In the one-pole approximation the electromagnetic transition form factor for ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit P}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ (${{\mathit P}}$ = ${{\mathit \pi}},{{\mathit \eta}}$) is given as a function of the ${{\mathit e}^{+}}{{\mathit e}^{-}}$ invariant mass squared, ${{\mathit q}^{2}}$, by the expression: $\vert \mathit F({{\mathit q}^{2}})\vert ^2$ = (1 $−$ ${{\mathit q}^{2}}/{{\mathit \Lambda}^{2}}){}^{-2}$, where vector meson dominance predicts parameter $\Lambda \approx{}$0.770 GeV ($\Lambda {}^{-2}\approx{}$1.687 GeV${}^{-2}$). The slope of this form factor, $\beta $ = d$\mathit F$/d${{\mathit q}^{2}}({{\mathit q}^{2}}$=0), equals $\Lambda {}^{-2}$ in this approximation.
The measurements below obtain $\beta $ in the one-pole approximation.