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

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## Abstract

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. cpvall.pdf [431 KiB] HiDef png [671 KiB] Thumbnail [102 KiB] *.C file 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] HiDef png [77 KiB] Thumbnail [25 KiB] *.C file asymAPBs.pdf [110 KiB] HiDef png [104 KiB] Thumbnail [32 KiB] *.C file Animated gif made out of all figures. PAPER-2013-018.gif Thumbnail

## 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_I.pdf [54 KiB] HiDef png [116 KiB] Thumbnail [13 KiB] tex code

Created on 16 February 2019.Citation count from INSPIRE on 16 February 2019.