Dispersive analysis of $τ^-\toπ^-π^0ν_τ$ Belle data

We analyse Belle data on the decay $τ^- \to π^- π^0 ν_τ$ using a dispersive representation of the vector form factor which is consistent with chiral symmetry and preserves analyticity and unitarity exactly. We fit the unknown theoretical parameters from the data, determining the values of the related low-energy observables $\left\langle r^2\right\rangle^π_V$ and $c_V^π$. The implementation of isospin breaking effects is also discussed.

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