# First observation and measurement of the branching fraction for the decay $B^0_s \to D_s^{*\mp} K^{\pm}$

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## Abstract

The first observation of the $B^0_s \to D_s^{*\mp} K^{\pm}$ decay is reported using 3.0$fb^{-1}$ of proton-proton collision data collected by the LHCb experiment. The $D_s^{*\mp}$ mesons are reconstructed through the decay chain $D_s^{*\mp} \to \gamma D_s^{\mp}(K^{\mp}K^{\pm}\pi^{\mp})$. The branching fraction relative to that for $B^0_s \to D_s^{*-} \pi^{+}$ is measured to be $0.068 \pm 0.005 ^{+0.003}_{-0.002}$, where the first uncertainty is statistical and the second is systematic. Using a recent measurement of $BR(B^0_s \to D_s^{*-} \pi^{+})$ the absolute branching fraction of $B^0_s \to D_s^{*\mp} K^{\pm}$ is measured as ( 16.3 $\pm$ 1.2 (stat) $^{+0.7}_{-0.5}$ (syst) $\pm$ 4.8 (norm) ) $\times$ 10$^{-5}$, where the third uncertainty is due to the uncertainty on the branching fraction of the normalisation channel.

## Figures and captions

 Feynman diagrams of the processes under study. The upper diagrams represent the two tree topologies ($b \rightarrow c$ and $b \rightarrow u$ transitions, respectively) by which a $B ^0_ s$ meson decays into the $D ^{*\mp}_ s$ $K ^\pm$ final state; the lower diagrams show the tree diagram of $B ^0_ s$ $\rightarrow$ $D ^{*-}_ s$ $\pi ^+$ and the $W$ -exchange topology of $B ^0_ s$ $\rightarrow$ $D ^{*-}_ s$ $K ^+$ . fig-1a.pdf [89 KiB] HiDef png [66 KiB] Thumbnail [32 KiB] *.C file fig-1b.pdf [89 KiB] HiDef png [62 KiB] Thumbnail [31 KiB] *.C file fig-1c.pdf [89 KiB] HiDef png [59 KiB] Thumbnail [29 KiB] *.C file fig-1d.pdf [89 KiB] HiDef png [99 KiB] Thumbnail [45 KiB] *.C file (left) The $K ^-$ $K ^+$ $\pi ^-$ invariant mass and (right) mass difference $\Delta_{M}$ of the $B ^0_ s$ $\rightarrow$ $D ^{*-}_ s$ $\pi ^+$ candidates. The points represent data. On the right plot the solid line represents the signal expected from the simulations. DsDeltaM.pdf [22 KiB] HiDef png [157 KiB] Thumbnail [145 KiB] *.C file Invariant mass distribution of (top) $B ^0_ s$ $\rightarrow$ $D ^{*-}_ s$ $\pi ^+$ and (bottom) $B ^0_ s$ $\rightarrow$ $D ^{*\mp}_ s$ $K ^\pm$ candidates with fit results superimposed. The fitted signal corresponding to the first observation of $B ^0_ s$ $\rightarrow$ $D ^{*\mp}_ s$ $K ^\pm$ is shown by the dotted line in the lower plot. fitPi-[..].pdf [37 KiB] HiDef png [307 KiB] Thumbnail [237 KiB] *.C file fitK-n[..].pdf [39 KiB] HiDef png [385 KiB] Thumbnail [289 KiB] *.C file Distributions of (left) $\eta$ and (right) $p_{\rm T}$ of the photons for the $D ^{*-}_ s$ $\pi ^+$ (blue) and $D ^{*\mp}_ s$ $K ^\mp$ (magenta) decays. Data, background-subtracted using the sPlot method, are represented by points, and simulations by solid lines. datavs[..].pdf [19 KiB] HiDef png [158 KiB] Thumbnail [122 KiB] *.C file Animated gif made out of all figures. PAPER-2015-008.gif Thumbnail

## Tables and captions

 Estimated systematic uncertainties on ${\cal R}^*$. Table_1.pdf [62 KiB] HiDef png [93 KiB] Thumbnail [43 KiB] tex code

Created on 16 August 2019.Citation count from INSPIRE on 20 August 2019.