Using a data sample of $(448.1 pm 2.9) imes10^6 ~psi(3686)$ decays
collected at an $e^+ e^-$ center-of-mass energy of $3.686~
m{GeV}$ by the
BESIII detector at BEPCII, we report an observation of the hindered
electromagnetic Dalitz decay $psi(3686) o e^+ e^- eta_c$ with a
significance of $7.9sigma$. The branching fraction is determined to be
$mathcal{B}(psi(3686) o e^+ e^- eta_c) = (3.77 pm 0.40_{
m stat.} pm
0.18_{
m syst.}) imes 10^{-5}$. This is the first measurement of the
electromagnetic Dalitz transition between the $psi(3686)$ and the $eta_c$,
which provides new insight into the electromagnetic properties of the
transition of the $psi(3686)$ into the $eta_c$, and offers new opportunities
to study the properties of the $eta_c$ meson using an absolute measurement
method.
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