Dispersion relations for hadronic light-by-light scattering in triangle kinematics

We present a new strategy for the dispersive evaluation of hadronic light-by-light contribution to anomalous magnetic moment muon $a_\mu$. The approach directly applies in kinematic limit relevant $a_\mu$: one photons is treated as an external electromagnetic field with vanishing momentum, so that kinematics corresponds triangle. derive expressions single-particle intermediate states, well tensor decompositions two-pion sub-processes appear addition those needed established approach. existing based on set dispersion relations four-point kinematics. At it not known how consistently include this framework resonant states spin 2 or larger, due appearance singularities can be traced back redundancy decomposition. show our circumvents problem and enables triangle are manifestly free from singularities, paving way towards data-driven all exclusive states.