Tetraquarks: new configurations of subatomic particles discovered

Researchers using the Large Hadron Collider at CERN say they’ve recently discovered a new exotic form of subatomic particles called tetraquarks. They exist for just fractions of a second, so they’re hard to spot and study. But, luckily, University of Cambridge physicist Harry Cliff, is working on the experiment that discovered them, as he described to Chris Smith...

Harry - Very simply a tetraquark is a particle made of two quarks and two antiquarks. So, sort of four objects inside it basically, hence the tetra. It probably requires a bit of unpacking though, because well, you might ask what's a quark? Well, every atom has a nucleus, and you probably know that the nucleus is made of protons and neutrons. And we've known for more than half a century now, that protons and neutrons are made of three quarks each, bound together. Until recently we only knew of combinations of quarks that came in threes, or in pairs. So tetraquarks are a whole new class of particle that we didn't know about until relatively recently.

Chris - It's funny, isn't it? Because the reason we call atoms atoms, is the Greek word tomos means cut. And atomos means can't be cut down further. So initially scientists thought atoms were the smallest possible things. And then people realised that atoms have got things inside them, protons and neutrons, and thought they were the smallest things. And then people like you started smashing things together at very high energies and realised that those tiny things inside atoms, have actually got tiny things inside those. Those are the quarks.

Harry - Yeah, that's exactly right. And there was this big mystery in the 1940s and 1950s, when in particle colliders and in astrophysics experiments, people were discovering all these new subatomic particles, with all strange names. They're all usually, with Greek letters, like omegas and deltas and lambdas, and it was called the Particle Zoo. No one really understood what these particles were, and it was eventually realised you could explain this huge number of different particles, as being combinations of basically six different types of even smaller particles called quarks. So that's our most up-to-date understanding of the basic ingredients of matter today.

Chris - Why don't these things hang around for very long though?

Harry - Basically it's to do with the forces that bind them together. So actually there's only one particle made of quarks, as far as we know, that's completely stable and that's the proton. So that's the thing that's inside every atom. And that is a good thing that the proton's stable because if it wasn't, well, you know, every atom would disintegrate. But anything apart from the proton basically is unstable. It can always collapse, and it can either sort of break apart, or annihilate with itself and turn into other particles. So these things, because they're bound together, but not in a way that allows them to exist for any length of time. And they very quickly disintegrate into other sets of particles, which we then detect in our experiments.

Chris - Are they therefore a real and important part of physics? Or do you think they're just an artifact of the fact that you have, in an unnatural way, slammed particles together? You can detect these things transiently, but because they disappear, they play no part in genuine physics, or do you think they really do have a role to play in the way that the universe works?

Harry - Well, they tell us something about one of the four forces of nature. So we know about four forces, gravity, the electromagnetic force, something called the weak force, and the strong force. And the strong force is the force that holds the nucleus together, holds quarks together. But the strong force is really mysterious. Despite being, you know, a force we've known about for ages. It's really hard to understand, and it's very hard to make predictions using our theory of the strong force. So we can't really work out in advance, how quarks come together, and what combinations are allowed and which ones aren't allowed. So discovering new particles like these tetraquarks tells us something about the way this strong force interacts, and as one of the four basic forces in nature, that's a really important thing to get a deeper understanding of, and has impacts on the rest of our understanding of particle physics as well.