Studies of the reaction γπ → ππ, in the context of the ongoing Primakoff program of the COM-PASS experiment at CERN, give access to the radiative couplings of the ρ(770) and ρ3(1690) resonances. We provide a vector-meson-dominance estimate of the respective radiative width of the ρ3, Γρ 3 →πγ = 48(18) keV, as well as its impact on the F -wave in γπ → ππ. For the ρ(770), we establish the formalism necessary to extract its radiative coupling directly from the residue of the resonance pole by analytic continuation of the γπ → ππ amplitude to the second Riemann sheet, without any reference to the vector-meson-dominance hypothesis.

Estimating the contribution from axial-vector intermediate states to hadronic light-by-light scattering requires input on their transition form factors (TFFs). Due to the Landau–Yang theorem, any experiment sensitive to these TFFs needs to involve at least one virtual photon, which complicates their measurement. Phenomenologically, the situation is best for the f1(1285) resonance, for which information is available from e+e− → e+e−f1, f1 → 4π, f1 → ργ, f1 → ϕγ, and f1 → e+e−. We provide a comprehensive analysis of the f1 TFFs in the framework of vector meson dominance, including short-distance constraints, to determine to which extent the three independent TFFs can be constrained from the available experimental input — a prerequisite for improved calculations of the axial-vector contribution to hadronic light-by-light scattering. In particular, we focus on the process f1 → e+e−, evidence for which has been reported recently by SND for the first time, and discuss the impact that future improved measurements will have on the determination of the f1 TFFs.

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