Using pp collision data, corresponding to an integrated luminosity of $1.0 \mathrm{fb}^{1}$, collected by LHCb in 2011 at a centerofmass 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.
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. 
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Animated gif made out of all figures. 
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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. 
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Created on 16 February 2019.Citation count from INSPIRE on 16 February 2019.