Observation of the decay $D^0 to omega phi$

Using a data sample corresponding to an integrated luminosity of 2.93 fb$^{-1}$ collected at a center-of-mass energy $sqrt{s}=3.773$ GeV by the BESIII detector, the decay $D^0toomegaphi$ is observed for the first time. The branching fraction is measured to be $(6.48 pm 0.96 pm 0.38)times 10^{-4}$ with a significance of $6.3 sigma$, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the $phi$ and $omega$ mesons from the $D^0 to omega phi$ decay are transversely polarized.