Analysis of the B⁰_s → J/ψK-bar*⁰ channel and constraints on penguin processes in B⁰_s → J/ψφ decays at LHCb detector

Within the Standard Model, the decay B⁰_s → J/ψφ can occur via two main processes: the predominant tree and the sub-leading penguin. Penguin contributions have been so far ignored. Controlling them is crucial in order to disentangle Standard Model effects from possible New Physics in the measurement of φ_s. We studied the B⁰_s → J/ψK-bar*⁰ channel where penguin diagrams are not negligible compared to the tree process. Using 3 fb^-1 of data collected by LHCb, we measured the branching ratio: B(B⁰_s → J/ψK-bar*⁰) = (4.17 ± 0.18 (stat) ± 0.26 (syst) ± 0.24 (f_d/f_s)) x 10^-5, where f_d(s) is the production fraction of the B⁰ (B⁰_s) meson. We performed an angular analysis of the decay products J/ψ and K-bar*⁰ to extract the polarisation fractions and the first estimate of the polarization-dependent direct CP asymmetry. Our results show no evidence for direct CP violation in B⁰_s → J/ψK-bar*⁰, as expected in the Standard Model. We measured: f₀ = 0.497^+0.024_-0.025(stat) ± 0.025(syst) and f_|| = 0.179^+0.027_-0.026(stat) ± 0.013(syst). We determined the first constraints on penguin diagrams in φ_s phase using the B⁰_s → J/ψK-bar*⁰ channel. Combining with the B⁰ → J/ψρ⁰ decays, the shift on φ_s, due to penguin processes, is below 19 mrad for the three polarizations. The effect of penguin processes is therefore sufficiently small to keep the φ_s measurement (σ_φs =35 mrad) among the top priorities of LHCb for the upcoming years, to search for New Physics. (author)