Observation of the doubly charmed baryon $\Xi_{cc}^{++}$

Abstract

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 $1334.94 \pm 0.72(stat.) \pm 0.27(syst.)  MeV/c^2$, and the $\Xi_{cc}^{++}$ mass is then determined to be $3621.40 \pm 0.72(stat.) \pm 0.27(syst.) \pm 0.14(\Lambda^+_c)  MeV/c^2$, 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  fb$^{−1}$, and confirmed in an additional sample of data collected at 8 TeV.