Observation of $D^{+}\to \eta \eta {\pi }^{+}$ and improved measurement of $D^{0(+)}\to \eta {\pi }^{+}{\pi }^{-(0)}$

BESIII Collaboration

Using an $e^{+}{e}^{-}$ annihilation data sample corresponding to an integrated luminosity of $2.93\mathrm{f}{\mathrm{b}}^{-1}$ collected at the center-of-mass energy of 3.773\,GeV with the BESIII det…