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Measurement of the CP-violating phase $\phi_s$ in $\overline{B}^0_s\rightarrow J/\psi \pi^+\pi^-$ decays

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Abstract

The mixing-induced CP-violating 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 centre-of-mass $pp$ collisions at the LHC. A time-dependent flavour-tagged 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.

Figures and captions

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) short-dashed line shows the $B^{\mp}$ background, the (magenta) dot-dashed 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) dot-dashed line is the $\optbar{B}{} ^0\rightarrow J/\psi K^{\mp} \pi^{\pm}$ reflection and the (blue) solid line is the total.

fitm-ft.pdf [50 KiB]
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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]
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mpp6N0.pdf
cospp6N0.pdf [39 KiB]
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cosH6N0.pdf [40 KiB]
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cosH6N0.pdf
chi6N0.pdf [47 KiB]
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chi6N0.pdf

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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etaSS_1d.pdf
etaOS_1d.pdf [32 KiB]
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etaOS_1d.pdf

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]
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fitkst.pdf
eff-ratio.pdf [35 KiB]
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eff-ratio.pdf

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]
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time6NII5.pdf

Animated gif made out of all figures.

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Tables and captions

Acceptance function parameter values and their correlations.

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Systematic uncertainties. The total is the sum in quadrature of each entry.

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Table_2.pdf

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.