  A highly significant structure is observed in the $\Lambda_c^+K^-\pi^+\pi^+$
mass spectrum, where the $\Lambda_c^+$ baryon is reconstructed in the decay
mode $pK^-\pi^+$. The structure is consistent with originating from a weakly
decaying particle, identified as the doubly charmed baryon $\Xi_{cc}^{++}$. The
difference between the masses of the $\Xi_{cc}^{++}$ and $\Lambda_c^+$ states
is measured to be $1334.94 \pm 0.72 (\mathrm{stat}) \pm 0.27
(\mathrm{syst}~\mathrm{MeV}/c^2$, and the $\Xi_{cc}^{++}$ mass is then
determined to be $3621.40 \pm 0.72 (\mathrm{stat}) \pm 0.27 (\mathrm{syst} \pm
0.14 \, (\Lambda_c^+)~\mathrm{MeV}/c^2$, where the last uncertainty is due to
the limited knowledge of the $\Lambda_c^+$ mass. The state is observed in a
sample of proton-proton collision data collected by the LHCb experiment at a
center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of
1.7 $\mathrm{fb}^{-1}$, and confirmed in an additional sample of data collected
at 8 TeV.
