Photon–photon scattering: a tutorial

Long-established results for low-energy photon–photon scattering, γγ → γγ, have recently been questioned. We analyze that claim and demonstrate that it is inconsistent with experience. We demonstrate that the mistake originates from an erroneous manipulation of divergent integrals and discuss the connection with another recent claim about the Higgs decay into two photons. We show a simple way of correctly computing low-energy γγ scattering.