Measurement of the mixinginduced CPviolating phase phi_s in Bs decays is of prime importance in probing new physics. Here 7421 +/ 105 signal events from the dominantly CPodd 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 timedependent fit to the data yields a value of phi_s=0.019^{+0.173+0.004}_{0.1740.003} rad, consistent with the Standard Model expectation. No evidence of direct CP violation is found.
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 [430 KiB] Thumbnail [467 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 shortdashed line), $\overline{ B }{} ^0 \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ (purple dotdashed), $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \eta'$ and $\overline{ B }{} ^0_ s \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \phi$ when $\phi\rightarrow \pi^+\pi^\pi^0$ (black dotlongdashed), and $\overline{ B }{} ^0 \rightarrow { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} K^ \pi^+$ (lightblue longdashed). 
fitmas[..].pdf [40 KiB] HiDef png [448 KiB] Thumbnail [252 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. 
mpipiBDT.pdf [34 KiB] HiDef png [286 KiB] Thumbnail [167 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 longlived component, and the solid curve the total. 
fit2f0.pdf [42 KiB] HiDef png [218 KiB] Thumbnail [188 KiB] *.C file 

Distribution of the estimated time resolution $\sigma_t$ for oppositesign $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \pi^+\pi^$ signal events after background subtraction, and for likesign background. 
cmp.pdf [35 KiB] HiDef png [137 KiB] Thumbnail [164 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. 
massp[..].pdf [40 KiB] HiDef png [218 KiB] Thumbnail [142 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 [269 KiB] Thumbnail [191 KiB] *.C file 

Loglikelihood 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. 
dlldg[..].pdf [37 KiB] HiDef png [206 KiB] Thumbnail [158 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 [262 KiB] Thumbnail [197 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 [109 KiB] Thumbnail [115 KiB] *.C file 

Animated gif made out of all figures. 
PAPER2012006.gif Thumbnail 
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 finalstate helicity of the Dwave is denoted by $\Lambda$. Only statistical uncertainties are quoted. 
Table_1.pdf [45 KiB] HiDef png [65 KiB] Thumbnail [29 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 [91 KiB] Thumbnail [37 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 [55 KiB] Thumbnail [24 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 [110 KiB] Thumbnail [49 KiB] tex code 
Created on 20 February 2021.