# 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. pengui[..].pdf [9 KiB] HiDef png [198 KiB] Thumbnail [38 KiB] *.C file pengui[..].pdf [9 KiB] HiDef png [196 KiB] Thumbnail [37 KiB] *.C file 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. four-b[..].pdf [49 KiB] HiDef png [871 KiB] Thumbnail [86 KiB] *.C file 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. masskkbs.pdf [18 KiB] HiDef png [212 KiB] Thumbnail [69 KiB] *.C file masskpibs.pdf [18 KiB] HiDef png [201 KiB] Thumbnail [65 KiB] *.C file masskkbd.pdf [16 KiB] HiDef png [202 KiB] Thumbnail [72 KiB] *.C file masskpibd.pdf [16 KiB] HiDef png [194 KiB] Thumbnail [67 KiB] *.C file 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. angles_Bs.pdf [9 KiB] HiDef png [130 KiB] Thumbnail [48 KiB] *.C file 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] HiDef png [214 KiB] Thumbnail [72 KiB] *.C file cth2.pdf [17 KiB] HiDef png [217 KiB] Thumbnail [72 KiB] *.C file Animated gif made out of all figures. PAPER-2013-012.gif Thumbnail

## Tables and captions

 Results of the fit to the sample of selected candidates. Table_1.pdf [52 KiB] HiDef png [80 KiB] Thumbnail [12 KiB] tex code Input values for the branching fraction computation. Table_2.pdf [73 KiB] HiDef png [70 KiB] Thumbnail [11 KiB] tex code Sources of systematic uncertainty in the branching fraction measurement. The total uncertainty is the addition in quadrature of the individual sources. Table_3.pdf [45 KiB] HiDef png [68 KiB] Thumbnail [11 KiB] tex code Systematic uncertainties of the angular parameters. Table_4.pdf [38 KiB] HiDef png [60 KiB] Thumbnail [9 KiB] tex code

Created on 23 February 2019.Citation count from INSPIRE on 23 February 2019.