Isospin violating effects in e+e− vs. τ measurements of the pion form factor |Fπ|2(s)

AbstractWe 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 aμhad(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