Radiative resonance couplings in $\gamma\pi\to\pi\pi$

Understand

Studies of the reaction $\gamma\pi\to\pi\pi$, in the context of the ongoing Primakoff program of the COMPASS experiment at CERN, give access to the radiative couplings of the $\rho(770)$ and $\rho_3(1690)$ resonances.

• We provide a vector-meson-dominance estimate of the respective radiative width of the $\rho_3$, $\Gamma_{\rho_3\to\pi\gamma}=48(18)$ keV, as well as its impact on the $F$-wave in $\gamma\pi\to\pi\pi$.
• For the $\rho(770)$, we establish the formalism necessary to extract its radiative coupling directly from the residue of the resonance pole by analytic continuation of the $\gamma\pi\to\pi\pi$ amplitude to the second Riemann sheet, without any reference to the vector-meson-dominance hypothesis.