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First measurement of time-dependent $C\!P$ violation in $B^0_s \to K^+K^-$ decays

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Abstract

Direct and mixing-induced CP-violating asymmetries in Bs -> K+K- decays are measured for the first time using a data sample of pp collisions, corresponding to an integrated luminosity of 1.0 fb^{-1}, collected with the LHCb detector at a centre-of-mass energy of 7 TeV. The results are C_{KK} = 0.14 \pm 0.11 \pm 0.03 and S_{KK} = 0.30 \pm 0.12 \pm 0.04, where the first uncertainties are statistical and the second systematic. The corresponding quantities are also determined for B0 -> pi+pi- decays to be C_{pipi} = -0.38 \pm 0.15 \pm 0.02 and S_{pipi} = -0.71 \pm 0.13 \pm 0.02, in good agreement with existing measurements.

Figures and captions

Fits to the (a) $\pi^+\pi^-$ and (b) $K^+K^-$ invariant mass spectra, after applying preselection and PID requirements. The components contributing to the fit model are shown.

normal[..].pdf [160 KiB]
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normalizationPIPI.pdf
normal[..].pdf [174 KiB]
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normalizationKK.pdf

Decay time distributions corresponding to (a, b, c) high- and (d, e, f) low-mass sidebands from the (a and d) $K^\pm\pi^\mp$, (b and e) $\pi^+\pi^-$ and (c and f) $K^+K^-$ mass spectra, with the results of fits superimposed. In the bottom plots, the combinatorial background component (dashed) and the three-body background component (dotted) are shown.

kpi_comb.pdf [127 KiB]
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kpi_comb.pdf
pipi_comb.pdf [128 KiB]
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pipi_comb.pdf
kk_comb.pdf [124 KiB]
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kk_comb.pdf
kpi_phys.pdf [130 KiB]
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kpi_phys.pdf
pipi_phys.pdf [130 KiB]
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pipi_phys.pdf
kk_phys.pdf [128 KiB]
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kk_phys.pdf

Distributions of $K^\pm\pi^\mp$ (a) mass and (b) decay time, with the result of the fit overlaid. The main components contributing to the fit model are also shown.

mass_b[..].pdf [183 KiB]
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mass_bdkpi_pipisel.pdf
KPI_time.pdf [173 KiB]
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KPI_time.pdf

Raw mixing asymmetries for candidates in the $ B ^0 \!\rightarrow K ^+ \pi ^- $ signal mass region, corresponding to the five tagging categories, with the result of the fit overlaid.

KPI_asym1.pdf [233 KiB]
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KPI_asym1.pdf
KPI_asym2.pdf [222 KiB]
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KPI_asym2.pdf
KPI_asym3.pdf [205 KiB]
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KPI_asym3.pdf
KPI_asym4.pdf [193 KiB]
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KPI_asym5.pdf [192 KiB]
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KPI_asym5.pdf

Distributions of $K^+K^-$ (a) mass and (b) decay time, with the result of the fit overlaid. The main components contributing to the fit model are also shown.

mass_b[..].pdf [163 KiB]
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mass_bskk_kksel.pdf
time_b[..].pdf [161 KiB]
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time_bskk_kksel.pdf

Time-dependent raw asymmetry for candidates in the $ B ^0_ s \!\rightarrow K ^+ K ^- $ signal mass region with the result of the fit overlaid. In order to enhance the visibility of the oscillation, only candidates belonging to the first two tagging categories are used. The offset $t_0 = 0.6$ $ {\rm \,ps}$ corresponds to the preselection requirement on the decay time.

KK_2cat.pdf [150 KiB]
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KK_2cat.pdf

Distributions of $\pi^+\pi^-$ (a) mass and (b) decay time, with the result of the fit overlaid. The main components contributing to the fit model are also shown.

mass_b[..].pdf [180 KiB]
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mass_bdpipi_pipisel.pdf
PIPI_time.pdf [160 KiB]
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PIPI_time.pdf

Time-dependent raw asymmetry for candidates in the $ B ^0 \!\rightarrow \pi ^+ \pi ^- $ signal mass region with the result of the fit overlaid. Tagged candidates belonging to all tagging categories are used.

cA_sig[..].pdf [215 KiB]
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cA_sig_OMPI_PIPI.pdf

Animated gif made out of all figures.

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Tables and captions

Kinematic requirements applied by the event preselection.

Table_1.pdf [57 KiB]
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Table_1.pdf

Definition of the five tagging categories determined from the optimization algorithm, in terms of ranges of the mistag probability $\eta$.

Table_2.pdf [28 KiB]
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Table_2.pdf

Signal tagging efficiencies, mistag probabilities and associated asymmetries, corresponding to the five tagging categories, as determined from the $K^\pm\pi^\mp$ mass and decay time fit. The uncertainties are statististical only.

Table_3.pdf [47 KiB]
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Table_3.pdf

Systematic uncertainties affecting the $B^0_s \rightarrow K^+K^-$ and $B^0 \rightarrow \pi^+\pi^-$ direct and mixing-induced $ C\!P$ asymmetry coefficients. The total systematic uncertainties are obtained by summing the individual contributions in quadrature.

Table_4.pdf [36 KiB]
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Table_4.pdf

Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.