First Observation of the Doubly Charmed Baryon ${\Xi }_{cc}^{+}$

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.

Figure 1

A candidate event with production target, $1\sigma$ track error corridors, and vertex error ellipses. This is a plan view of three-dimensional tracks and vertices. Three additional found tracks which form the primary vertex with the beam track are not shown.

Figure 2

(a) The ${\Lambda }_c^{+}{K}^{-}{\pi }^{+}$ mass distribution in $5\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ bins. The shaded region $3.400–3.640\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ contains the signal peak and is shown in more detail in (c). (b) The wrong-sign combination ${\Lambda }_c^{+}{K}^{+}{\pi }^{-}$ mass distribution in $5\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ bins. (c) The signal (shaded) region (22 events) and sideband mass regions with 162 total events in $2.5\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ bins. The fit is a Gaussian plus linear background.

Figure 3

The signal significance as a function of (a) the vertex significance of the $K^{-}{\pi }^{+}$ vertex, (b) the half-width of the signal region. (c) the mass window around the ${\Lambda }_c^{+}$ mass, (d) the half-width of the sideband region. The square points are the values used in this analysis.