The mixinginduced CPviolating phase $\phi_s$ in ${B}^0_s$ and $\overline{B}^0_s$ decays is measured using the $J/\psi \pi^+\pi^$ final state in data, taken from 3 fb$^{1}$ of integrated luminosity, collected with the LHCb detector in 7 and 8 TeV centreofmass $pp$ collisions at the LHC. A timedependent flavourtagged amplitude analysis, allowing for direct \CP violation, yields a value for the phase $\phi_s=70\pm 68\pm 8$ mrad. This result is consistent with the Standard Model expectation and previous measurements.
Invariant mass of $J/\psi \pi^+\pi^$ combinations. The data are fitted with double Crystal Ball signal functions and several background functions. The (red) solid line shows the $\optbar{B}{} ^0_{\!\!s}$ signal, the (brown) dotted line shows the exponential combinatorial background, the (green) shortdashed line shows the $B^{\mp}$ background, the (magenta) dotdashed line shows the $\optbar{B}{} ^0$ signal, the (light blue) dashed line is the sum of $\optbar{B}{} ^0_{\!\!s}\rightarrow J/\psi\eta'$, $\optbar{B}{} ^0_{\!\!s}\rightarrow J/\psi\phi$, $\phi\rightarrow \pi^+\pi^\pi^0$ backgrounds, and the $\Lambda ^0_ b \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^ p$ plus $\ensuremath\overline{{\Lambda ^0_ b }} \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^+\overline{p}$ reflections, the (black) dotdashed line is the $\optbar{B}{} ^0\rightarrow J/\psi K^{\mp} \pi^{\pm}$ reflection and the (blue) solid line is the total. 
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Projections of (a) $m(\pi^+\pi^)$, (b) $\cos\theta_{\pi\pi}$, (c) $\cos \theta_{J/\psi}$ and (d) $\chi$ [13]. The points with error bars are data, the signal fits are shown with (red) dashed lines, the background with a (black) dotted lines, and the (blue) solid lines represent the total fits. The difference between the data and the fits divided by the uncertainty on the data is shown below. 
mpp6N0.pdf [40 KiB] HiDef png [428 KiB] Thumbnail [80 KiB] *.C file 

cospp6N0.pdf [39 KiB] HiDef png [368 KiB] Thumbnail [78 KiB] *.C file 

cosH6N0.pdf [40 KiB] HiDef png [376 KiB] Thumbnail [81 KiB] *.C file 

chi6N0.pdf [47 KiB] HiDef png [312 KiB] Thumbnail [69 KiB] *.C file 

Estimated fractions of mistag probabilities from (a) the SSK tagger, $\eta^{\rm SSK}$, and (b) the OS tagger, $\eta^{\rm OS}$. 
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etaOS_1d.pdf [32 KiB] HiDef png [170 KiB] Thumbnail [45 KiB] *.C file 

Distributions of (a) decay time of $\optbar{B}{} ^0 \rightarrow J/\psi \optbar{K}{} ^{\!*0}$ candidates in data, (b) ratio of time acceptance between $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ and $\overline{ B }{} ^0 \rightarrow J/\psi \overline{ K }{} ^{*0} $ decays from simulation. 
fitkst.pdf [35 KiB] HiDef png [271 KiB] Thumbnail [53 KiB] *.C file 

effratio.pdf [35 KiB] HiDef png [288 KiB] Thumbnail [61 KiB] *.C file 

Decay time distribution of $\optbar{B}{} ^0\rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ candidates. The signal PDF is shown with a (red) dashed line, the background with a (black) dotted line, and the (blue) solid line represents the total. 
time6NII5.pdf [39 KiB] HiDef png [429 KiB] Thumbnail [83 KiB] *.C file 

Animated gif made out of all figures. 
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Acceptance function parameter values and their correlations. 
Table_1.pdf [45 KiB] HiDef png [81 KiB] Thumbnail [11 KiB] tex code 

Systematic uncertainties. The total is the sum in quadrature of each entry. 
Table_2.pdf [40 KiB] HiDef png [113 KiB] Thumbnail [17 KiB] tex code 
Created on 15 February 2019.Citation count from INSPIRE on 15 February 2019.