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Title: What determines the hierarchy of quark and lepton masses?
Plain: 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.
Precise: The SM Yukawa couplings span $y_e \sim 3e-6$ to $y_t \sim 1$, CKM angles are small ($\theta_C \sim 0.23$, |V_ub| ~ 4e-3) while PMNS angles are large ($\theta_{12} \sim 34\,\mathrm{deg}$, $\theta_{23} \sim 45\,\mathrm{deg}$). An answer is a mechanism (Froggatt-Nielsen charges, modular or discrete flavor symmetries, extra-dimensional localization, partial compositeness) that predicts these values with fewer parameters than it explains.
Settled by: A model with fewer free parameters than observables whose extra predictions (mixing sum rules, flavor-violating rates, new states) are confirmed.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)