Measurement of the Bc meson lifetime using Bc -> J/psi mu nu X decays with the LHCb detector at CERN

The precision measurement of the \Bc meson lifetime provides an essential test of the models describing the unique open-flavour state composed of two heavy quarks. It is also a necessary input for all measurements of \Bc production and decay branching fractions. The first measurement of the \Bc lifetime achieved by the LHCb Collaboration is presented in this Thesis. The data sample collected in 2012, in $pp$ collisions at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of 2 fb$^{-1}$, is analysed to select \BctojmX decays. A two-dimensional data-model is developed combining the information on the invariant mass of the \jpsimup combination and the decay time, as measured in the rest-frame of the \jpsimup combination. Data-driven techniques are proposed to model the background sources, including the candidates selected because of the misidentification of a hadron as the muon produced in a \Bc decay, for which an original technique has been developed. The template distribution for \BctojmX decays relies on realistic dynamical models including feed-down decays, and depends on the \Bc lifetime through a statistical correction between the \Bc and \jpsimup rest frames, known as $k$-factor. Data-driven cross-checks are used to test the dynamical model and assess the related uncertainties. The measured lifetime is $%\begin{equation} \tau_{\Bc} = 509 \pm 8 \stat \pm 12 \syst \ \mathrm{fs}, $ %\end{equation} where the largest systematic uncertainty is due to the statistical model used to describe the combinatorial background, which is the only one relying on simulation.