If you weren’t too busy waiting for photos of Pluto, you might have heard some news coming out of CERN, Geneva, this week. LHCb, an experiment based at the Large Hadron Collider particle accelerator there, announced that it had found evidence for a completely new type of particle – a pentaquark. That might sound slightly diabolical, but, in fact, finally seeing a pentaquark neatly closes a particle physics puzzle almost 50 years old.
Particle physicists are in the business of working out what anything – everything – is made up of. You might already know that everything you see around you is made, if you look deeply enough, of collections of different types of atoms. Inside atoms is smaller stuff still – peer inside at large enough magnification and you’ll see that the nucleus of an atom is made of smaller particles, protons, and neutrons, and these in turn are actually three quarks stuck together. So far we don’t think that quarks are made of anything else; this is where our Russian doll-like idea of matter stops.
This might sound neat, but there’s still a lot we don’t understand about the way quarks combine to form other particles. They seem to come in combos of twos or threes. When quarks were first identified in the 1960s, it seemed possible that they could link up in much greater numbers. We first saw evidence of very rare particles containing four quarks a few years ago. Many experiments thought they saw evidence of the elusive five quark particle – the pentaquark – too, but these claims have always faded on closer examination.
Until now.
If this LHCb measurement holds up, then we can expect to find a lot more pentaquarks. And if we do, this is going to strengthen our understanding of how tiny quarks link up, and – ultimately – why all the stuff in the universe is constructed the way it is. For a discipline that wants to understand the structure and composition of the universe, this measurement is a big deal. It might not be the answer to some of our biggest mysteries, but it cements our understanding of that bit of the universe we do know about.
Tara Shears is a particle physicist and Professor of Physics at the University of Liverpool. She has spent her career investigating the behaviour of fundamental forces and particles, and currently researches (with 699 other people!) using the LHCb experiment at CERN’s Large Hadron Collider. She will be a guest on a future episode of Beyond the (Micro)scope. Follow her on twitter @TaraShears!