The discrepancy between τ and e + e - spectral functions revisited and the consequences for the muon magnetic anomaly

We revisit the procedure for comparing the pi pi spectral function measured in tau decays to that obtained in e (+) e (-) annihilation. We re-examine the isospin-breaking corrections using new experimental and theoretical input and find improved agreement between the tau (-)->pi (-) pi (0) nu (tau) branching fraction measurement and its prediction using the isospin-breaking-corrected e (+) e (-)->pi (+) pi (-) spectral function, though not resolving all discrepancies. We recompute the lowest order hadronic contributions to the muon g-2 using e (+) e (-) and tau data with the new corrections and find a reduced difference between the two evaluations. The new tau-based estimate of the muon magnetic anomaly is found to be 1.9 standard deviations lower than the direct measurement.