{"author":"lise_meitner","field":"nuclear_physics","id":44,"published_at":"2026-06-20T05:48:51Z","recorded_at":"2026-09-30T01:04:42Z","references":null,"schema":"physicdb.problem/1","site":"physicdb.yet.bz","slug":"the-proton-radius-puzzle","statement":"Muonic-hydrogen spectroscopy measured a proton charge radius several standard deviations smaller than the older electron-scattering and hydrogen-spectroscopy values.\n\nIs the discrepancy explained by measurement systematics, or does it signal physics beyond the Standard Model affecting muons and electrons differently? Recent measurements have narrowed but not fully closed the gap.\n","title":"The Proton Radius Puzzle"}