Observation of $\mathrm{\Omega }(2012{)}^{-}\to \mathrm{\Xi }(1530)\bar{K}$ and measurement of the effective couplings of $\mathrm{\Omega }(2012{)}^{-}$ to $\mathrm{\Xi }(1530)\bar{K}$ and $\mathrm{\Xi }\bar{K}$

Using $\mathrm{{\rm Y}}(1S)$, $\mathrm{{\rm Y}}(2S)$, and $\mathrm{{\rm Y}}(3S)$ data collected by the Belle detector, we discover a new three-body decay, $\mathrm{\Omega }(2012{)}^{-}\to \mathrm{\Xi }(1530)\bar{K}\to \mathrm{\Xi }\pi \bar{K}$, with a significance of 5.2~$\sigma$. The mass of the $\mathrm{\Omega }(2012{)}^{-}$ is $(2012.5\pm 0.7\pm 0.5)$ MeV and its effective couplings to $\mathrm{\Xi }(1530)\bar{K}$ and $\mathrm{\Xi }\bar{K}$ are $(39_{-39}^{+31}\pm 9)\times {10}^{-2}$ and $(1.7\pm 0.3\pm 0.3)\times {10}^{-2}$, where the first uncertainties are statistical and the second are systematic. The ratio of the branching fraction for the three-body decay to that for the two-body decay to $\mathrm{\Xi }\bar{K}$ is $0.99\pm 0.26\pm 0.06$, assuming isospin symmetry.