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 2023 AN   BES3 ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}}$ 4.7M 2 ANASTASI 2016 A   KLOE ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit \phi}}$ $\rightarrow$ ${{\mathit \eta}}{{\mathit \gamma}}$ 79k ABLIKIM 2015 G   BES3 ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ $\rightarrow$ ${{\mathit J / \psi}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \eta}}$ 174k ADLARSON 2014 A   WASA ${{\mathit p}}$ ${{\mathit d}}$ $\rightarrow$ ${{\mathit \eta}}{}^{3}\mathrm {He}$ 1.34M AMBROSINO 2008 D   KLOE 3230 3 ABELE 1998 D   CBAR ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}^{0}}{{\mathit \pi}^{0}}{{\mathit \eta}}$ at rest 1077 4 AMSLER 1995   CBAR ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \pi}^{+}}{{\mathit \pi}^{-}}{{\mathit \eta}}$ at rest 81k LAYTER 1973   ASPK 220k LAYTER 1972   ASPK 1138 CARPENTER 1970   HBC 349 DANBURG 1970   DBC 7250 GORMLEY 1970   WIRE 526 BAGLIN 1969   HLBC 7170 CNOPS 1968   OSPK 37k GORMLEY 1968 C   WIRE 1300 CLPWY 1966   HBC 705 LARRIBE 1966   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