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Rare decay<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup><mml:mo>→</mml:mo><mml:msup><mml:mi>e</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>e</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:math>: Theory confronts KTeV data
Within the dispersive approach to the amplitude of the rare decay ${\ensuremath{\pi}}^{0}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$ the nontrivial dynamics is contained only in the subtraction constant. We express this constant, in the leading order in $({m}_{e}/\ensuremath{\Lambda}{)}^{2}$ perturbative series, in terms of the inverse moment of the pion transition form factor given in symmetric kinematics. …