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Title: Why do e+e- to pi+ pi- cross-section measurements disagree?
Plain: Measurements of how often electron-positron collisions make two pions disagree between experiments, and with supercomputer calculations. This number feeds the muon magnet prediction.
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.
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.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: Unresolved as of 2025; the 2025 White Paper did not use data-driven HVP for its central value.
Posed since: 2023