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Flavor puzzle: masses, mixing and three generations

Matter particles come in three copies (generations) with identical charges but masses spread over more than ten powers of ten. Nobody knows why there are three, or why the masses and mixings take their values.

Why it matters

Most of the SM's free parameters are flavor parameters, and their pattern is unexplained.

Review

F. Feruglio, Pieces of the flavour puzzle, European Physical Journal C, 2015. https://doi.org/10.1140/epjc/s10052-015-3576-5 reference checked

Timestamp record

Signed record: bf83146b3162…
Public log: Entry 242, in checkpoint 2,123
In the log by: Block 969,698, 2026-10-03

This text was in the PubPhys log by that date. The date says nothing about when the problem was first posed.

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3 problems

Does muon-to-electron conversion occur at rates near $10^{-16}$?

Open

The SM forbids a muon from turning into an electron without neutrinos, except at hopelessly tiny rates. Seeing it would prove new physics acting on flavor.

Why are there exactly three fermion generations?

Open

Experiments show three light neutrino types and three sets of quarks and leptons, and a fourth ordinary copy is ruled out. Why three, and not one or four, is unknown.

What determines the hierarchy of quark and lepton masses?

Open

The top quark is about 340000 times heavier than the electron, and quark mixing is small while neutrino mixing is large. No theory predicts these numbers.