Observation of ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^{0}J\mathrm{/}\psi$ and evidence for ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^0\psi (2S)$ decays

The first search for the beauty baryon decays ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^{0}J\mathrm{/}\psi$ and ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^0\psi (2S)$ is presented using the proton-proton collision dataset collected by the LHCb experiment between 2016 and 2018, corresponding to an integrated luminosity of $5.4{\mathrm{f}\mathrm{b}}^{-1}$. The first observation of the decay ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^{0}J\mathrm{/}\psi$ is reported and evidence of the decay ${\mathrm{\Xi }}_b^0\to {\mathrm{\Xi }}^0\psi (2S)$ is presented. The ${\mathrm{\Xi }}^0$ hyperon is fully reconstructed for the first time at an LHC experiment, which is achieved using the ${\mathrm{\Xi }}^0\to \mathrm{\Lambda }{\pi }^0$ decay. The ratio of the branching fractions is measured as $\frac{\mathcal{B}({\mathcal{\Xi }}_{\mathcal{b}}^{\mathcal{0}}\to {\mathcal{\Xi }}^{\mathcal{0}}\mathcal{\psi }(\mathcal{2}\mathcal{S}))}{\mathcal{B}({\mathcal{\Xi }}_{\mathcal{b}}^{\mathcal{0}}\to {\mathcal{\Xi }}^{\mathcal{0}}\mathcal{J}\mathcal{/}\mathcal{\psi })}=0.59\pm 0.19\text{(stat)}\pm 0.04\text{(syst)}$.