Measurement of the branching fraction and time dependent CP asymmetry in B0⟶D*-D*+K0s decays at the belle experiment

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.