Earlier this week, the LHCb experiment at the Large Hadron Collider announced that, after painstakingly analyzing the data from earlier runs, they have decisive evidence of a previously unobserved particle: the pentaquark.
What’s a pentaquark? In simple terms, it’s five quarks stuck together. Stick two up quarks and a down quark together, and you get a proton. Stick two quarks together, you get a meson of some sort. Five, you get a pentaquark.
(In this case, if you’re curious: two up quarks, one down quark, one charm quark and one anti-charm quark.)
Crucially, this means pentaquarks are not fundamental particles. Fundamental particles aren’t like species, but composite particles like pentaquarks are: they’re examples of a dizzying variety of combinations of an already-known set of basic building blocks.
So why is this discovery exciting? If we already knew that quarks existed, and we already knew the forces between them, shouldn’t we already know all about pentaquarks?
Well, not really. People definitely expected pentaquarks to exist, they were predicted fifty years ago. But their exact properties, or how likely they were to show up? Largely unknown.
Quantum field theory is hard, and this is especially true of QCD, the theory of quarks and gluons. We know the basic rules, but calculating their large-scale consequences, which composite particles we’re going to detect and which we won’t, is still largely out of our reach. We have to supplement first-principles calculations with experimental data, to take bits and pieces and approximations until we get something reasonably sensible.
This is an important point in general, not just for pentaquarks. Often, people get very excited about the idea of a “theory of everything”. At best, such a theory would tell us the fundamental rules that govern the universe. The thing is, we already know many of these rules, even if we don’t yet know all of them. What we can’t do, in general, is predict their full consequences. Most of physics, most of science in general, is about investigating these consequences, coming up with models for things we can’t dream of calculating from first principles, and it really does start as early as “what composite particles can you make out of quarks?”
Pentaquarks have been a long time coming, long enough that someone occasionally proposed a model that explained that they didn’t exist. There are still other exotic states of quarks and gluons out there, like glueballs, that have been predicted but not yet observed. It’s going to take time, effort, and data before we fully understand composite particles, even though we know the rules of QCD.