Isospin violating effects in $e^+$$ e^-$ versus $\tau$ measurements of the pion form-factor $|F_\pi|^2(s)$

We study possible so far unaccounted isospin breaking effects in the relation between the pion form factor as determined in e + e − experiments and the corresponding quantity obtained after accounting for known isospin breaking effects by an isospin rotation from the τ -decay spectra. In fact the observed 10% discrepancy in the respective pion form factors may be explained by the isospin breaking which is due to the difference between masses and widths of the charged and neutral ρ mesons. Since the hadronic contribution to the muon anomalous magnetic moment can be calculated directly in terms of the e + e − -data the corresponding evaluation seems to be more reliable. Our estimate is a μ had(1) =(694.8±8.6)×10 −10 . The τ -data are useful at the presently aimed level of accuracy only after appropriate input from theory