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Study of $B_{\scriptscriptstyle (s)}^0 \to K_{\rm \scriptscriptstyle S}^0 h^{+} h^{\prime -}$ decays with first observation of $B_{\scriptscriptstyle s}^0 \to K_{\rm \scriptscriptstyle S}^0 K^{\pm} \pi^{\mp}$ and $B_{\scriptscriptstyle s}^0 \to K_{\rm \scriptscriptstyle S}^0 \pi^{+} \pi^{-}$

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Abstract

A search for charmless three-body decays of $B^0$ and $B_{s}^0$ mesons with a $K_{\rm S}^0$ meson in the final state is performed using the $pp$ collision data, corresponding to an integrated luminosity of $1.0 fb^{-1}$, collected at a centre-of-mass energy of $7\mathrm{ Te V}$ recorded by the LHCb experiment. Branching fractions of the $B_{(s)}^0 \to K_{\rm S}^0 h^{+} h^{\prime -}$ decay modes ($h^{(\prime)} = \pi, K$), relative to the well measured $B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-}$ decay, are obtained. First observation of the decay modes $B_s^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp}$ and $B_s^0 \to K_{\rm S}^0 \pi^{+} \pi^{-}$ and confirmation of the decay $B^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp}$ are reported. The following relative branching fraction measurements or limits are obtained \begin{eqnarray*} \nonumber \frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp})}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &=& 0.128 \pm 0.017 ({\rm stat.}) \pm 0.009 ({\rm syst.}) , \\ \nonumber \frac{{\cal B}(B^0 \to K_{\rm S}^0 K^{+} K^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &=& 0.385 \pm 0.031 ({\rm stat.}) \pm 0.023 ({\rm syst.}) , \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 \pi^{+} \pi^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &=& 0.29\phantom{0} \pm 0.06\phantom{0} ({\rm stat.}) \pm 0.03\phantom{0} ({\rm syst.}) \pm 0.02 (f_s/f_d) , \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 K^{\pm} \pi^{\mp})}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &=& 1.48\phantom{0} \pm 0.12\phantom{0} ({\rm stat.}) \pm 0.08\phantom{0} ({\rm syst.}) \pm 0.12 (f_s/f_d) , \\ \nonumber \frac{{\cal B}(B_s^0 \to K_{\rm S}^0 K^{+} K^{-} )}{{\cal B}(B^0 \to K_{\rm S}^0 \pi^{+} \pi^{-})} &\in& [0.004;0.068] \; {\rm at \;\; 90\% \; CL} . \end{eqnarray*}

Figures and captions

Invariant mass distributions of (top) $ K ^0_{\rm\scriptscriptstyle S} K ^+ K ^- $ , (middle) $ K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ , and (bottom) $ K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^- $ candidate events, with the loose selection for (left) Downstream and (right) Long $ K ^0_{\rm\scriptscriptstyle S}$ reconstruction categories. In each plot, data are the black points with error bars and the total fit model is overlaid (solid black line). The $ B ^0$ ( $ B ^0_ s $ ) signal components are the black short-dashed (dotted) lines, while fully reconstructed misidentified decays are the black dashed lines close to the $ B ^0$ and $ B ^0_ s $ peaks. The partially reconstructed contributions from $ B $ to open charm decays, charmless hadronic decays, $ B ^0 \rightarrow \eta ^{\prime} (\rightarrow \rho ^0 \gamma) K ^0_{\rm\scriptscriptstyle S} $ and charmless radiative decays are the red dash triple-dotted, the blue dash double-dotted, the violet dash single-dotted, and the pink short-dash single-dotted lines, respectively. The combinatorial background contribution is the green long-dash dotted line.

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Invariant mass distributions of (top) $ K ^0_{\rm\scriptscriptstyle S} K ^+ K ^- $ , (middle) $ K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ , and (bottom) $ K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^- $ candidate events, with the tight selection for (left) Downstream and (right) Long $ K ^0_{\rm\scriptscriptstyle S}$ reconstruction categories. In each plot, data are the black points with error bars and the total fit model is overlaid (solid black line). The $ B ^0$ ( $ B ^0_ s $ ) signal components are the black short-dashed (dotted) lines, while fully reconstructed misidentified decays are the black dashed lines close to the $ B ^0$ and $ B ^0_ s $ peaks. The partially reconstructed contributions from $ B $ to open charm decays, charmless hadronic decays, $ B ^0 \rightarrow \eta ^{\prime} (\rightarrow \rho ^0 \gamma) K ^0_{\rm\scriptscriptstyle S} $ and charmless radiative decays are the red dash triple-dotted, the blue dash double-dotted, the violet dash single-dotted, and the pink short-dash single-dotted lines, respectively. The combinatorial background contribution is the green long-dash dotted line.

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Likelihood profiles of the $ B ^0_ s \rightarrow K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^- $ signal yield for the (left) Downstream and (right) Long $ K ^0_{\rm\scriptscriptstyle S}$ samples. The dashed red line is the statistical-only profile, while the solid blue line also includes the fit model systematic uncertainties. The significance of the Downstream and Long signals are $3.4 \sigma$ and $4.8 \sigma$, respectively, including systematic uncertainties. Combining Downstream and Long $ K ^0_{\rm\scriptscriptstyle S}$ samples, an observation with $5.9 \sigma$, including systematic uncertainties, is obtained.

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Efficiency-corrected Dalitz-plot distributions, produced using the sPlot procedure, of (top) $ B ^0_ s \rightarrow K ^0_{\rm\scriptscriptstyle S} \pi ^+ \pi ^- $ , (middle) $ B ^0_ s \rightarrow K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ and (bottom) $ B ^0 \rightarrow K ^0_{\rm\scriptscriptstyle S} K ^\pm \pi ^\mp $ events. Bins with negative content appear empty.

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Animated gif made out of all figures.

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Tables and captions

Yields obtained from the simultaneous fit corresponding to the chosen optimisation of the selection for each mode, where the uncertainties are statistical only. The average selection efficiencies are also given for each decay mode, where the uncertainties are due to the limited simulation sample size.

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Systematic uncertainties on the ratio of branching fractions for Downstream and Long $ K ^0_{\rm\scriptscriptstyle S}$ reconstruction. All uncertainties are relative and are quoted as percentages.

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Created on 17 August 2019.Citation count from INSPIRE on 20 August 2019.