First observation of $CP$ violation in the decays of $B^0_s$ mesons

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Using pp collision data, corresponding to an integrated luminosity of $1.0 \mathrm{fb}^{-1}$, collected by LHCb in 2011 at a center-of-mass energy of 7 TeV, we report the measurement of direct CP violation in $B^0_s \to K^-\pi^+$ decays, $A_{C/!P}(B^0_s \rightarrow K^- \pi^+)=0.27 \pm 0.04 \mathrm{(stat)} \pm 0.01 \mathrm{(syst)}$, with significance exceeding five standard deviations. This is the first observation of CP violation in the decays of $B^0_s$ mesons. Furthermore, we provide an improved determination of direct CP violation in $B^0 \to K^+\pi^-$ decays, $A_{C/!P}(B^0\rightarrow K^+ \pi^-)=-0.080 \pm 0.007 \mathrm{(stat)} \pm 0.003 \mathrm{(syst)}$, which is the most precise measurement of this quantity to date.

Figures and captions

Invariant mass spectra obtained using the event selection adopted for the best sensitivity on (a, b) $A_{C P}(B^0 \rightarrow K^+\pi^-)$ and (c, d) $A_{C P}(B^0_s \rightarrow K^-\pi^+)$. Panels (a) and (c) represent the $K^+\pi^-$ invariant mass, whereas panels (b) and (d) represent the $K^-\pi^+$ invariant mass. The results of the unbinned maximum likelihood fits are overlaid. The main components contributing to the fit model are also shown.

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Raw asymmetries as a function of the decay time for (a) $B^0 \rightarrow K^+\pi^-$ and (b) $B^0_s \rightarrow K^-\pi^+$ decays. In (b), the offset $t_0=1.5$ $ {\rm ps}$ corresponds to the minimum value of the decay time required by the $B^0_s \rightarrow K^-\pi^+$ event selection. The curves represent the asymmetry projections of fits to the decay time spectra.

asymAPBd.pdf [94 KiB]
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asymAPBs.pdf [110 KiB]
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Animated gif made out of all figures.


Tables and captions

Systematic uncertainties on $A_{C P}(B^0 \rightarrow K^+ \pi^-)$ and $A_{C P}(B^0_s \rightarrow K^-\pi^+)$. The total systematic uncertainties are obtained by summing the individual contributions in quadrature.

Table_1.pdf [54 KiB]
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Created on 20 February 2021.