# 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] HiDef png [433 KiB] Thumbnail [491 KiB] *.C file 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] HiDef png [456 KiB] Thumbnail [261 KiB] *.C file 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. mpipi-BDT.pdf [34 KiB] HiDef png [310 KiB] Thumbnail [168 KiB] *.C file 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. fit2-f0.pdf [42 KiB] HiDef png [227 KiB] Thumbnail [194 KiB] *.C file 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] HiDef png [147 KiB] Thumbnail [187 KiB] *.C file (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] HiDef png [220 KiB] Thumbnail [153 KiB] *.C file 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] HiDef png [272 KiB] Thumbnail [197 KiB] *.C file 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] HiDef png [212 KiB] Thumbnail [166 KiB] *.C file 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] HiDef png [265 KiB] Thumbnail [203 KiB] *.C file $C P$ asymmetry as a function of decay time modulo $2\pi/\Delta m_s$. The curve shows the expectation for $\phi_s=-0.019$ rad. asym_modu.pdf [59 KiB] HiDef png [108 KiB] Thumbnail [127 KiB] *.C file Animated gif made out of all figures. PAPER-2012-006.gif Thumbnail

## 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] HiDef png [78 KiB] Thumbnail [36 KiB] tex code 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] HiDef png [108 KiB] Thumbnail [45 KiB] tex code 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] HiDef png [65 KiB] Thumbnail [31 KiB] tex code 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] HiDef png [133 KiB] Thumbnail [63 KiB] tex code

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