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First observation of the decay $B_s^0 \rightarrow \phi \bar{K}^{*0}$

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Abstract

The first observation of the decay $B_s^0 \rightarrow \phi \bar{K}^{*0}$ is reported. The analysis is based on a data sample corresponding to an integrated luminosity of 1.0 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 7 TeV$, collected with the LHCb detector. A yield of $30 \pm 6$ $B_s^0 \rightarrow (K^+K^-)(K^-\pi^+)$ decays is found in the mass windows $1012.5 < M(K^+K^-) < 1026.5 MeV/c^2$ and $746 < M(K^-\pi^+)< 1046 MeV/c^2$. The signal yield is found to be dominated by $B_s^0 \rightarrow \phi \bar{K}^{*0}$ decays, and the corresponding branching fraction is measured to be ${\cal B}(B_s^0 \rightarrow \phi \bar{K}^{*0}) = (1.10 \pm 0.24 (stat) \pm 0.14 (syst) \pm 0.08 (f_d/f_s)) \times 10^{-6}$, where the uncertainties are statistical, systematic and from the ratio of fragmentation fractions $f_d/f_s$ which accounts for the different production rate of $B^0$ and $B_s^0$ mesons. The significance of $B_s^0 \rightarrow \phi \bar{K}^{*0}$ signal is 6.1 standard deviations. The fraction of longitudinal polarization in $B_s^0 \rightarrow \phi \bar{K}^{*0}$ decays is found to be $f_0 = 0.51 \pm 0.15 (stat) \pm 0.07 (syst)$.

Figures and captions

Feynman diagrams for the $ B ^0_ s \rightarrow \phi \overline{ K }{} ^{*0} $ and the $ B ^0 \rightarrow \phi K ^{*0} $ decays.

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penguinBphiKst.pdf
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Four-body $ K^+K^- K^-\pi^+ $ invariant mass distribution. The points show the data, the blue solid line shows the overall fit, the solid dark red shaded region is the $ B ^0_ s \rightarrow \phi \overline{ K }{} ^{*0} $ signal, the light blue shaded region corresponds to the $ B ^0 \rightarrow \phi K ^{*0} $ signal, the grey dotted line is the combinatorial background and the green dashed line and magenta dashed-dotted lines are the partially reconstructed and misidentified $\Lambda ^0_ b $ backgrounds.

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four-body-mass.pdf

Invariant mass distributions for (left) $ K^+K^-$ and (right) $K^{\mp}\pi^{\pm}$ pairs in a $\pm 30 {\mathrm{\,MeV\!/}c^2} $ window around the (top) $ B ^0_ s $ and (bottom) $ B ^0$ mass. The solid blue line is the overall fit, the green dashed line corresponds to $ B ^0$ cross-feed into the $ B ^0_ s $ mass window, the red dotted line is the {\rm S-wave} contribution and the light blue is the combinatorial background.

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masskkbs.pdf
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masskpibd.pdf

Definition of the angles in $ B ^0_ s \rightarrow \phi \overline{ K }{} ^{*0} $ decays where $\theta_1$ ($\theta_2$) is the $K^+$ ($K^-$) emission angle with respect to the direction opposite to the $ B ^0_ s $ meson in the $\phi$ ( $\overline{ K }{} ^{*0}$ ) rest frame and $\varphi$ is the angle between the $\overline{ K }{} ^{*0}$ and $\phi$ decay planes in the $ B ^0_ s $ rest frame.

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angles_Bs.pdf

Result of the fit to the angular distribution of the $ B ^0_ s \rightarrow \phi \overline{ K }{} ^{*0} $ candidates in (left) $\cos\theta_1$ and (right) $\cos\theta_2$. The red dotted line corresponds to the combinatorial background under the $ B ^0_ s $ signal, the green dashed line is the $ B ^0 \rightarrow \phi K ^{*0} $ signal in the $ B ^0_ s $ region and the grey dotted-dashed line corresponds to the sum of the {\rm S-wave} and the interference terms.

cth1.pdf [17 KiB]
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cth1.pdf
cth2.pdf [17 KiB]
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Animated gif made out of all figures.

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Tables and captions

Results of the fit to the sample of selected candidates.

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Table_1.pdf

Input values for the branching fraction computation.

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Sources of systematic uncertainty in the branching fraction measurement. The total uncertainty is the addition in quadrature of the individual sources.

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Table_3.pdf

Systematic uncertainties of the angular parameters.

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Table_4.pdf

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.