Towards a data-driven analysis of hadronic light-by-light scattering
  The hadronic light-by-light contribution to the anomalous magnetic moment of the muon was recently analyzed in the framework of dispersion theory, providing a systematic formalism where all input quantities are expressed in terms of on-shell form factors and scattering amplitudes that are in principle accessible in experiment. We briefly review the main ideas behind this framework and discuss the various experimental ingredients needed for the evaluation of one- and two-pion intermediate states. In particular, we identify processes that in the absence of data for doubly-virtual pion-photon interactions can help constrain parameters in the dispersive reconstruction of the relevant input quantities, the pion transition form factor and the helicity partial waves for $\gamma^*\gamma^*\to\pi\pi$. 
Gilberto Colangelo, Martin Hoferichter, Bastian Kubis, Massimiliano   Procura, Peter Stoffer
hep-ph hep-ex nucl-ex nucl-th