${{\boldsymbol \eta}^{\,'}{(958)}}$ $\rightarrow$ ${{\boldsymbol \gamma}}{{\boldsymbol \ell}^{+}}{{\boldsymbol \ell}^{-}}$ TRANSITION FORM FACTOR SLOPE INSPIRE search

Related to the effective virtual meson mass $\Lambda $, via slope $\approx{}$ $\Lambda {}^{-2}$. See e.g. LANDSBERG 1985 , eq. (3.8), for a detailed definition.
VALUE (GeV${}^{-2}$) EVTS DOCUMENT ID TECN  COMMENT
$\bf{ 1.62 \pm0.17}$ OUR AVERAGE
$1.60$ $\pm0.17$ $\pm0.08$ 864 1
ABLIKIM
2015O
BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
$1.7$ $\pm0.4$ 33 1
VIKTOROV
1980
25,33 ${{\mathit \pi}^{-}}$ ${{\mathit p}}$ $\rightarrow$ 2 ${{\mathit \mu}}{{\mathit \gamma}}$
1  In the single-pole Ansatz where slope = 1/($\Lambda {}^{2}$ + $\gamma {}^{2}$) with $\Lambda $, $\gamma $ being a Breit-Wigner mass, width for the effective contributing vector meson.
  References:
ABLIKIM 2015O
PR D92 012001 Observation of the Dalitz Decay $\rightarrow$ ${{\mathit \gamma}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
VIKTOROV 1980
SJNP 32 520 Further Study of the ${{\mathit \eta}^{\,'}{(958)}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \gamma}}$ Decay