Reconstruction of the Tau Lepton and the Study of B0 D*-[tau]+[nu][tau] at LHCb

The LHCb experiment at CERN has been designed to perform high precision measurements of B meson decays. In this thesis the LHCb magnetic field parameterization and the emulation of the LHCb Silicon Tracker noise reduction algorithms are discussed. Both studies have contributed to the general performance of the LHCb detector. The ability of the LHCb experiment to accurately measure B meson production and decay vertices allows the experiment to perform precision time-dependent studies. Most τ leptons detected by LHCb either originate from B mesons or Ds mesons, which causes the τ decay vertex to be significantly displaced. This makes the LHCb experiment a great detector for τ physics studies. Tauonic decays are thought to be highly sensitive to New Physics contribu- tions due to the high τ mass. Each known decay containing a τ lepton, also includes at least one τ neutrino, due to lepton flavour conservation. Decays containing τ leptons are often disregarded due to reconstruction difficulties with neutrinos passing straight through the LHCb detector, carrying undetectable energy. However, if one can overcome these difficulties, there are numerous interesting decays with τ leptons that are highly sensitive to New Physics contributions, but are unexplored at LHCb. In the first part of this thesis the exclusive reconstruction of a number of interesting τ physics channels is performed. These decays all suffer from missing energy and some even contain unreconstructable vertices. It is shown that despite these difficulties, the full τ momentum can be calculated up to a certain ambiguity in all cases considered. The analytical calculation is then extended to a constrained fit that exploits measurement errors to obtain the best result. The constrained fit improves the reconstruction efficiency and may also improve the accuracy of the calculation. In the second part, the developed τ reconstruction method is tested on LHCb data collected in 2011. A specific tauonic decay is chosen on which the exclusive τ reconstruction is performed in an attempt to extract its signal. It is shown that the goodness of the constrained fit can be used as an excellent new selection cut to reduce background. However, to confidently extract the observed number of signal events, the study can be improved. The foundations are laid for exclusive τ reconstruction at LHCb and the developed methods in this thesis will continue to be used to perform τ physics analyses at LHCb.