The amplitude for the process ${{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \gamma}}$ contains a form factor F($\mathit x$) at the ${{\mathit \pi}^{0}}{{\mathit \gamma}}{{\mathit \gamma}}$ vertex, where $\mathit x$ = [${\mathit m}_{\mathrm {{{\mathit e}^{+}} {{\mathit e}^{-}}}}/{\mathit m}_{{{\mathit \pi}^{0}}}]{}^{2}$. The parameter $\mathit a$ in the linear expansion F($\mathit x$) = 1 + $\mathit a{}\mathit x$ is listed below. All the measurements except that of BEHREND 1991 are in the time-like region of momentum transfer.
LINEAR COEFFICIENT OF ${{\mathit \pi}^{0}}$ ELECTROMAGNETIC FORM FACTOR
INSPIRE search
1BEHREND 1991 estimates that their systematic error is of the same order of magnitude as their statistical error, and so we have included a systematic error of this magnitude. The value of $\mathit a$ is obtained by extrapolation from the region of large space-like momentum transfer assuming vector dominance.
2TUPPER 1983 is a theoretical analysis of FISCHER 1978 including 2-photon exchange in the corrections.
3
The FISCHER 1978 error is statistical only. The result without radiation corrections is $+0.05$ $\pm0.03$.
References:
LAZZERONI
2017
PL B768 38
Measurement of the ${{\mathit \pi}^{0}}$ Electromagnetic Transition Form Factor Slope
FARZANPAY
1992
PL B278 413
Measurement of the Slope of the ${{\mathit \pi}^{0}}$ Electromagnetic Formfactor
MEIJERDREES
1992B
PR D45 1439
Measurement of the ${{\mathit \pi}^{0}}$ Electromagnetic Transition Formfactor
BEHREND
1991
ZPHY C49 401
A Measurement of the ${{\mathit \pi}^{0}}$, ${{\mathit \eta}}$ and ${{\mathit \eta}^{\,'}}$ Electromagnetic Form Factors
