${{\mathit \eta}^{\,'}{(958)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$ 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.
M002FFL
VALUE (GeV${}^{-2}$) EVTS DOCUMENT ID TECN  COMMENT
$\bf{ 1.53 \pm0.04}$ OUR AVERAGE
$1.524$ $\pm0.041$ 7611 1
ABLIKIM
2024M
 BES3 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}^{\,'}}$
$1.7$ $\pm0.4$ 33 1
VIKTOROV
1980
25,33 ${{\mathit \pi}^{-}}$ ${{\mathit p}}$ $\rightarrow$ 2 ${{\mathit \mu}}{{\mathit \gamma}}$
• • • We do not use the following data for averages, fits, limits, etc. • • •
$1.30$ $\pm0.19$ 2
ABLIKIM
2024AK
 BES3 ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ , ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
$1.60$ $\pm0.17$ $\pm0.08$ 864 1
ABLIKIM
2015O
 BES3 ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit e}^{+}}{{\mathit e}^{-}}$
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.
2  From a weighted average of the ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$ values. The uncertainty is obtained by combining statistical and systematic uncertainties. TFF slope values go between $1.10$ $\pm0.21$ GeV${}^{-2}$ and $1.61$ $\pm0.71$ GeV${}^{-2}$ for ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit e}^{+}}{{\mathit e}^{-}}$, and between $2.37$ $\pm0.49$ GeV${}^{-2}$ and $2.88$ $\pm0.25$ GeV${}^{-2}$ for ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \mu}^{+}}{{\mathit \mu}^{-}}$, according to the solution of the fitting function model.
  References:
ABLIKIM 2024M
PR D109 072001 Improved measurements of the Dalitz decays $\eta/\eta'\rightarrow\gamma e^{+}e^{-}$
ABLIKIM 2024AK
JHEP 2407 135 Measurement of the Electromagnetic Transition Form-factors in the decays $\eta'\rightarrow\pi^+\pi^-l^+l^-$
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