  The chiral anomaly is a fundamental property of quantum chromodynamics (QCD).
It governs the transition amplitudes for processes involving an odd number of
Goldstone bosons of chiral symmetry breaking. In case of the coupling of three
pions to a photon, the magnitude of the resulting coupling is $F_{3\pi}$ and
the value is predicted by chiral perturbation theory with small uncertainty. It
can experimentally be measured in $\pi^-\gamma \to \pi^- \pi^0$ scattering.
  Here, we report on a precision experiment on $F_{3\pi}$ using the COMPASS
experiment at CERN where pion-photon scattering is mediated via the Primakoff
effect using heavy nuclei as target. We exploit the interference of the
production of the $\pi^- \pi^0$ final state via the chiral anomaly with the
photo-production of the $\rho(770)$ resonance over a wide mass range ($M_{\pi^-
\pi^0}<1\textrm{ GeV}/c^2$). This is in contrast to previous measurements
restricting themselves to the threshold region ($M_{\pi^- \pi^0}<370\textrm{
MeV}$) only. Our analysis allows to simultaneously extract the radiative width
of the $\rho(770)$ resonance and gives a stronger handle on $F_{3\pi}$ in a
unified approach thereby minimizing systematic effects rarely addressed
previously.
