First Observation of the Doubly Charmed Baryon Decay Ξcc++→Ξc+π+

The doubly charmed baryon decay ${\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{+}{\pi }^{+}$ is observed for the first time, with a statistical significance of $5.9\sigma$, confirming a recent observation of the baryon in the ${\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+}$ final state. The data sample used corresponds to an integrated luminosity of $1.7{\mathrm{fb}}^{-1}$, collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of 13 TeV. The ${\mathrm{\Xi }}_{cc}^{++}$ mass is measured to be $3620.6\pm 1.5(\mathrm{stat})\pm 0.4(\mathrm{syst})\pm 0.3({\mathrm{\Xi }}_c^{+})\mathrm{MeV}/c^2$ and is consistent with the previous result. The ratio of branching fractions between the decay modes is measured to be $[\mathcal{B}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Xi }}_c^{+}{\pi }^{+})\times \mathcal{B}({\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+})]/[\mathcal{B}({\mathrm{\Xi }}_{cc}^{++}\to {\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+})\times \mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+})]=0.035\pm 0.009(\mathrm{stat})\pm 0.003(\mathrm{syst})$.