Observation of the doubly charmed baryon $\hat{I}{ž}_{cc}^{++}$

Abstract

A highly significant structure is observed in the $\hat{I}{›}_c^{+}{K}^{-}\ddot{I}{€}^{+}\ddot{I}{€}^{+}$ mass spectrum, where the $\hat{I}{›}_c^{+}$ baryon is reconstructed in the decay mode $p{K}^{-}\ddot{I}{€}^{+}$. The structure is consistent with originating from a weakly decaying particle, identified as the doubly charmed baryon $\hat{I}{ž}_{cc}^{++}$. The difference between the masses of the $\hat{I}{ž}_{cc}^{++}$ and $\hat{I}{›}_c^{+}$ states is measured to be $1334.94\pm 0.72(\mathrm{stat})\pm 0.27(\mathrm{syst}\mathrm{MeV}/c^2$, and the $\hat{I}{ž}_{cc}^{++}$ mass is then determined to be $3621.40\pm 0.72(\mathrm{stat})\pm 0.27(\mathrm{syst}\pm 0.14(\hat{I}{›}_c^{+})\mathrm{MeV}/c^2$, where the last uncertainty is due to the limited knowledge of the $\hat{I}{›}_c^{+}$ 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.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2017-018.html