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Measurement of $CP$ asymmetry in $B^0_s \rightarrow D^{\mp}_s K^{\pm}$ decays

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Abstract

We report on measurements of the time-dependent CP violating observables in $B^0_s\rightarrow D^{\mp}_s K^{\pm}$ decays using a dataset corresponding to 1.0 fb$^{-1}$ of pp collisions recorded with the LHCb detector. We find the CP violating observables $C_f=0.53\pm0.25\pm0.04$, $A^{\Delta\Gamma}_f=0.37\pm0.42\pm0.20$, $A^{\Delta\Gamma}_{\bar{f}}=0.20\pm0.41\pm0.20$, $S_f=-1.09\pm0.33\pm0.08$, $S_{\bar{f}}=-0.36\pm0.34\pm0.08$, where the uncertainties are statistical and systematic, respectively. Using these observables together with a recent measurement of the $B^0_s$ mixing phase $-2\beta_s$ leads to the first extraction of the CKM angle $\gamma$ from $B^0_s \rightarrow D^{\mp}_s K^{\pm}$ decays, finding $\gamma$ = (115$_{-43}^{+28}$)$^\circ$ modulo 180$^\circ$ at 68% CL, where the error contains both statistical and systematic uncertainties.

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Feynman diagrams for $\overline{ B }{} {}^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^+ K^-$ without (left) and with (right) $ B ^0_ s $ mixing.

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Mass distributions for $ D ^-_ s $ candidates passing (black, open circles) and failing (red, crosses) the PID selection criteria. In reading order: $ D ^-_ s \!\rightarrow K ^- K ^+ \pi ^- $ , $ D ^-_ s \!\rightarrow \pi ^- \pi ^+ \pi ^- $ , and $ D ^-_ s \!\rightarrow K ^- \pi ^+ \pi ^- $ .

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The multivariate fit to the (left) $ B ^0_ s \!\rightarrow D ^-_ s \pi ^+ $ and (right) $ B ^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^\mp K^\pm$ candidates for all $ D ^-_ s $ decay modes combined. From top to bottom: distributions of candidates in $ B ^0_ s $ mass, $ D ^-_ s $ mass, companion PID log-likelihood difference. The solid, blue, line represents the sum of the fit components.

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Measured mistag rate against the average predicted mistag rate for the (left) OS and (right) SSK taggers in $ B ^0_ s \!\rightarrow D ^-_ s \pi ^+ $ decays. The error bars represent only the statistical uncertainties. The solid curve is the linear fit to the data points, the shaded area defines the 68% confidence level region of the calibration function (statistical only).

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Result of the \it sFit to the decay-time distribution of $ B ^0_ s \!\rightarrow D ^-_ s \pi ^+ $ candidates, which is used to measure the decay-time acceptance in $ B ^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^\mp K^\pm$ decays. The solid curve is measured decay-time acceptance.

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Result of the decay-time (top left) \it sFit and (top right) \it cFit to the $ B ^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^\mp K^\pm$ candidates; the \it cFit plot groups $ B ^0_ s \!\rightarrow D ^{*-}_ s \pi ^+ $ and $ B ^0_ s \!\rightarrow D ^-_ s \rho^{+}$ , and also groups $ B ^0 \!\rightarrow D ^- K ^+ $ , $ B ^0 \!\rightarrow D ^- \pi ^+ $ , $\overline{\Lambda} {}^0_ b \!\rightarrow \overline{\Lambda} {}^-_ c K ^+ $ , $\overline{\Lambda} {}^0_ b \!\rightarrow \overline{\Lambda} {}^-_ c \pi ^+ $ , $\Lambda ^0_ b \!\rightarrow D ^-_ s p$ , $\Lambda ^0_ b \!\rightarrow D ^{*-}_ s p$ , and $ B ^0 \!\rightarrow D ^-_ s K ^+ $ together for the sake of clarity. The folded asymmetry plots for (bottom left) $ D ^+_ s K^-$, and (bottom right) $ D ^-_ s K^+$ are also shown.

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Graph showing $ 1-{\rm CL}$ for $\gamma$ , together with the central value and the 68.3% CL interval as obtained from the frequentist method described in the text (top). Profile likelihood contours of $ r_{ D_{\hspace{-0.0625em}s} \hspace{-0.0625em} K }$ vs. $\gamma$ (bottom left), and $\delta$ vs. $\gamma$ (bottom right). The contours are the $1\sigma$ ($2\sigma$) profile likelihood contours, where $\Delta\chi^2=1$ $(\Delta\chi^2=4)$, corresponding to 39% CL (86% CL) in Gaussian approximation. The markers denote the best-fit values.

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Animated gif made out of all figures.

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Tables and captions

Calibration parameters of the combined OS tagger extracted from different control channels. In each entry the first uncertainty is statistical and the second systematic.

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Flavour tagging performance for the three different tagging categories for $ B ^0_ s \!\rightarrow D ^-_ s \pi ^+ $ candidates.

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Fitted values of the $ C\!P$ observables to the $ B ^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^\mp K^\pm$ time distribution for (left) \it sFit and (right) \it cFit , where the first uncertainty is statistical, the second is systematic. All parameters other than the $ C\!P$ observables are constrained in the fit.

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Statistical correlation matrix of the $ B ^0_ s \!\rightarrow D_{\hspace{-0.0625em}s} ^\mp K^\pm$ (top) \it sFit and (bottom) {\it cFit } $ C\!P$ parameters. Other fit parameters have negligible correlations with the $ C\!P$ parameters and are omitted for brevity.

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Systematic errors, relative to the statistical error, for (top) \it sFit and (bottom) \it cFit . The daggered contributions ( $\Gamma_{ s }$ , $\Delta\Gamma_{ s }$ ) are given separately for comparison (see text) with the other uncertainties and are not added in quadrature to produce the total.

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Systematic uncertainty correlations for (top) \it sFit and (bottom) \it cFit .

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2014-038. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.jpg, *.eps, *.root, *.C: The figures in variuous formats

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