Reconstruction of exclusive $B$ decays into $J/\psi$ using data from the DELPHI experiment and implications to future CP violation measurements at the LHC

Since the discovery of the beauty quark in 1977, the physics of particles containing beauty quarks has played a dominant role in high energy physics research. Studying the properties of B decays can help to determine some of the fundamental parameters of the Standard Model and offers the possibility to better understand the mechanism of weak decays of the b quark. Particles containing a b quark have a lifetime of the order of 1 ps and if produced at high enough energies they will fly several millime- ters before decaying. That makes it possible to identify B hadrons by reconstructing their separated decay vertices. Decays of B particles into charmonium states are of special interest for exclusive reconstructions. Charmonium states can be easily identi- fied through their decay into lepton pairs and reconstructed with good efficiency and relatively low background. The decay channel of the neutral B meson into J/ψKS0 is of particular interest for the search for CP violation in the B system. The importance of CP violation in physics is in no doubt. Within the Standard Model, there is only one CP-violating parameter: the phase of the CKM matrix. This is an important parameter and, as such, has the importance within the model of the mass of the Z0 or that of the top quark. Although CP violation has been observed in the neutral kaon system, strong-interaction uncertainties render the measurements rather inadequate as a means of determining the phase. Therefore, the CKM phase remains one of the poorest-determined parameters of the Standard Model. The system of neutral B mesons seems to be an excellent environment to measure CP violation and specially the decay Bd0 → J/ψKS0 is a good candidate. The Large Electron Positron collider, LEP, at the European Laboratory for High Energy Physics, CERN, is an excellent machine for the study of beauty hadrons, since a substantial fraction of all Z0 decay modes consist of decays into beauty quarks. In order to reconstruct these B decays a detector with a vertex detector, which permits a high track extrapolation resolution to the collision point is required. Furthermore hadron identification and a good lepton reconstruction efficiency is needed. The DELPHI experiment is performed by a large international collaboration. One of it’s members is the Institute for High Energy Physics of the Austrian Academy of Sciences. I had the privilege to join the institute and to work at CERN for the DELPHI experiment. As a member of the Austrian group I was involved in the maintenance and operation of the Forward Chamber A, where I worked mainly on online-monitoring and calibration. The following analysis on reconstruction of exclusive B meson decays is based on data recorded at the DELPHI experiment. The DELPHI detector is well suited for studying fully reconstructed B decays, since the vertex detector provides a high track extrapolation resolution and the Ring Imaging Cherenkov detectors enable particle identification over a wide momentum range. Since decays of the type B → J/ψX have small branching ratios the main limitation in the search for fully reconstructed B events at LEP is statistics, and even under the most optimistic circumstances a measurement of CP violation in the neutral B system is not possible. Therefore I was studying the possibilities of future experiments in the search for CP violation, with special emphasis on the planned experiments at the LHC. Since the early design phase I was involved in the CMS collaboration, where I was working in the physics group and performed a study on CP violation measurements at the LHC. This led to a couple of publications and conference reports and showed that even with a general purpose detector like CMS it will be possible to measure CP violation in the B system with very high precision. My thesis is summarizing the analyses I performed at DELPHI and CMS.