Experimental measurement of theγ→3πcoupling constant

We present the results of an experimental investigation of the production in the nuclear Coulomb field of low-invariant-mass pions pairs by the reaction ${\pi }^{\mathrm{-}}$Z→${\pi }^{\mathrm{-}}$ ${\pi }^0$Z at 40 GeV. We have obtained for the first time the experimental value of the γ→3π coupling constant $F^{3\pi }$(0)=13.0±0.9±1.3 ${\mathrm{GeV}}^{\mathrm{-}3}$, where the first (second) error is statistical (systematic). This result is in agreement with calculations based on the theory of chiral anomalies.