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Precision measurement of $CP$ violation in $B_s^0 \to J/\psi K^+K^-$ decays

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Abstract

The time-dependent $CP$ asymmetry in $B_s^0 \to J/\psi K^+K^-$ decays is measured using $pp$ collision data, corresponding to an integrated luminosity of $3.0$fb$^{-1}$, collected with the LHCb detector at centre-of-mass energies of $7$ and $8$TeV. In a sample of 96 000 $B_s^0 \to J/\psi K^+K^-$ decays, the $CP$-violating phase $\phi_s$ is measured, as well as the decay widths $\Gamma_{L}$ and $\Gamma_{H}$ of the light and heavy mass eigenstates of the $B_s^0-\bar{B}_s^0$ system. The values obtained are $\phi_s = -0.058 \pm 0.049 \pm 0.006$ rad, $\Gamma_s \equiv (\Gamma_{L}+\Gamma_{H})/2 = 0.6603 \pm 0.0027 \pm 0.0015$ps$^{-1}$, and$\Delta\Gamma_s \equiv \Gamma_{L} - \Gamma_{H} = 0.0805 \pm 0.0091 \pm 0.0032$ps$^{-1}$, where the first uncertainty is statistical and the second systematic. These are the most precise single measurements of those quantities to date. A combined analysis with $B_s^{0} \to J/\psi \pi^+\pi^-$ decays gives $\phi_s = -0.010 \pm 0.039 $rad. All measurements are in agreement with the Standard Model predictions. For the first time the phase $\phi_s$ is measured independently for each polarisation state of the $K^+K^- $ system and shows no evidence for polarisation dependence.

Figures and captions

(a) Background-subtracted invariant mass distributions of the $K^{+}K^{-}$ system in the selected $ B ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^{+}K^{-}$ candidates (black points). The vertical red lines denote the boundaries of the six bins used in the maximum likelihood fit. (b) Distribution of $m( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ K ^- )$ for the data sample (black points) and projection of the maximum likelihood fit (blue line). The $ B ^0_ s $ signal component is shown by the red dashed line and the combinatorial background by the green long-dashed line. Background from misidentified $ B ^0$ and $\Lambda ^0_ b $ decays is subtracted, as described in the text.

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Decay-time and helicity-angle distributions for $ B ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ K ^- $ decays (data points) with the one-dimensional fit projections overlaid. The solid blue line shows the total signal contribution, which is composed of $ C\!P$ -even (long-dashed red), $ C\!P$ -odd (short-dashed green) and S-wave (dotted-dashed purple) contributions.

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Tables and captions

Values of the principal physics parameters determined from the polarisation-independent fit.

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Values of the polarisation-dependent parameters $\phi_s^k$ and $|\lambda^k|$ determined from the polarisation-dependent fit.

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Statistical and systematic uncertainties for the polarisation-independent result.

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Statistical and systematic uncertainties for the polarisation-dependent result.

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Statistical correlation matrix from the polarisation-independent fit.

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Total statistical and systematic correlation matrix from the polarisation-independent fit.

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Statistical correlation matrix for the $\phi_s^{k}$ and $|\lambda^k|$ parameters from the polarisation-dependent fit.

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Total statistical and systematic correlation matrix for the $\phi_s^{k}$ and $|\lambda^k|$ parameters from the polarisation-dependent fit.

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S-wave parameter estimates in each $m( K ^+ K ^- )$ bin from the polarisation-independent fit. Only statistical uncertainties are shown.

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Supplementary Material [file]

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