2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/188292The anomaly in the vacuum expectation value of the product of axial and two vector currents (AVV) in QCD is investigated. The goal is to determine from its value the $π^0 \to 2 γ$ decay width with high precision. The sum rule for AVV formfactor is studied. The difference $f_{π^0} - f_{π^+}$ caused by strong interaction is calculated and appears to be small. The $π^0 - η$ mixing is accounted. The $π^0 \to 2 γ$ decay width determined theoretically from the axial anomaly is found to be $Γ(π^0 \to 2 γ) = 7.93 eV$ with an error $\sim 1.5%$. The measurement of the $π^0 \to 2 γ$ decay width at the same level of accuracy would allow one to achieve a high precision test of QCD.8 pages, few misprints are correctedHigh Energy Physics - Phenomenologytext