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This number feeds the muon magnet prediction.","posed_since":"2023","precise":"Identify the cause of the disagreement among $\\sigma(e+e- \\to \\pi+ \\pi-)$ measurements near the $\\rho$ peak (CMD-3 2023 versus KLOE, BaBar, SND) and between data-driven and lattice a_mu^HVP,LO (difference $\\sim 2\\ \\text{to}\\ 3\\ \\text{percent}$ of HVP). An answer locates the systematic error, the radiative-correction issue, or a physical effect responsible.","problem_ref":null,"references":"","settled_by":"New $\\pi\\,\\pi$ measurements (BaBar reanalysis, Belle II, BESIII, KLOE reanalysis, CMD-3 crosscheck) together with lattice windows and $\\tau$-decay data with isospin-breaking corrections.","status_note":"Unresolved as of 2025; the 2025 White Paper did not use data-driven HVP for its central value.","title":"Why do e+e- to pi+ pi- cross-section measurements 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