$\boldsymbol a_{2}$ = $\boldsymbol M2/\sqrt {\boldsymbol E1{}^{2}+\boldsymbol M2{}^{2} }$ Magnetic quadrupole fractional transition amplitude INSPIRE search

VALUE ($10^{-2}$) EVTS DOCUMENT ID TECN  COMMENT
$\bf{ -6.7 \pm0.9}$ OUR AVERAGE  Error includes scale factor of 2.6.
$-7.40$ $\pm0.33$ $\pm0.34$ 164k 1
 2017 N
BES3 ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$
$-6.26$ $\pm0.63$ $\pm0.24$ 39k
 2009
CLEO ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \ell}^{+}}{{\mathit \ell}^{-}}$
$0.2$ $\pm3.2$ $\pm0.4$ 2090
 2002
E835 ${{\mathit p}}$ ${{\overline{\mathit p}}}$ $\rightarrow$ ${{\mathit \chi}_{{c1}}}$ $\rightarrow$ ${{\mathit J / \psi}}{{\mathit \gamma}}$
$-0.2$ ${}^{+0.8}_{-2.0}$ 921
 1982
CBAL ${{\mathit \psi}{(2S)}}$ $\rightarrow$ ${{\mathit \chi}_{{c1}}}{{\mathit \gamma}}$ $\rightarrow$ ${{\mathit J / \psi}}{{\mathit \gamma}}{{\mathit \gamma}}$
1  Correlated with ${{\mathit b}_{{2}}}$ with correlation coefficient $\rho _{ {{\mathit a}_{{2}}} {{\mathit b}_{{2}}} }$ = 0.133.

$\mathit a_{2}$ = $\mathit M2/\sqrt {\mathit E1{}^{2}+\mathit M2{}^{2} }$
References:
 ABLIKIM 2017N
PR D95 072004 Measurement of Higher-Order Multipole Amplitudes in ${{\mathit \psi}{(3686)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \chi}_{{c1,2}}}$ with ${{\mathit \chi}_{{c1,2}}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit J / \psi}}$ and Search for the Transition ${{\mathit \eta}_{{c}}{(2S)}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit J / \psi}}$
 ARTUSO 2009
PR D80 112003 Higher-Order Multipole Amplitudes in Charmonium Radiative Transitions
 AMBROGIANI 2002
PR D65 052002 Study of the Angular Distributions of the Reactions ${{\overline{\mathit p}}}$ ${{\mathit p}}$ $\rightarrow$ ${{\mathit \chi}_{{c1}}}$ , ${{\mathit \chi}_{{c2}}}$ $\rightarrow$ ${{\mathit J / \psi}}{{\mathit \gamma}}$ $\rightarrow$ ${{\mathit e}^{+}}{{\mathit e}^{-}}{{\mathit \gamma}}$
 OREGLIA 1982
PR D25 2259 Study of the Reaction ${{\mathit \psi}^{\,'}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit J / \psi}}$