Chiral symmetry, ππ scattering and aμ

The hadronic vacuum polarization contributions to (g−2)μ below one GeV are dominated by the two-pion contribution. The latter can be described by the electromagnetic pion form factor. Unitarity and analyticity strongly constrain the latter. Unitarity, e.g., implies that the phase of the form factor in the elastic region coincides with the P-wave π+π− phase shift. We show that these theoretical constraints, and in particular the precise knowledge of the ππ scattering amplitude, obtained using chiral symmetry and Roy equations, play an important role in the evaluation of the hadronic vacuum polarization contributions to (g−2)μ.