First Observation of the Doubly Charmed BaryonΞcc+

We observe a signal for the doubly charmed baryon ${\Xi }_{cc}^{+}$ in the charged decay mode ${\Xi }_{cc}^{+}\to {\Lambda }_c^{+}{K}^{-}{\pi }^{+}$ in data from SELEX, the charm hadroproduction experiment at Fermilab. We observe an excess of 15.9 events over an expected background of $6.1\pm 0.5$ events, a statistical significance of $6.3\sigma$. The observed mass of this state is $3519\pm 1\mathrm{M}\mathrm{e}\mathrm{V}/c^2$. The Gaussian mass width of this state is $3\mathrm{M}\mathrm{e}\mathrm{V}/c^2$, consistent with resolution; its lifetime is less than 33 fs at 90% confidence.