| Sumario: | B0 s ! K 0K 0, using the first data acquired by LHCb during 2010 and 2011 from the LHC proton-proton collisions at a centre-of-mass energy p s = 7 TeV, corresponding to an integrated luminosity of 37 pb1 and 1 fb1 respectively. B0 s ! K 0K 0 is a Flavour Changing Neutral Current b ! sdd¯ process. Within the Standard Model (SM), it proceeds through a penguin transition dominated by a virtual intermediate top quark coupling to a W boson. Extensions of the SM predict additional one-loop contributions that could introduce sizeable effects on the dynamics of this transition, and produce measurable deviations from the SM predictions of its observables. The most relevant SM predictions concerning this mode have been summarised. Furthermore, the LHCb experiment has been described. LHCb is one of the experiments operating at the Large Hadron Collider (LHC) at CERN. It is dedicated to the study of CP violation and rare decays in hadrons containing b quarks. The experimental conditions during the relevant data taking period have also been explained. As a result of the work presented here, the first observation of the B0 s ! K 0K 0 decay was performed. With the data collected during 2010 a clear B0 s ! (K+p)(Kp+) signal was found with an statistical significance of more than 10 s. Normalising to the B0 d ! J/yK 0 decay channel, the branching ratio of B0 s ! K 0K 0 was measured, taking as input the polarisation fractions measured through a simplified angular analysis of the K 0 decay products. With the extended data sample collected in 2011, a more precise time integrated and untagged study was carried out of the decay B0 s ! (K+p)(Kp+). A combined angular and mass analysis was performed to asses the fraction of the different amplitudes contributing to this final state. The polarisation fractions of the B0 s ! K 0K 0 were determined, obtaining for the longitudinal component fL = 0.201 0.057(stat.) 0.040(syst.), and the S–wave contribution was measured to be large, 0.665 0.067(stat.) 0.030(syst.). The branching fraction for the vector-vector mode was also measured to be B(B0 s ! K 0K 0) = (10.6 1.8(stat.) 1.0(syst.) 0.6 ( fd/ fs)) 106, in agreement with the Standard Model prediction. Finally, a determination was performed, in a model independent way, of the 8 angular asymmetries sensitive to CP violation accessible to the untagged sample. They are four triple product asymmetries and four direct CP asymmetries that arise in the interference of the CP-even and CP-odd amplitudes contributing to the B0 s ! (K+p)(Kp+) decay. No signs of CP-violation were found, in agreement with the Standard Model expectation.
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