Electromagnetic Form Factor of the Pion

The Standard Model prediction for the magnetic moment of the muon requires a determination of the electromagnetic form factor of the pion at high precision. It is shown that the recent progress in ππ scattering allows us to obtain an accurate representation of this form factor on the basis of the data on e+e− → π+π−. The same method also applies to the form factor of the weak vector current, where the data on the decay τ → π−π0 ντ are relevant. Unfortunately, however, the known sources of isospin breaking do not explain the difference between the two results. The discrepancy implies that the Standard Model prediction for the magnetic moment of the muon is currently subject to a large uncertainty.