Search for doubly charmed baryons and study of charmed strange baryons at Belle

We report results of a study of doubly charmed baryons and charmed strange baryons. The analysis is performed using a $980{\mathrm{fb}}^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We search for doubly charmed baryons ${\mathrm{\Xi }}_{cc}^{+(+)}$ with the ${\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}({\pi }^{+})$ and ${\mathrm{\Xi }}_c^0{\pi }^{+}({\pi }^{+})$ final states. No significant signal is observed. We also search for two excited charmed strange baryons, ${\mathrm{\Xi }}_c(3055{)}^{+}$ and ${\mathrm{\Xi }}_c(3123{)}^{+}$ with the ${\mathrm{\Sigma }}_c^{++}(2455)K^{-}$ and ${\mathrm{\Sigma }}_c^{++}(2520)K^{-}$ final states. The ${\mathrm{\Xi }}_c(3055{)}^{+}$ signal is observed with a significance of 6.6 standard deviations including systematic uncertainty, while no signature of the ${\mathrm{\Xi }}_c(3123{)}^{+}$ is seen. We also study properties of the ${\mathrm{\Xi }}_c(2645{)}^{+}$ and measure a width of $2.6\pm 0.2(\text{stat})\pm 0.4(\text{syst})\mathrm{MeV}/c^2$, which is the first significant determination.

Figure 1

Invariant mass distribution of the ${\mathrm{\Xi }}_{cc}$ candidates for (a) $M({\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+})$, (b) $M({\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}{\pi }^{+})$; the vertical error bars are from data and the dashed histogram are from signal MC for the ${\mathrm{\Xi }}_{cc}$ signal generated with a mass of $3.60\mathrm{GeV}/c^2$ and a production cross section $\sigma (e^{+}{e}^{-}\to {\mathrm{\Xi }}_{cc}^{+(+)}X)$ of 500 fb and $\mathcal{B}({\mathrm{\Xi }}_{cc}^{+(+)}\to {\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+}({\pi }^{+}))$ of 5%. 95% C.L. upper limit of ${\sigma }_{\mathcal{B}}$ as a function of the mass with a $1\mathrm{MeV}/c^2$ step for (c) ${\mathrm{\Xi }}_{cc}^{+}$ and (d) ${\mathrm{\Xi }}_{cc}^{++}$.

Figure 2

Reconstruction efficiency as a function of the ${\mathrm{\Xi }}_{cc}$ mass, for (a) ${\mathrm{\Xi }}_{cc}^{+}$, (b) ${\mathrm{\Xi }}_{cc}^{++}$. Circular points are for ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ and square points are for ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0$. The lines show the result of the fit with a linear function.

Figure 3

(a) The $p^{*}({\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+})$ distribution from data. (b) The $M({\mathrm{\Lambda }}_c^{+}{\pi }^{+})$ distribution. The vertical lines show the selected regions of the ${\mathrm{\Sigma }}_c(2455{)}^{++}$ and ${\mathrm{\Sigma }}_c(2520{)}^{++}$. (c) The $M({\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+})$ distribution with ${\mathrm{\Sigma }}_c(2455{)}^{++}$ selection. The dots with error bars show the distribution for the ${\mathrm{\Sigma }}_c(2455{)}^{++}$ selected region whereas the rectangles show the distribution for the ${\mathrm{\Sigma }}_c(2455{)}^{++}$ sideband region. The solid line shows the fit result. The dashed, dotted, and dash-dotted lines show the contributions from the background, ${\mathrm{\Xi }}_c(3055{)}^{+}$, and ${\mathrm{\Xi }}_c(2980{)}^{+}$, or ${\mathrm{\Xi }}_c(3080{)}^{+}$, respectively. (d) The $M({\mathrm{\Lambda }}_c^{+}{K}^{-}{\pi }^{+})$ distribution with ${\mathrm{\Sigma }}_c(2520{)}^{++}$ selection. The dots with error bars show the distribution for ${\mathrm{\Sigma }}_c(2520{)}^{++}$ selected region whereas the rectangles show the distribution for the ${\mathrm{\Sigma }}_c(2520{)}^{++}$ sideband region. The solid line shows the fit result. The dashed, dotted, and dash-dotted lines show the contributions from the background, ${\mathrm{\Xi }}_c(3123{)}^{+}$, and ${\mathrm{\Xi }}_c(3080{)}^{+}$, respectively.

Figure 4

$M({\mathrm{\Xi }}_c^0{\pi }^{+})$ distributions below the ${\mathrm{\Xi }}_{cc}$ search region for (a) ${\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+}$, (b) ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{K}^{-}{\pi }^{+}$, (c) ${\mathrm{\Xi }}_c^0\to p{K}^{-}{K}^{-}{\pi }^{+}$. The solid lines show the fit result. The dashed, dotted, and dash-dotted lines show the contributions from background, ${\mathrm{\Xi }}_c(2645{)}^{+}$, and ${\mathrm{\Xi }}_c(2790{)}^{+}$, respectively.

Figure 5

Reconstruction efficiencies for the ${\mathrm{\Xi }}_{cc}$ as a function of the ${\mathrm{\Xi }}_{cc}$ mass for (a) ${\mathrm{\Xi }}_{cc}^{+}$, (b) ${\mathrm{\Xi }}_{cc}^{++}$. Circles, square, and triangle points are for ${\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+}$, $\mathrm{\Lambda }{K}^{-}{\pi }^{+}$, and $p{K}^{-}{K}^{-}{\pi }^{+}$, respectively. The lines are the result of the fit with a linear function.

Figure 6

(a)–(c): $M({\mathrm{\Xi }}_c^0{\pi }^{+})$ distribution in the ${\mathrm{\Xi }}_{cc}^{+}$ search region for ${\mathrm{\Xi }}_c^0\to$ (a) ${\mathrm{\Xi }}^{-}{\pi }^{+}$, (b) $\mathrm{\Lambda }{K}^{-}{\pi }^{+}$, (c) $p{K}^{-}{K}^{-}{\pi }^{+}$. The vertical error bars are from data. The dashed histograms are from signal MC. (d): 95% C.L. upper limit of the ${\sigma }_{{\mathcal{B}}^2}$ for ${\mathrm{\Xi }}_{cc}^{+}$ as a function of the mass with a $1\mathrm{MeV}/c^2$ step.

Figure 7

(a)–(c): $M({\mathrm{\Xi }}_c^0{\pi }^{+}{\pi }^{+})$ distribution in the ${\mathrm{\Xi }}_{cc}^{++}$ search region for ${\mathrm{\Xi }}_c^0\to$ (a) ${\mathrm{\Xi }}^{-}{\pi }^{+}$, (b) $\mathrm{\Lambda }{K}^{-}{\pi }^{+}$, (c) $p{K}^{-}{K}^{-}{\pi }^{+}$. The vertical error bars are from data. The dashed histograms are from signal MC. (d): 95% C.L. upper limit of the ${\sigma }_{{\mathcal{B}}^2}$ for ${\mathrm{\Xi }}_{cc}^{++}$ as a function of the mass with a $1\mathrm{MeV}/c^2$ step.