Abstract
The decay-time-dependent CP asymmetry in {{B} ^0_{s}} rightarrow J/psi {{K} ^+} {{K} ^-} decays is measured using proton–proton collision data, corresponding to an integrated luminosity of 1.9,mathrm{fb}^{-1} , collected with the LHCb detector at a centre-of-mass energy of 13,mathrm {TeV} in 2015 and 2016. Using a sample of approximately 117 000 signal decays with an invariant {{K} ^+} {{K} ^-} mass in the vicinity of the phi (1020) resonance, the CP-violating phase phi _s is measured, along with the difference in decay widths of the light and heavy mass eigenstates of the {{B} ^0_{s}} -{{overline{B}{}} {}^0_{s}} system, Delta Gamma _s. The difference of the average {{B} ^0_{s}} and {{B} ^0} meson decay widths, Gamma _s-Gamma _d, is determined using in addition a sample of {{B} ^0} rightarrow J/psi {{K} ^+} {{pi } ^-} decays. The values obtained are phi _s = -0.083pm 0.041pm 0.006mathrm { ,rad} , Delta Gamma _s = 0.077 pm 0.008 pm 0.003 {mathrm { ,ps^{-1}}} and Gamma _s-Gamma _d = -0.0041 pm 0.0024 pm 0.0015{mathrm { ,ps^{-1}}} , where the first uncertainty is statistical and the second systematic. These are the most precise single measurements of these quantities to date and are consistent with expectations based on the Standard Model and with a previous LHCb analysis of this decay using data recorded at centre-of-mass energies 7 and 8 TeV. Finally, the results are combined with recent results from {{B} ^0_{s}} rightarrow J/psi {{pi } ^+} {{pi } ^-} decays obtained using the same dataset as this analysis, and with previous independent LHCb results.
Highlights
The existence of new phenomena beyond those predicted by the Standard Model (SM), hereafter referred to as New Physics (NP), could introduce sizeable effects on CP-violating observables
Various input quantities are used in the boosted decision tree (BDT) to exploit the features of the signal decay in order to distinguish it from background, namely the track-fit χ 2 of the final-state particles, the particle identification probability as provided mainly from the RICH and muon systems, the quality of the candidate J/ψ and Bs0 decay vertices, the pT of the Bs0 candidate and of the K + K − combination and the Bs0 impact parameter (IP) with respect to its origin vertex
The dashed blue lines define the boundaries of the six m(K + K −) bins that are used in the analysis resolution function is modelled with a Gaussian distribution with a mean of zero and a width σeff, where σeff is determined using a sample of candidates constructed from combinations of J/ψ, K + and K − candidates that originate predominantly in the primary interaction
Summary
The existence of new phenomena beyond those predicted by the Standard Model (SM), hereafter referred to as New Physics (NP), could introduce sizeable effects on CP-violating observables. The ratio of the average decay width of Bs0 and B0 mesons, s/ d , is usually the preferred observable to compare with experimental measurements as it allows the suppression of common uncertainties in the calculation. The analysis procedure gives access to s − d rather than s due to the dependence of the time efficiency parametrisation on d This allows the determination of s − d with a significant reduction of the systematic uncertainty associated with lifetime-biasing selection requirements compared to the previous measurement. Taking as an input the precisely known value of d [32], the ratio s/ d may be determined with higher precision with respect to measuring the two lifetimes independently In this analysis, the polarisation-independent CP-violating parameter λr , associated with each polarisation state r , is defined such that λr = ηr λ, where ηr = +1 for r ∈ {0, } and ηr = −1 for r ∈ {⊥, S}.
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