Measurement of $CP$ violation in $B^0 \rightarrow J/\psi K^0_S$ decays

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Measurements are presented of the $CP$ violation observables $S$ and $C$ in the decays of $B^0$ and $\overline{B}{}^0$ mesons to the $J/\psi K^0_S$ final state. The data sample corresponds to an integrated luminosity of $3.0 \text{fb}^{-1}$ collected with the LHCb experiment in proton-proton collisions at center-of-mass energies of $7$ and $8 \text{TeV}$. The analysis of the time evolution of $41500$ $B^0$ and $\overline{B}{}^0$ decays yields $S = 0.731 \pm 0.035 \text{(stat)} \pm 0.020 \text{(syst)}$ and $C = -0.038 \pm 0.032 \text{(stat)} \pm 0.005 \text{(syst)}$. In the Standard Model, $S$ equals $\sin(2\beta)$ to a good level of precision. The values are consistent with the current world averages and with the Standard Model expectations.

Figures and captions

Distribution of (a) the reconstructed mass and (b) logarithmic distribution of the decay time of tagged $B^0 \rightarrow {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} {K^0_{\rm\scriptscriptstyle S}}$ candidates. The solid black lines show the fit projections, while the dashed (dotted) lines show the projections for the signal (background) components only.

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Time-dependent signal-yield asymmetry $(N_{{\overline{ B}}{}^0} - N_{B^0})/(N_{{\overline{ B}}{}^0} + N_{B^0})$. Here, $N_{B^0}$ ($N_{{\overline{ B}}{}^0}$) is the number of $B^0 \rightarrow {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} {K^0_{\rm\scriptscriptstyle S}}$ decays with a $B^0$ (${\overline{ B}}{}^0$) flavor tag. The data points are obtained with the sPlot technique [37], assigning signal weights to the events based on a fit to the reconstructed mass distribution. The solid curve is the projection of the signal PDF.

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(a) Raw mixing asymmetry $(N_{\text{unmixed}} - N_{\text{mixed}})/(N_{\text{unmixed}} + N_{\text{mixed}})$ for all SS$\pi$ tagged $B^0 \rightarrow {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} K^{*0}$ decay candidates. Here, $N_{\text{unmixed}}$ ($N_{\text{mixed}}$) is the number of $B^0 \rightarrow {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} K^{*0}$ decays with a final state that does (not) correspond to the flavor tag. The black line shows the fit projection. (b) The linear calibration of the SS$\pi$ mistag probability with $B^0 \rightarrow {J\mskip -3mu/\mskip -2mu\psi\mskip 2mu} K^{*0}$ decays.

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Animated gif made out of all figures.


Tables and captions

Systematic uncertainties $\sigma_{S}$ and $\sigma_{C}$ on $S$ and $C$. Entries marked with a dash represent studies where no significant effect is observed.

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Created on 18 January 2020.