PARAMETER $\beta $ IN ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit P}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ DECAYS

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.

PARAMETER $\beta $ IN ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ DECAY

INSPIRE search
VALUE (GeV${}^{-2}$) EVTS DOCUMENT ID TECN  COMMENT
$\bf{ 1.29 \pm0.13}$ OUR AVERAGE
$1.28$ $\pm0.10$ ${}^{+0.09}_{-0.08}$ 30k
BABUSCI
2015
KLOE 1.02 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
$3.8$ $\pm1.8$ 213 1
ACHASOV
2001B
SND 1.02 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
1  The uncertainty is statistical only. The systematic one is negligible, in comparison.
References:
BABUSCI 2015
PL B742 1 Study of the Dalitz Decay ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ with the KLOE Detector
ACHASOV 2001B
PL B504 275 Study of Conversion Decay ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and ${{\mathit \eta}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ in the Experiment with SND Detector at the VEPP-2M Collider