See the “Note on ${{\mathit \eta}}$ Decay Parameters,” page 1454, in our 1994 edition (Physical Review D50 1173 (1994)). The following experiments fit to one or more of the coefficients $\mathit a$, $\mathit b$, $\mathit c$, $\mathit d$, $\mathit e$, $\mathit f$ or $\mathit g$ for $\vert $matrix element$\vert ^2$ = 1 + $\mathit a{}\mathit y$ + $\mathit b{}\mathit y{}^{2}$ + $\mathit c{}\mathit x$ + $\mathit d{}\mathit x{}^{2}$ + $\mathit e{}\mathit x{}\mathit y$ + $\mathit f{}\mathit y{}^{3}$ + $\mathit g{}\mathit x{}^{2}{}\mathit y$.

VALUE EVTS DOCUMENT ID TECN  COMMENT • • We do not use the following data for averages, fits, limits, etc. • • 631k 1 ABLIKIM 02 AN   BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}}$ 4.7M 2 ANASTASI 01 A   KLOE ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit \gamma}}$ 79k ABLIKIM 01 G   BES3 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}}$ 174k ADLARSON 01 A   WASA ${{\mathit p}}$ ${{\mathit d}}$ $\rightarrow$ ${{\mathit \eta}}{}^{3}\mathrm {He}$ 1.34M AMBROSINO 00 D   KLOE 3230 3 ABELE 99 D   CBAR ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit \eta}}$ at rest 1077 4 AMSLER 99   CBAR ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \eta}}$ at rest 81k LAYTER 97   ASPK 220k LAYTER 97   ASPK 1138 CARPENTER 97   HBC 349 DANBURG 97   DBC 7250 GORMLEY 97   WIRE 526 BAGLIN 96   HLBC 7170 CNOPS 96   OSPK 37k GORMLEY 96 C   WIRE 1300 CLPWY 96   HBC 705 LARRIBE 96   HBC 1  ABLIKIM 2023AN fit the Dalitz plot density distribution with two parameter sets ($\mathit a,~b,~d,~f$), and ($\mathit a,~b,~d,~f,~g$). 2  ANASTASI 2016A measure the Dalitz parameters $\mathit a$, $\mathit b$, $\mathit d$, $\mathit f$, and $\mathit g$. This is the first measurement of $\mathit g$. 3  ABELE 1998D obtains $\mathit a$ =$-1.22$ $\pm0.07$ and $\mathit b$ = $0.22$ $\pm0.11$ when $\mathit c$ (or$~\mathit d$) is fixed at $0.06$. 4  AMSLER 1995 fits to (1$+\mathit ay+\mathit by{}^{2}$) and obtains $\mathit a=-0.94$ $\pm0.15$ and $\mathit b=0.11$ $\pm0.27$.

References