I'm trying to understand how LFU arises mathematically within the framework of the Standard Model. Is it a direct consequence of the theory, or is it simply an assumption or empirical observation without a detailed theoretical explanation?
Thank you in advance to anyone who can help clarify this!
LFU is a direct implication and consequence of the Standard Model because the Standard Model believes that the three leptons differ in mass but not in their Weak Force interactions. Basically implying the three Leptons are all the same besides their mass.
For example, electricity is the flow of charged particles, including electrons. So sometimes electricity is "made of" electrons so to speak in some cases. Muons are supposed to be the same thing except heavier, but Muons aren't nearly as common in nature so they are harder to do tests on.
The Standard Model typically does very well when tested against real-world experiments. There are some examples where evidence of B-Meson decay has contradicted LFU, implying that some leptons interact differently than others with regards to the Weak Force. But it's possible these were just bad readings within the margin of error I think. In summary, LFU is a consequence of the Standard Model because the SM believes that electrons muons and taus are all the same thing but with different masses, and therefore their quantum numbers are also the same except mass, and they shouldn't differ in their quantum interactions, except mass related ones.
