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Measurement of the CP-violating phase $\phi_s$ in $\overline{B}^0_s \to J/\psi\pi^+\pi^-$ decays

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Abstract

Measurement of the mixing-induced CP-violating phase phi_s in Bs decays is of prime importance in probing new physics. Here 7421 +/- 105 signal events from the dominantly CP-odd final state J/\psi pi+ pi- are selected in 1/fb of pp collision data collected at sqrt{s} = 7 TeV with the LHCb detector. A time-dependent fit to the data yields a value of phi_s=-0.019^{+0.173+0.004}_{-0.174-0.003} rad, consistent with the Standard Model expectation. No evidence of direct CP violation is found.

Figures and captions

Distributions of the BDT variable for both training and test samples of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi\pi$ signal and background events. The signal samples are from simulation and the background samples derived from data.

overtr[..].pdf [52 KiB]
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overtrain_BDT-a.pdf

Mass distribution of the selected $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ combinations in the $f_{\rm odd}$ region. The blue solid curve shows the result of a fit with a double Gaussian signal (red solid curve) and several background components: combinatorial background (brown dotted line), background from $B^-\rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^-$ and $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^-$ (green short-dashed line), $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ (purple dot-dashed), $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \eta'$ and $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \phi$ when $\phi\rightarrow \pi^+\pi^-\pi^0$ (black dot-long-dashed), and $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^- \pi^+$ (light-blue long-dashed).

fitmas[..].pdf [40 KiB]
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fitmass_bdt_sel.pdf

Mass distribution of selected $\pi^+\pi^-$ combinations shown as the (solid black) histogram for events in the $\overline{ B }{} ^0_ s $ signal region. The (dashed red) line shows the background determined by fitting the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ mass in bins of $\pi^+\pi^-$ mass. The arrows designate the limits of the $f_{\rm odd}$ region.

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mpipi-BDT.pdf

Decay time distribution of prompt $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ candidates in the $f_0$ region. The dashed (red) line shows the long-lived component, and the solid curve the total.

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fit2-f0.pdf

Distribution of the estimated time resolution $\sigma_t$ for opposite-sign $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ signal events after background subtraction, and for like-sign background.

cmp.pdf [35 KiB]
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cmp.pdf

(a) Mass distribution of $\overline{ B }{} ^0 \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \overline{ K }{} ^{*0} $ candidates. The dashed (red) line shows the background, and the solid (blue) curve the total. (b) Decay time distribution, where the small background has been subtracted using the $\overline{ B }{} ^0 $ mass sidebands. The (blue) curve shows the lifetime fit.

mass-p[..].pdf [40 KiB]
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mass-psikst.pdf

Decay time distribution of $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_0$ candidates fitted with a single exponential function multiplied by the acceptance and convolved with the resolution. The dashed line is signal and the shaded area background.

fitlif[..].pdf [37 KiB]
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fitlifetime.pdf

Log-likelihood difference as a function of $\phi_s$ for $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_{\rm odd}$ events.

dll-dg[..].pdf [37 KiB]
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dll-dgp-blind.pdf

Decay time distribution of $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} f_{\rm odd}$ candidates. The solid line shows the result of the fit, the dashed line shows the signal, and the shaded region the background.

time.pdf [37 KiB]
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time.pdf

$ C\!P$ asymmetry as a function of decay time modulo $2\pi/\Delta m_s$. The curve shows the expectation for $\phi_s=-0.019$ rad.

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asym_modu.pdf

Animated gif made out of all figures.

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Tables and captions

Resonance fractions in $\overline{ B }{} ^0_ s \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ over the full mass range [11]. The final-state helicity of the D-wave is denoted by $\Lambda$. Only statistical uncertainties are quoted.

Table_1.pdf [45 KiB]
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Table_1.pdf

Parameters of the decay time resolution function determined from fits to $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi^+\pi^-$ prompt data samples.

Table_2.pdf [44 KiB]
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Table_2.pdf

Parameters used in the functions for the invariant mass and decay time describing the signal and background. These parameters are fixed to their central values in the fit for $\phi_s$.

Table_3.pdf [64 KiB]
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Table_3.pdf

Summary of systematic uncertainties on $\phi_s$. Quantities fixed in the fit that are not included here give negligible uncertainties. The total uncertainty is found by adding in quadrature all the positive and negative contributions separately.

Table_4.pdf [66 KiB]
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Table_4.pdf

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