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Measurement of the $CP$ asymmetry in $B^0_s -\overline B {^0 _s}$ mixing

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Abstract

The $CP$ asymmetry in the mixing of $B_s^0$ and $\overline{B}{}_s^0$ mesons is measured in proton-proton collision data corresponding to an integrated luminosity of 3.0 fb$^{-1}$, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Semileptonic $B_s^0$ and $\overline{B}{}_s^0$ decays are studied in the inclusive mode $D_s^{\mp} \mu^{\pm} \nu_{\mu} X$ with the $D_s^{\mp}$ mesons reconstructed in the $K^+ K^- \pi^{\mp}$ final state. Correcting the observed charge asymmetry for detection and background effects, the $CP$ asymmetry is found to be $a_{\text{sl}}^s = (0.39 \pm 0.26 \pm 0.20) \% $, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $a_{\text{sl}}^s$ to date. It is consistent with the prediction from the Standard Model and will constrain new models of particle physics.

Figures and captions

Dalitz plot of the $ D ^{\mp}_ s \rightarrow K ^\pm K ^\mp \pi ^\mp $ decay for selected $ D ^{\mp}_ s \mu^{\pm}$ candidates, with the three selection regions indicated. To suppress combinatorial background, a narrow invariant mass window, between 1950 and 1990 $ {\mathrm{\,MeV\!/}c^2}$ , is required for the $ D ^{\mp}_ s $ candidates in this plot.

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Distributions of $ K ^+ K ^- \pi ^\mp $ mass in the three Dalitz plot regions, summed over both magnet polarities and data-taking periods. Overlaid is the result of the fit, with signal and combinatorial background components as indicated in the legend.

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Overview of the most precise measurements of $ a_{\rm sl}^{d}$ and $ a_{\rm sl}^{s}$ . The horizontal and vertical bands indicate the naive averages of pure $ a_{\rm sl}^{s}$ and $ a_{\rm sl}^{d}$ measurements [22,30,31,32,33,34]. The yellow ellipse represents the D0 dimuon measurement with $\Delta\Gamma_{ d } /\Gamma_{ d } $ set to its SM expectation value [5]. The error bands and contours correspond to 68% confidence level.

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Tables and captions

Overview of contributions in the determination of $ a_{\rm sl}^{s}$ , averaged over Dalitz plot regions, magnet polarities and data taking periods, with their statistical and systematic uncertainties. All numbers are in percent. The central value of $ a_{\rm sl}^{s}$ is calculated according to Eq. 3. The uncertainties are added in quadrature and multiplied by $2/(1- f_{\rm bkg} )$, which is the same for all twelve subsamples, to obtain the uncertainties on $ a_{\rm sl}^{s}$ .

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Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

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supplementary.pdf

This ZIP file contains supplementary material for the publication LHCb-PAPER-2016-013. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps, *.C : The figures in various formats

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