First Observation of Doubly Charmed Baryons

The SELEX experiment (E781) at Fermilab has observed two statistically compelling high mass states near 3.6 GeV/c^2, decaying to Lambda_c+ K- pi+ and Lambda_c+ K- pi+ pi+. These final states are Cabibbo-allowed decay modes of doubly charmed baryons $\Xi_{cc}^+$ and $\Xi_{cc}^{++}$, respectively. The masses are in the range expected from theoretical considerations, but the spectroscopy is surprising. SELEX also has weaker preliminary evidence for a state near 3.8 GeV/$\rm{c}^2$, a high mass state decaying to Lambda_c+ K- pi+ pi+, possibly an excited $\Xi_{cc}^{++}$ (ccu*). Data are presented and discussed.