Dissecting the Hadronic Contributions to (g-2)_{μ} by Schwinger's Sum Rule.

The theoretical uncertainty of (g-2)_{μ} is currently dominated by hadronic contributions. In order to express those in terms of directly measurable quantities, we consider a sum rule relating g-2 to an integral of a photoabsorption cross section. The sum rule, attributed to Schwinger, can be viewed as a combination of two older sum rules: Gerasimov-Drell-Hearn and Burkhardt-Cottingham. The Schwinger sum rule has an important feature, distinguishing it from the other two: the relation between the anomalous magnetic moment and the integral of a photoabsorption cross section is linear, rather than quadratic. The linear property makes it suitable for a straightforward assessment of the hadronic contributions to (g-2)_{μ}. From the sum rule, we rederive the Schwinger α/2π correction, as well as the formula for the hadronic vacuum-polarization contribution. As an example of the light-by-light contribution, we consider the single-meson exchange.