Searches for Doubly Charmed Baryons at LHCb

The results of two separate searches for the doubly-charmed $\Xi$$_{cc}^{++}$ particle through the $\Xi$$_{cc}^{++}$ → $\Lambda$$_{c}^{+}$K−$\pi$+$\pi$+ and $\Xi$$_{cc}^{++}$ → D+pK−$\pi$+ decay modes are presented in this thesis. Both analyses examine proton-proton collision data collected by the LHCb detector at the European Laboratory for Particle Physics. The $\Xi$$_{cc}^{++}$ → $\Lambda$$_{c}^{+}$K−$\pi$+$\pi$+ analysis resulted in the first-ever observation of a doubly-charmed baryon, namely the $\Xi$$_{cc}^{++}$ state. Using data recorded in 2016 at LHCb, the mass of the $\Xi$$_{cc}^{++}$ state was measured to be $m$($\Xi$$_{cc}^{++}$) = 3621.40 ± 0.72 (stat) ± 0.27 (syst) ± 0.14 ($\Lambda$$_{c}^{+}$) MeV/c$^{2}$, where the first and second uncertainties are statistical and systematic, respectively, and the last uncertainty is due to the limited knowledge on the mass of the $\Lambda$$_{c}^{+}$ baryon. The signal was determined to be compatible with a weak decay. The second $\Xi$$_{cc}^{++}$ analysis did not yield any evidence of the $\Xi$$_{cc}^{++}$ → D+pK−$\pi$+ decay within the data recorded in 2016 so, instead, the following branching fraction ratio upper limit is set B($\Xi$$_{cc}^{++}$ → D+pK−$\pi$+)/B($\Xi$$_{cc}^{++}$ → $\Lambda$$_{c}^{+}$K−$\pi$+$\pi$+) < 1.7 (2.1) × 10$^{−2}$ at the 90% (95%) confidence level and at the measured mass of the $\Xi$$_{cc}^{++}$ particle from the $\Xi$$_{cc}^{++}$ → $\Lambda$$_{c}^{+}$K−$\pi$+$\pi$+ analysis. No signal is observed in the mass range 3300−3800 MeV/c$^{2}$. Additionally, a new multivariate-based trigger line was developed for reconstructing $\Xi$$_{c}^{+}$ → pK−$\pi$+ decays at LHCb. The new trigger makes use of the Bonsai Boosted Decision Tree method and is designed with searches for doubly-charmed particles in mind. This trigger gives a 36% increase in the signal yield of potential $\Xi$$_{c}^{+}$ particles compared to an original trigger line that was designed for the similar purposes. Aside from searches for doubly-charmed baryons, a short study is conducted into the calibration of the Ring Imaging Cherenkov (RICH) subdetectors at the LHCb detector. A method was successfully developed to obtain high momentum tracks without any knowledge of the magnetic field, which will be used in the future to test the hypothesis that the magnetic field is causing discrepancies seen between data and simulation for the RICH-1 Cherenkov angle resolution.