Meson electro-/photo-production from QCD

I present the calculation of the $π^+γ^\star\toπ^+π^0$ transition amplitude from quantum chromodynamics performed by the Hadron Spectrum Collaboration. The amplitude is determined for a range of values of the photon virtuality and the final state energy. One observes a clear dynamical enhancement due to the presence of the $ρ$ resonance. By fitting the transition amplitude and analytically continuing it onto the $ρ$-pole, the $ρ\toπγ^\star$ form factor is obtained. This exploratory calculation, performed using lattice quantum chromodynamics, constitutes the very first determination of an electroweak decay of a hadronic resonance directly from the fundamental theory of quarks and gluons. In this talk, I highlight some of the necessary steps that made this calculation possible, placing emphasis on recently developed formalism. Finally, I discuss the status and outlook of the field for the study of $Nγ^\star\to N^\star\to Nπ$ transitions.

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