Axial anomaly and the precise value of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup><mml:mo>→</mml:mo><mml:mn>2</mml:mn><mml:mi>γ</mml:mi></mml:math> decay width

The 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 π0→2γ decay width with high precision. The sum rule for AVV formfactor is studied. The difference fπ0−fπ+ caused by strong interaction is calculated and appears to be small. The π0–η mixing is accounted. The π0→2γ decay width determined theoretically from the axial anomaly is found...