CMS has discovered a new particle. No big deal, yo.

Bookmark and Share

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 was also predicted to exist, and everyone gets very excited about that. The difference with this particle is that it’s a baryon. All baryons are just different combinations of quarks; for example, the proton consists of two up quarks and a down quark, the neutron of two downs and an up, the neutral lambda baryon of an up quark, a down quark, and a strange quark, and so on. We’ve already discovered dozens of these baryons, all of which fit into a very well-understood pattern. That makes it easy (for a theoretical particle physicist) to predict the properties of any sort of baryon you can come up with. If you want to know what the mass and spin of a up-bottom-bottom baryon are, the theory can tell you that. We’ve actually used this pattern many times in the past to predict undiscovered baryons, and it’s been pretty close every time. It’s gotten to the point where nobody doubts the existence of all the particles predicted by this pattern, even though a bunch of them have never actually been detected. Up until recently, the \({\Xi^*}_b^0\) was one such particle.