The muon anomalous magnetic moment and the pion polarizability

We compute the charged pion loop contribution to the muon anomalous magnetic moment aμaμ, taking into account the previously omitted effect of the charged pion polarizability, (α1−β1)π+(α1−β1)π+. We evaluate this contribution using two different models that are consistent with the requirements of chiral symmetry in the low-momentum regime and perturbative quantum chromodynamics in the asymptotic region. The result may increase the disagreement between the present experimental value for aμaμ and the theoretical, Standard Model prediction by as much as ∼60×10−11∼60×10−11, depending on the value of (α1−β1)π+(α1−β1)π+ and the choice of the model. The planned determination of (α1−β1)π+(α1−β1)π+ at Jefferson Laboratory will eliminate the dominant parametric error, leaving a theoretical model uncertainty commensurate with the error expected from planned Fermilab measurement of aμaμ.