Lepton fizz

While one can safely open a can of soda, if you add energy to it by shaking it, it will fizz all over. If the familiar electron and muon turn out to be made of smaller objects, we can add energy to them, which might be emitted as photons.

Grab a can of soda, shake it thoroughly, and open it up. What do you get? It’s an explosion of fizz. Today’s column is about a similar thing in the subatomic world.

Scientists have spent centuries searching for the ultimate building blocks of matter. And they first found that molecules gave way to atoms. Then, in the early 1900s, scientists realized that atoms were actually composed of smaller particles called protons, neutrons and electrons. And about 50 years ago, scientists realized that the protons and neutrons were themselves made of quarks. The electron has resisted all attempts to find a smaller object that contains it and it is now classified as a lepton, a label that also includes the muon and tau lepton.

As of this article, it appears that the quarks and the leptons are the smallest objects in nature… objects that are point-like, which means they contain nothing inside them. With the right mix of these particles, we can construct the entire universe. This understanding is called the Standard Model and is rightfully considered a triumph of human ingenuity.

However, if history teaches us anything, it teaches us that we don’t know everything. It is entirely reasonable to ask if these familiar building blocks are made of even smaller things. If they are, it is possible to pump energy into a quark or lepton. That energy will then be emitted and detected. This is kind of like how you can safely open a can of soda, but if you pump energy into it by shaking, it will fizz all over. Analogously, the CMS experiment searched for excited electrons or muons, which emit a photon and then settle down into one of our familiar particles.

Events containing a lepton and photon are expected from understood physics. However, if we found an excess of events where the lepton and photon came from a parent particle with a specific mass, this would be powerful evidence that we have discovered an even smaller particle. It’d be another layer in the subatomic onion. Such a discovery would have profound impact on our understanding of the world. While this study found no evidence for excited leptons, it was done with about one percent of the data recorded so far. This paper surely isn’t the last word we’ll hear about excited leptons.

Don Lincoln

These physicists contributed to this analysis.
Sushil Singh Chauhan from UC Davis has been taking care of the online beam position determination. This is important feedback from CMS to the LHC and also important for triggers that look for tracks that don’t originate directly from the collision.