# Measurement of the flavour-specific $CP$-violating asymmetry $a_{\rm sl}^s$ in $B_s^0$ decays

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## Abstract

The CP-violating asymmetry a_{sl}^s is studied using semileptonic decays of Bs and anti-Bs mesons produced in pp collisions at a centre-of-mass energy of 7 TeV at the LHC, exploiting a data sample corresponding to an integrated luminosity of 1.0/fb. The reconstructed final states are Ds^(+/-)\mu ^(-/+), with the Ds^(+/-) particle decaying in the \phi\pi^(+/-) mode. The Ds^(+/-)\mu ^{-/+) yields are summed over Bs and anti-Bs initial states, and integrated with respect to decay time. Data-driven methods are used to measure efficiency ratios. We obtain $a_{sl}^s$ = (-0.06 +/- 0.50 +/- 0.36)%, where the first uncertainty is statistical and the second systematic.

## Figures and captions

 Invariant mass distributions for: (a) $K ^+ K ^- \pi ^+$ and (b) $K ^+ K ^- \pi ^-$ candidates for magnet up, (c) $K ^+ K ^- \pi ^+$ and (d) $K ^+ K ^- \pi ^-$ candidates for magnet down with $K ^+ K ^-$ invariant mass within $\pm$20 $\mathrm{ Me V}$ of the $\phi$ meson mass. The $D ^+_ s$ [yellow (grey) shaded area] and $D ^+$ [red (dark) shaded area] signal shapes are described in the text. The $\chi ^2$/ndf for these fits are 1.28, 1.25, 1.53, and 1.27 respectively, the corresponding p-values are 7%, 8%, 4%, 7%. Figure1.pdf [81 KiB] HiDef png [533 KiB] Thumbnail [314 KiB] *.C file Invariant $\mu ^+ \mu ^-$ mass distributions of the kinematically-selected ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates in magnet up data, where the red (open) circles represent entries where the muon candidate, kinematically selected, is rejected and the black (filled) circles those where it is accepted by the muon identification algorithm. The dashed lines represent the combinatorial background. Figure2.pdf [51 KiB] HiDef png [358 KiB] Thumbnail [189 KiB] *.C file Relative muon efficiency as a function of muon momentum determined using the kinematically-selected ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ sample. Figure3.pdf [31 KiB] HiDef png [280 KiB] Thumbnail [147 KiB] *.C file (a) Spectrum of the logarithm of the IP calculated with respect to the primary vertex for $D ^+_ s$ candidates in combination with muons; the insert shows a magnified view of the region where the prompt $D ^+_ s$ contribution peaks. The blue dashed line is the component coming from $B$ hadron decays, the black dashed line the false $D ^+_ s$ background, the red line the prompt background, (b) the invariant mass distributions for $D ^+_ s \rightarrow \phi\pi$ candidates. These distributions are for the magnet down sample. (For interpretation of the reference to colour in this figure legend, the reader is referred to the web version of this Letter.) fig4n1.pdf [33 KiB] HiDef png [356 KiB] Thumbnail [156 KiB] *.C file Asymmetries corrected for relative muon efficiencies, $A_{\mu}^{\text{c}}$, examined in the five muon momentum intervals for (a) magnet up data, (b) magnet down data and (c) average, using the KS muon calibration method. Then (d) magnet up data, (e) magnet down data and (f) average, using the MS muon calibration method in the two different binning scheme. Figure5.pdf [43 KiB] HiDef png [283 KiB] Thumbnail [327 KiB] *.C file Measurements of semileptonic decay asymmetries. The bands correspond to the central values $\pm$1 standard deviation uncertainties, defined as the sum in quadrature of the statistical and systematic errors. The solid dot indicates the SM prediction. Figure6.pdf [9 KiB] HiDef png [1 MiB] Thumbnail [520 KiB] *.C file Animated gif made out of all figures. PAPER-2013-033.gif Thumbnail

## Tables and captions

 Yields for $D ^+_ s \mu ^-$ and $D ^-_ s \mu ^+$ events separately for magnet up and down data. These yields contain very small contributions from prompt $D _s$ and b-hadron backgrounds. Table_1.pdf [49 KiB] HiDef png [60 KiB] Thumbnail [27 KiB] tex code Muon efficiency ratio corrected asymmetry $A_{\mu}^c$. The errors account for the statistical uncertainties in the $B ^0_ s$ signal yields. Table_2.pdf [47 KiB] HiDef png [54 KiB] Thumbnail [27 KiB] tex code Sources of systematic uncertainty on $A_{\rm meas}$. Table_3.pdf [33 KiB] HiDef png [94 KiB] Thumbnail [45 KiB] tex code

Created on 17 August 2019.Citation count from INSPIRE on 20 August 2019.