Revision 1

Accepted · first revision · record

Title: Why are there exactly three fermion generations?
Plain: 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.
Precise: LEP Z-width data give $N_\nu = 2.9963 \pm 0.0074$ light neutrino species after the 2020 luminosity correction (Janot and Jadach), and Higgs production rates exclude a fourth sequential chiral generation. An answer is a principle (anomaly cancellation in an extended gauge group, string compactification topology, or a symmetry argument) that requires $N_{\mathrm{g}} = 3$ and makes a further testable prediction.
Settled by: A framework deriving $N_g = 3$ whose additional predictions are verified experimentally.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)