Hooray, it’s time for another blog post! What I’m writing about this time is kind of old news — and don’t worry, there’s more to come on just why I haven’t been able to write about it for so long — but very interesting nonetheless.

A few weeks ago, two separate physics experiments announced that they had discovered a tetraquark, a composite particle made of four quarks. Or rather, that’s what all the popular news coverage said. But what really happened? The discovery of a real tetraquark would be huge news, so I’m sure not going to trust the media reports on this one. As always, I’m going straight to the source: the original papers by the BES III and Belle collaborations.

Two or Three is Company, Four’s a Crowd

Before delving into the discovery itself, I’m going to tackle the burning question on everybody’s mind: what’s so special about a particle made of four quarks?

To understand that, we have to look to quantum chromodynamics (QCD), the theory of how “color-charged” particles interact. In some ways, QCD is superficially similar to quantum electrodynamics, the theory of how electrically charged …

OK, actually it is kind of a big deal. Discovering a new particle is not something that happens every day, and it’s a concrete result of having a well-tuned detector. Besides, it’s just cool. So congratulations to the CMS collaboration!

In case you haven’t heard the story, late last week CMS announced that they had a statistically significant observation of the \({\Xi^*}_b^0\) baryon, a particle made up of an up quark, a strange quark, and a bottom quark. In this case, “statistically significant” means that they detected this particular decay signature 21 times, of which only \(3\pm 1.4\) of them can be attributed to random coincidences in the detector. So they’re about as sure as you can be in physics that they are seeing signs of a real particle. They’ve also managed to reconstruct various properties of this particle by examining the decay products, and everything matches up with the predicted properties of the \({\Xi^*}_b^0\).

Now, why isn’t this a bigger deal, and why didn’t I write about it right away? Well, as I just mentioned, this particle was predicted to exist. Of course, the Higgs boson …