Search for the doubly charmed baryon at LHCb

The doubly charmed baryon $\Xi_{cc}^+$, containing two charm quarks, is a baryon predicted by the SU(4) quark model. Experimentally its existence has not been established yet. Many Quantum Chromodynamics (QCD) based theoretical models have predicted its properties with a mass in the range 3500-3700 MeV/$c^2$ and a lifetime in the range 110-250 fs. The experimental searches for the $\Xi_{cc}^+$ baryon and the measurements of its properties can test these models directly, providing an important input for the understanding of the non-perturbative aspect of QCD. The SELEX collaboration claimed the observation of the $\Xi_{cc}^+$ baryon in the $\Xi_{cc}^+ \to \Lambda_{c}^+K^-\pi^+$ decay in 2003. However, the measured lifetime was much shorter than theoretical predictions. Searches for the $\Xi_{cc}^+$ baryon in the same decay mode by FOCUS, Belle and BaBar experiments failed to reproduce the results. This does not mean that the SELEX result is excluded, however, since production environments at these experiments were different from that of SELEX. The Large Hadron Collider (LHC) at CERN is a high energy proton-proton collider with an expected $\Xi_{cc}^+$ production cross-section of the order of 0.1 $\mu$b. LHCb is the dedicated heavy flavor physics experiment at the LHC and has good tracking, flexible trigger and particle identification systems. Using the 0.65 fb$^{-1}$ data collected by LHCb in 2011, a search for the $\Xi_{cc}^+$ baryon in the decay $\Xi_{cc}^+ \to (\Lambda_c^+ \to p K^- \pi^+) K^- \pi^+$ is reported in this thesis. No signal is observed. Since the mass and lifetime of the $\Xi_{cc}^+$ baryon are a priori unknown, the upper limit of production cross-section of the $\Xi_{cc}^+$ baryon relative to that of the $\Lambda_c^+$ baryon is given as a function of the invariant mass for 5 different lifetime hypotheses. The measured upper limits have no direct contradiction with theoretical predictions, but much smaller than the value reported by SELEX. This could be explained by the fact that the production environments of LHCb and SELEX are different, or that the lifetime of $\Xi_{cc}^+$ is indeed smaller than theoretical predictions. In future the $\Xi_{cc}^+$ baryon will be searched with 2011 and 2012 data using more decay modes, and the trigger for the signal will be much improved. It is expected that the sensitivity of the $\Xi_{cc}^+$ baryon search will be much enhanced.