For 25 years, a discrepancy between theoretical predictions and experimental results for the muon's wobble in a magnetic field hinted at new particles. This "muon g-2" puzzle suggested an unexpected one-part-in-a-million deviation.

In 2021, a new theoretical calculation using lattice quantum chromodynamics (QCD) appeared to resolve the mystery. The BMW group, a collaboration of physicists from Budapest, Marseille, and Wuppertal, published results suggesting the muon's wobble was precisely as predicted by known physics, aligning with experimental data from Fermilab. This calculation, refined over a decade using advanced computational techniques, brought theoretical predictions to a precision of one part in 100 billion.

However, this resolution has created a new puzzle. Older theoretical calculations, inferred from other experiments using a data-driven method, still conflict with the latest experimental results. This suggests either an issue with the underlying experimental data used in the older calculations or, perhaps, a return of the original hint of new, undiscovered particles. Physicists are now scrutinizing the electron-positron collision data that underpins the data-driven method to understand this ongoing divergence.