First measurements of absolute branching fractions of the ${\mathrm{\Xi }}_c^{+}$ baryon at Belle

We present the first measurements of the absolute branching fractions of ${\mathrm{\Xi }}_c^{+}$ decays into ${\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ and $p{K}^{-}{\pi }^{+}$ final states. Our analysis is based on a data set of $(772\pm 11)\times {10}^{6}B\overline{B}$ pairs collected at the $\mathrm{\Upsilon }(4S)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. We measure the absolute branching fraction of ${\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+}$ with the ${\mathrm{\Xi }}_c^{+}$ recoiling against ${\overline{\mathrm{\Lambda }}}_c^{-}$ in ${\overline{B}}^0$ decays resulting in $\mathcal{B}({\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+})=[1.16\pm 0.42(\mathrm{stat}.)\pm 0.15(\mathrm{syst}.)]\times {10}^{-3}$. We then measure the product branching fractions $\mathcal{B}({\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+})\mathcal{B}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+})$ and $\mathcal{B}({\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+})\mathcal{B}({\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+})$. Dividing these product branching fractions by ${\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+}$ yields $\mathcal{B}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+})=[2.86\pm 1.21(\mathrm{stat}.)\pm 0.38(\mathrm{syst}.)]\%$ and $\mathcal{B}({\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+})=[0.45\pm 0.21(\mathrm{stat}.)\pm 0.07(\mathrm{syst}.)]\%$. Our result for $\mathcal{B}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+})$ can be combined with ${\mathrm{\Xi }}_c^{+}$ branching fractions measured relative to ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ to set the absolute scale for many ${\mathrm{\Xi }}_c^{+}$ branching fractions.

Figure 1

The distribution of $M_{\mathrm{bc}}^{\mathrm{tag}}$ of the $B_{\mathrm{tag}}^0$ versus $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ for selected ${\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+}$ candidates with ${\mathrm{\Xi }}_c^{+}\to \mathrm{anything}$ and ${\overline{\mathrm{\Lambda }}}_c^{-}\to \overline{p}{K}^{-}{\pi }^{+}$ (left) and the fit to the $M_{B_{\mathrm{tag}}^0{\overline{\mathrm{\Lambda }}}_c^{-}}^{\mathrm{rec}}$ distribution (right). The solid box shows the selected signal region. The blue dashed and red dash-dotted boxes define the $M_{\mathrm{bc}}^{\mathrm{tag}}$ and $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ sidebands described in the text. The points with error bars are the data in the signal box, the solid blue curve is the best fit, the dashed curve is the fitted background, the cyan shaded histogram is the normalized $M_{\mathrm{bc}}^{\mathrm{tag}}$ and $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ sidebands, the red open histogram is the sum of the MC-simulated contributions for $e^{+}{e}^{-}\to q\overline{q}$, and $\mathrm{\Upsilon }(4S)\to B\overline{B}$ generic-decay backgrounds with the number of events normalized to the number of events from the normalized $M_{\mathrm{bc}}^{\mathrm{tag}}$ and $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ sidebands.

Figure 2

The distributions of (a) $M_{{\mathrm{\Xi }}_c^{+}}$ versus $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$, and the fits to the (b) $M_{\mathrm{bc}}$ and (c) $\mathrm{\Delta }E$ distributions of the selected ${\overline{B}}^0\to {\overline{\mathrm{\Lambda }}}_c^{-}{\mathrm{\Xi }}_c^{+}$ candidates for (b1-c1) the ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Xi }}^{-}{\pi }^{+}{\pi }^{+}$ and (b2-c2) the ${\mathrm{\Xi }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay modes. In plots (a1-a2), the central solid boxes are the signal regions, and the red dash-dotted and blue dashed boxes show the $M_{{\mathrm{\Xi }}_c^{+}}$ and $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ sidebands used to estimate of the backgrounds (see text). The dots with error bars are the data, the blue solid curves represent the best fits, and the dashed curves represent the fit background contributions. The shaded histograms are the normalized as in the text $M_{{\mathrm{\Xi }}_c^{+}}$ and $M_{{\overline{\mathrm{\Lambda }}}_c^{-}}$ sidebands, the red open histograms represent the generic background described in the caption of Fig. 1.