We report the first observation of the Cabibbo-suppressed decays $Xi_c^0rightarrow Xi^- K^+$, $Xi_c^0raLambda K^+ K^-$ and $Xi_c^0rightarrow Lambda phi$, using a data sample of $711, mathrm{fb}^{-1}$ collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We measure the ratios of branching fractions to be $frac{{cal B}(Xi_c^0ra Xi^- K^+)}{{cal B}(Xi^0_craXi^-pi^+)}=(2.75pm 0.51pm 0.25)times 10^{-2}$, $frac{{cal B}(Xi_c^0raLambda K^+ K^-)}{{cal B}(Xi_c^0rightarrowXi^- pi^+)}=(2.86pm 0.61pm 0.24)times 10^{-2}$ and $frac{{cal B}(Xi_c^0ra Lambdaphi)}{{cal B}(Xi^0_craXi^-pi^+)}=(3.43pm 0.58pm 0.32)times 10^{-2}$, where the first uncertainty is statistical and the second is systematic.
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