Analyse du canal $B^0 _s \to J/\psi {\overline{k}}^{*0}$ et contraintes sur les processus pingouins dans les désintégrations $B^0 _s \to J/\psi\phi$ auprès du détecteur LHCb

Within the Standard Model, the decay Bs->Jpsi Phi 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 \phis. We studied the Bs->Jpsi K*bar channel where penguin diagrams are not negligible compared to tree. Using 3 fb^{-1} of data collected by LHCb, we measured the branching ratio: $BRBs−>JpsiK∗bar=(4.17±0.18(stat)±0.26(syst)±0.24(fd/fs))×10−5,\\$ where $fd(s)$ is the production fraction of the Bd(Bs) meson. We performed an angular analysis of the decay products Jpsi and K*bar 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 Bs->Jpsi K*bar, as expected in the Standard Model. We measured: $f_0=0.497+0.024−0.025(stat)±0.025(syst)$ and $f_∥=0.179+0.027−0.026(stat)±0.013(syst)$. We determined the first constraints on penguin diagrams in \phis phase using the Bs->Jpsi K*bar channel. Combining with the Bd->Jpsi rho^0 decays, the shift on \phis, due to penguin processes, is below 19\,mrad for the three polarizations. The effect of penguin processes is therefore sufficiently small to keep the \phis measurement ($\sigma_{\phi_s}=35\,mrad$) among the top priorities of LHCb for the upcoming years, to search for New Physics.