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Ritter, Martin (2014): Measurement of the branching fraction and time dependent CP asymmetry in B0⟶D*-D*+K0s decays at the belle experiment. Dissertation, LMU München: Faculty of Physics



Why do we exist? CP violation is an integral part of this question as its understanding is crucial to explain the matter-antimatter asymmetry observed in our universe. Several experiments were designed and carried out to precisely measure CP violation, especially in the B meson system where large asymmetries where predicted and found. With Belle II and LHCb, two new experiments are going to improve the existing measurements. Belle II will be based on the very successful Belle experiment at the KEKB collider, currently holding the world record on luminosity with 2.11×10³⁴ cm⁻²2s⁻¹. The B meson system has a very rich decay topology and many of theses decay modes and their CP asymmetry parameters have already been measured at Belle. The most famous decay channel, B0⟶J/ψK0s, poses very tight constraints on sin 2φ₁ but leaves a twofold ambiguity on the actual value of the angle φ₁ in the CKM triangle. The decay mode B0⟶D*-D*+K0s, while experimentally much more challenging, offers the unique possibility to also extract cos2φ₁ and thus resolve this ambiguity. In the first chapters of this thesis we present the principle of this measurement and the results for the branching fraction and the time-dependent CP violation parameters of B0⟶D*-D*+K0s decays. These results are obtained from the final data sample of the Belle experiment containing 772 million BBbar pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain the branching fraction BR(B0⟶D*-D*+K0s) = (5.35+0.35−0.34(stat) ± 0.57(syst))×10⁻³, which is in agreement with the current world average. In a 3 parameter fit sensitive to cos2φ₁, we extract the currently most precise values for the CP parameters Jc/J0 = 0.37 ± 0.10(stat) ± 0.02(syst), (2Js1/J0) sin(2φ₁) = 0.30 ± 0.16(stat) ± 0.03(syst), (2Js2/J0) cos(2φ₁) = 0.16 ± 0.16(stat) ± 0.03(syst). This allows us to exclude a negative value for cos2φ₁ at a 85% confidence under the assumption that that (2Js2/J0) is positive. Finally, we describe the implementation of the vertex detector geometry for the upcoming Belle II experiment. The upgrade to Belle aims to increase the integrated luminosity by a factor of 50 and will receive, among other upgrades, a completely new vertex detector. To produce simulated events, a precise description of the sensor geometry and material budget is needed.