In an essay listing Good Problems in the Foundations of Physics, Sabine Hossenfelder describes the theory-led explanation about the prediction of the charm quark.
The charm quark, so they will tell you, was a prediction based on naturalness, which is an argument from beauty. However, we also know that the theories which particle physicists used at the time were not renormalizable and therefore would break down at some energy. Once electro-weak unification removes this problem, the requirement of gauge-anomaly cancellation will tell you that a fourth quark is necessary. But this isn’t a prediction based on beauty. It’s a prediction based on consistency.
The “beauty”-based or more often called symmetry-based argument always bugged me. I mean, really? One would expect a fourth quark just because they knew of four leptons back then and having four of each would be nice? Anyway, it is another interesting story in the name of charm quark, after the November Revolution.
To my surprise, Sabine mentiones the Matter-Antimatter Asymmetry as Baryon Asymmetry. I am looking forward to study some cosmology to find out why Leptons don’t play much role here.
PS: I have recently added kottke, one of the oldest blogs on the web to my RSS reader and this format of a blog post is inspired from it. Let’s see if it sticks.