${{\mathit \eta}^{\,'}{(958)}}$ $\beta $ PARAMETER $\vert $MATRIX ELEMENT$\vert ^2$ = (1 + 2$\beta {{\mathit Z}}$)

See the “Note on ${{\mathit \eta}}$ Decay Parameters” in our 1994 edition Physical Review D50 1173 (1994), p.$~$1454.

$\beta $ decay parameter

INSPIRE   PDGID:
M002B0
VALUE EVTS DOCUMENT ID TECN  COMMENT
$\bf{ -0.61 \pm0.08}$ OUR AVERAGE  Error includes scale factor of 1.2.
$-0.640$ $\pm0.046$ $\pm0.047$ 1.8k
ABLIKIM
2015G
 BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}$( ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$)
$-0.59$ $\pm0.18$ 235
BLIK
2008
GAMS 32 ${{\mathit \pi}^{-}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \eta}^{\,'}}{{\mathit n}}$
$-0.1$ $\pm0.3$
ALDE
1987B
 GAM2 38 ${{\mathit \pi}^{-}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit n}}$3 ${{\mathit \pi}^{0}}$
References:
ABLIKIM 2015G
PR D92 012014 Measurement of the Matrix Elements for the Decays ${{\mathit \eta}}$ $\rightarrow$ ${+}$ ${{\mathit \pi}^{-}}{{\mathit \pi}^{0}}$ and ${{\mathit \eta}}$ $/$ ${{\mathit \eta}^{\,'}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit \pi}^{0}}$
BLIK 2008
PAN 71 2124 Investigation of the Decay ${{\mathit \eta}^{\,'}}$ $\rightarrow$ 3 ${{\mathit \pi}^{0}}$ with the GAMS-4$\pi $ Spectrometer
ALDE 1987B
ZPHY C36 603 Study of Neutral Decays of the ${{\mathit \eta}^{\,'}{(958)}}$ Meson