhaps it does in yours.
Matthew - One good way of understanding this is to use this famous equation by Einstein, E=mc2. So, (E) is the energy in for example, a particle collision. (m) is the mass of the particles that you can create and (c) is the speed of light. By smashing together protons at the LHC at very, very high energies, you might produce pairs of dark matter particles. For example, one way that dark matter could be produced at the LHC is that even the Higgs Boson could decay into dark matter. So, you could have one of these collisions produce a Higgs Boson and then that Higgs Boson actually decays into dark matter. So, you donít see, you never see the Higgs Boson and you never see the decay happening, and this dark matter flies out of the detector.
Graihagh - So a proton hits a proton, they collide and Higgs Boson was created and that will create dark matter. That dark matter disappears undetected. Is that correct?
Matthew - Absolutely.
Graihagh - You're probably wondering how, if a newly created dark matter particle flies out of the detector undetected, how on Earth we can a.) know itís there and b.) learn anything about it? I put this to Will (Calderon), physicist at Oxford University whoís been working on just this conundrum.
Will - So, right now, weíre standing outside the atlas control room. Actually, with very good timing because right now, itís the first 13 TEV collisions ever. These first collisions are at the highest energies that humans have ever collided particles and will be used to calibrate the machine and the detectors to get it in the best shape ready for Ė well, when we start the real physics in the summer.
Graihagh - The real physics. I mean, looking at what's in front of us now, it does look like real physics is happening. There are perhaps 30 people, all avidly tapping away on their computers whilst there are some huge projections of all sorts of particles scattering and various simulations going on. No doubt, in a couple of weeks, when all the tests and configurations are done, you will be in there.
Will - Yeah, really soon. Actually, itís happening in less than a month now until we get the actual collisions that weíll, from which maybe weíll get hints of dark matter and new physics and who knows what.
Graihagh - Does that mean you'll be able to potentially hold the dark matter particle in your hand?
Will - I wish, but sadly, by their very nature, you can't hold them. They donít interact with us at all.
Graihagh - If you can't see it, you can't hold it, how on Earth do you go about detecting it?
Will - We look for this indirectly. So, we know that we have two protons flying towards each other. What we do then is, of this spray of everything flying out, we add up the momenta in all the different directions. So, if we imagine each particle as being a little arrow and the length of the arrow is its momentum, we stick all those arrows end to end, and they make a big wiggly line that comes back on itself. If that line doesnít come back on itself, that means momentum has not been conserved. One of the explanations that that could be is that there is a particle that has escaped our detection and that particle could be dark matter.
Graihagh - So essentially, you're looking for missing energy and that missing energy will have been stolen away by a dark matter particle.
Will - Exactly.
Graihagh - So, now that youíve seen this missing energy, and you think you might have witnessed the dark matter particle being created, what can you actually infer about it? What can you tell about its properties?
Will - Itís really a statistics game. We need to see lots of them before we can say anything conclusive. But you can tell things like the mass of this particle and what kind of things it interacts with.
Graihagh - I was going to say, so you can tell quite a lot then just from one Ė I say, one simple collision Ė a few collisions and I suppose they're not even all that simple either. You can start to build up a picture of what it is.
Will - Yeah, exactly. So, if we see something hopefully, it will progress as much as the Higgs discovery did.
Graihagh - It took 50 years to find the Higgs. When might we finally see a dark matter particle?
Will - Well, we have all the experience from the Higgs hunt. So, it could be that with this jump in energy, we can suddenly produce these dark matter particles in a sense itís up to the universe. We donít know what itís made of. If itís made of the right kind of things, maybe by the end of the summer, if itís made of the wrong kind of things, who knows.
Graihagh - Oxfordís Will Kalderon and before him CERNís Matthew McCulloch.