We present a measurement of the timedependent CPviolating asymmetry in B_s > J/psi phi decays, using data collected with the LHCb detector at the LHC. The decay time distribution of B_s > J/psi phi is characterized by the decay widths Gamma_H and Gamma_L of the heavy and light mass eigenstates of the B_sB_sbar system and by a CPviolating phase phi_s. In a sample of about 8500 B_s > J/psi phi events isolated from 0.37 fb^1 of pp collisions at sqrt(s)=7 TeV we measure phi_s = 0.15 +/ 0.18 (stat) +/ 0.06 (syst) rad. We also find an average B_s decay width Gamma_s == (Gamma_L + Gamma_H)/2 = 0.657 +/ 0.009 (stat) +/ 0.008 (syst) ps^1 and a decay width difference Delta Gamma_s == Gamma_L  Gamma_H} = 0.123 +/ 0.029 (stat) +/ 0.011 (syst) ps^1. Our measurement is insensitive to the transformation (phi_s,DeltaGamma_s > pi  phi_s,  Delta Gamma_s.
Invariant mass distribution for $ B ^0_s \to\mu^+\mu^ K^+K^$ candidates with the mass of the $\mu^+\mu^$ pair constrained to the nominal $J/\psi$ mass. Curves for fitted contributions from signal (dashed), background (dotted) and their sum (solid) are overlaid. 
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Projections for the decay time and transversity angle distributions for events with $m_B$ in a $\pm 20$ $\mathrm{ Me V}$ {} range around the $ B ^0_s$ {} mass. The points are the data. The dashed, dotted and solid lines represent the fitted contributions from signal, background and their sum. The remaining curves correspond to different contributions to the signal, namely the $ C P$ even Pwave (dashed with single dot), the $ C P$ odd Pwave (dashed with double dot) and the Swave (dashed with triple dot). 
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Likelihood confidence regions in the $\Delta\Gamma_{s}$$\phi_s$ plane. The black square and error bar corresponds to the Standard Model prediction [10,7]. 
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Animated gif made out of all figures. 
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Fit results for the solution with $\Delta\Gamma_{s}>0$ with statistical and systematic uncertainties. \\ 
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Created on 27 February 2021.