Search for $CP$ violation in the phase space of $D^0\to {\pi }^{-}{\pi }^{+}{\pi }^0$ decays with the energy test

A search for $CP$ violation in $D^0\to {\pi }^{-}{\pi }^{+}{\pi }^0$ decays is reported, using $pp$ collision data collected by the LHCb experiment from 2015 to 2018 corresponding to an integrated luminosity of 6$f{b}^{-1}$. An unbinned model-independent approach provides sensitivity to local $CP$ violation within the two-dimensional phase space of the decay. The method is validated using the Cabibbo-favoured channel ${\text{D}}^0\to {\text{K}}^{-}{\pi }^{+}{\pi }^0$ and background regions of the signal mode. The results are consistent with $CP$ symmetry in this decay.