Testing Predictions of the Chiral Anomaly in Primakoff Reactions at COMPASS

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.