Measurement of the mixinginduced CPviolating phase phi_s in Bs decays is of prime importance in probing new physics. Here 7421 +/ 105 signal events from the dominantly CPodd final state J/\psi pi+ pi are selected in 1/fb of pp collision data collected at sqrt{s} = 7 TeV with the LHCb detector. A timedependent fit to the data yields a value of phi_s=0.019^{+0.173+0.004}_{0.1740.003} rad, consistent with the Standard Model expectation. No evidence of direct CP violation is found.
% Distributions of the BDT variable for both training and test samples of $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi\pi$ signal and background events. The signal samples are from simulation and the background samples derived from data. 
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Mass distribution of the selected $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ combinations in the $f_{\rm odd}$ region. The blue solid curve shows the result of a fit with a double Gaussian signal (red solid curve) and several background components: combinatorial background (brown dotted line), background from $B^\rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^$ and $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^$ (green shortdashed line), $\overline{ B }{} ^0 \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ (purple dotdashed), $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \eta'$ and $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \phi$ when $\phi\rightarrow \pi^+\pi^\pi^0$ (black dotlongdashed), and $\overline{ B }{} ^0 \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^ \pi^+$ (lightblue longdashed). 
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Mass distribution of selected $\pi^+\pi^$ combinations shown as the (solid black) histogram for events in the $\overline{ B }{} ^0_ s $ signal region. The (dashed red) line shows the background determined by fitting the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ mass in bins of $\pi^+\pi^$ mass. The arrows designate the limits of the $f_{\rm odd}$ region. 
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Decay time distribution of prompt $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ candidates in the $f_0$ region. The dashed (red) line shows the longlived component, and the solid curve the total. 
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Distribution of the estimated time resolution $\sigma_t$ for oppositesign $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ signal events after background subtraction, and for likesign background. 
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(a) Mass distribution of $\overline{ B }{} ^0 \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} $ candidates. The dashed (red) line shows the background, and the solid (blue) curve the total. (b) Decay time distribution, where the small background has been subtracted using the $\overline{ B }{} ^0 $ mass sidebands. The (blue) curve shows the lifetime fit. 
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Decay time distribution of $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} f_0$ candidates fitted with a single exponential function multiplied by the acceptance and convolved with the resolution. The dashed line is signal and the shaded area background. 
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Loglikelihood difference as a function of $\phi_s$ for $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} f_{\rm odd}$ events. 
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Decay time distribution of $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} f_{\rm odd}$ candidates. The solid line shows the result of the fit, the dashed line shows the signal, and the shaded region the background. 
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$ C P$ asymmetry as a function of decay time modulo $2\pi/\Delta m_s$. The curve shows the expectation for $\phi_s=0.019$ rad. 
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Animated gif made out of all figures. 
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Resonance fractions in $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ over the full mass range [11]. The finalstate helicity of the Dwave is denoted by $\Lambda$. Only statistical uncertainties are quoted. 
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Parameters of the decay time resolution function determined from fits to $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ prompt data samples. 
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Parameters used in the functions for the invariant mass and decay time describing the signal and background. These parameters are fixed to their central values in the fit for $\phi_s$. 
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Summary of systematic uncertainties on $\phi_s$. Quantities fixed in the fit that are not included here give negligible uncertainties. The total uncertainty is found by adding in quadrature all the positive and negative contributions separately. 
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Created on 15 June 2019.Citation count from INSPIRE on 15 June 2019.