Flavor puzzle: masses, mixing and three generations

3 problems

• Does muon-to-electron conversion occur at rates near 1e-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.