Doubly charmed baryon searches at the LHCb experiment

The main analysis presented in this thesis is a search for the doubly charmed baryon $\it{\Xi}_{cc}^{+}$ in the $\it{\Xi}_{c}^{+}\pi^{-}\pi^{+}$ final state, where the $\it{\Xi}_{c}^{+}$ baryon is reconstructed in the $p K^{-} \pi^{+}$ final state. The search is based on the data collected at the LHCb experiment in 2016$-$2018 at a centre-of-mass energy of 13$\mathrm{\,Te\kern -0.1em V}$, corresponding to about 5.4$\,\mathrm{fb}^{-1}$ of integrated luminosity. Since the $\it{\Xi}_{cc}^{+}$ baryon has previously not been observed and this analysis presents the first search for this baryon in the $\it{\Xi}_{c}^{+}\pi^{-}\pi^{+}$ final state, the $\it{\Xi}_{c}^{+}\pi^{+}\pi^{-}$ invariant-mass region from 3.3 to 3.8$\mathrm{\,Ge\kern -0.1em V}/c^2$ was not examined until the full analysis was finalised in order to avoid any bias of the decisions made during the analysis. No significant signal for the $\it{\Xi}_{cc}^{+}$ baryon is observed in the invariant-mass range from 3.4 to 3.8$\mathrm{\,Ge\kern -0.1em V}/c^2$, therefore upper limits are set on the ratio of production cross-section times branching fraction with respect to the already observed $\it{\Xi}_{cc}^{++} \rightarrow (\it{\Xi}_{c}^{+} \rightarrow p K^{-} \pi^{+}) \pi^{+}$ decay, as a function of the assumed $\it{\Xi}_{cc}^{+}$ mass for different $\it{\Xi}_{cc}^{+}$ lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and the transverse momentum range from 2.5 to 25$\mathrm{\,Ge\kern -0.1em V}/c$. For the $\it{\Xi}_{cc}^{+}$ lifetime hypothesis of 80$\mathrm{\,fs}$, the evaluated upper limit on the ratio of production cross-section times branching fraction with respect to the $\it{\Xi}_{cc}^{++} \rightarrow \it{\Xi}_{c}^{+} \pi^{+}$ decay varies between 2 and 5 at 95\% confidence level in the invariant-mass range from 3.4 to 3.8$\mathrm{\,Ge\kern -0.1em V}/c^2$. The results are combined with the previously published search for the $\it{\Xi}_{cc}^{+}$ baryon in the $\it{\Lambda}_{c}^{+} K^{-} \pi^{+}$ final state at LHCb. A maximum local significances of 4.0 standard deviations, including systematic uncertainties, is found for the combination of the two decay channels around the mass of 3623$\mathrm{\,Me\kern -0.1em V}/c^2$. Taking into account the look-elsewhere effect in the invariant-mass range from 3.5 to 3.7$\mathrm{\,Ge\kern -0.1em V}/c^2$, the combined global significance is 2.9 standard deviations, including systematic uncertainties. A detector performance study described in this thesis is devoted to hit-resolution corrections of the VELO detector in the simulation, in order to improve the agreement with the corresponding resolutions measured in data. The method uses the hit resolutions in data to generate the hit uncertainties in simulation. A significant improvement in the resolutions of 2 and 3-strip clusters between the corrected simulation and data is achieved. As the method is not applicable on 1-strip clusters, a strategy to correct the 1-strip clusters is developed, in order to improve the agreement for the 1-strip clusters in the future. Lastly, an inclusive software trigger line is developed for the $\it{\Xi}_{c}^{0}$ baryon based on the multivariate selection, which was implemented for the 2018 data taking at the LHCb experiment. The developed selection is aimed to be efficient for the $\it{\Xi}_{c}^{0} \rightarrow p K^{-} K^{-} \pi^{+}$ decay, independent of the origin of the $\it{\Xi}_{c}^{0}$ baryon. The efficiency of the developed trigger line is improved with respect to the corresponding existing cut-based trigger line for the $\it{\Xi}_{c}^{0}$ baryon, in particular for the $\it{\Xi}_{cc}^{+} \rightarrow \it{\Xi}_{c}^{0} \pi^{+}$ decay, which is important for future searches and studies of the doubly charmed baryon $\it{\Xi}_{cc}^{+}$.