Observation of new $\Xi_c^0$ states decaying to $\Lambda_c^+K^-$, and development of the TORCH time-of-flight detector.

Using a data sample of proton-proton collisions at center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 5.6 fb$^{-1}$ collected by the LHCb experiment, the $\Lambda_c^+K^-$ mass spectrum is studied. Three new, excited $\Xi_c^{0}$ states are observed and their masses and natural widths are measured to be \begin{eqnarray*} m(\Xi_c(2923)^0)&=& 2923.04 \pm 0.25 \pm 0.18 \pm 0.14 MeV\\ \Gamma(\Xi_c(2923)^0) &=& 7.09 \pm 0.79 \pm 1.39 MeV,\\\\ m(\Xi_c(2939)^0) &=& 2938.55 \pm 0.21 \pm 0.17 \pm 0.14MeV,\\ \Gamma(\Xi_c(2939)^0) &=& 10.22 \pm 0.77 \pm 1.04MeV\\\\ m(\Xi_c(2965)^0) &=& 2964.88 \pm 0.26 \pm 0.14 \pm 0.14 MeV, \\ \Gamma(\Xi_c(2965)^0) &=& 14.07 \pm 0.91 \pm 1.34 MeV, \end{eqnarray*} where the uncertainties are statistical, systematic and the third one is due to the knowledge of the $\Lambda_c^+$ mass. All peaks have significances greatly exceeding $5\sigma$. Though the mass of $\Xi_c(2965)^0$ is close to the known $\Xi_c(2970)^0$, its natural width differs significantly and therefore it is interpreted as a different baryon. Furthermore, this thesis summarises the status of the TORCH time-of-flight detector by describing the characterisation of Micro-Channel Plate Photomultiplier tubes (MCP-PMTs), outlining the recent beam test campaigns as well as an analysis comparing the photon yield in simulation and data.