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Measurement of the time-dependent CP asymmetries in $B_s^0\rightarrow J/\psi K_{\rm S}^0$

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Abstract

The first measurement of decay-time-dependent CP asymmetries in the decay $B_s^0\rightarrow J/\psi K_{\rm S}^0$ and an updated measurement of the ratio of branching fractions $\mathcal{B}(B_s^0\rightarrow J/\psi K_{\rm S}^0)/\mathcal{B}(B_d^0\rightarrow J/\psi K_{\rm S}^0)$} are presented. The results are obtained using data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions recorded with the LHCb detector at centre-of-mass energies of 7 and 8 TeV. The results on the CP asymmetries are $A_{\Delta\Gamma}(B_s^0\rightarrow J/\psi K_{\rm S}^0) = 0.49_{-0.65}^{+0.77}(stat) \pm 0.06(syst)$, $C_{\rm dir}(B_s^0\rightarrow J/\psi K_{\rm S}^0) = -0.28 \pm 0.41(stat) \pm 0.08(syst)$ and $S_{\rm mix}(B_s^0\rightarrow J/\psi K_{\rm S}^0) = -0.08 \pm 0.40(stat) \pm 0.08(syst)$. The ratio $\mathcal{B}(B_s^0\rightarrow J/\psi K_{\rm S}^0)/\mathcal{B}(B_d^0\rightarrow J/\psi K_{\rm S}^0)$ is measured to be $0.0431 \pm 0.0017(stat) \pm 0.0012(syst) \pm 0.0025(f_s/f_d)$, where the last uncertainty is due to the knowledge of the $B_s^0$ and $B_d^0$ production fractions.

Figures and captions

Decay topologies contributing to the $ B_{(s)}^0\!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} {} K ^0_{\rm\scriptscriptstyle S} $ channel: (left) tree diagram and (right) penguin diagram.

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Mass distribution of $ B $ candidates at different stages of the event selection for the (left) long $ K ^0_{\rm\scriptscriptstyle S}$ and (right) downstream $ K ^0_{\rm\scriptscriptstyle S}$ sample. The data sample after initial selection (red, $+$), after the first neural net (green, $\times$) and after the second neural net (black, $\bullet$) are shown. Overlaid are projections of the fit described in Sec. 5. Shown components are $ B ^0_ s \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^0_{\rm\scriptscriptstyle S} $ (dark blue, dashed), $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^0_{\rm\scriptscriptstyle S} $ (red, dotted) and combinatorial background (turquoise, dash-dotted).

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Decay time distribution of $ B $ candidates in the (left) long $ K ^0_{\rm\scriptscriptstyle S}$ and (right) downstream $ K ^0_{\rm\scriptscriptstyle S}$ sample. The fit projection is shown as solid black line. Shown components are $ B ^0_ s \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^0_{\rm\scriptscriptstyle S} $ (dark blue, dashed), $ B ^0 \!\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^0_{\rm\scriptscriptstyle S} $ (red, dotted) and combinatorial background (turquoise, dash-dotted).

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Tables and captions

List of the observables describing the $ B ^0$ and $ B ^0_ s $ systems that are included as Gaussian constraints to the likelihood fit.

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Effective $ B $ production asymmetries specific to the data sample used in this analysis.

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Fitted yields from the unbinned maximum likelihood fit. The uncertainties are statistical only.

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Summary of systematic uncertainties.

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Supplementary Material [file]

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2015-005. The files are: Supplementary.pdf : Results from Feldman--Cousins Scan + Correlation matrix Fig4a.*, Fig4b.* : The 1-CL plots from the Feldman--Cousins Scan

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Created on 15 February 2019.Citation count from INSPIRE on 15 February 2019.