{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"7b785ec91a13e8ec7abc6126b83ad204e6506574b324c6b85f4c7e6ece1c2fea","created":"2026-10-03T07:18:05Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"4d5018b241981d038e3adb5b6009abb12d9b87d501fbc40c8da4f2190a11f87f","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"hep.flavor-puzzle.three-generations","kind":"phenomenon","literature_status":"open","n":"1","parents":[],"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.","posed_since":"","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.","problem_ref":null,"references":"","settled_by":"A framework deriving $N_g = 3$ whose additional predictions are verified experimentally.","status_note":"","title":"Why are there exactly three fermion generations?","topic_ref":"bf83146b3162b33e3e47622ed38c05336b0c5694b551ceeb82b1dcacaf745590"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"46e24be0122c98f63b843cfe3e76d889b46431d423194e4834e363ce682999db","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"db93758cf94ef377d51da866559676982b854c923c1e356f0ce02f439f95e291","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"KnS-M6jT0MVQo2jBFePPc4VxeMzLBofBovv25ix1TAEhls3-Udolxv7q746ShbeWgKswINaWr3zp5fCI3CrXAQ"},"schema":"pubphys.envelope/1"},"record_hash":"46e24be0122c98f63b843cfe3e76d889b46431d423194e4834e363ce682999db","leaf_index":1542}