 Integrated narrow vector resonance area

For a narrow vector meson formed and observed in $e^{+}{e}^{-}$, neglecting external masses and slowly varying kinematic factors gives ${\sigma }_{ee}=3\pi {\Gamma }_{ee}^2/[M^2((E-M{)}^2+{\Gamma }^2/4)]$. Integrating the Lorentz line shape gives the displayed result. A normalized instrumental convolution preserves this full area, although it changes the observed peak and width. A constant uncorrected acceptance multiplies the area and must be divided out before extracting the decay width.