Commissioning of the LHCb Inner Tracker and measurement of $V^0$-particle production in $pp$ collisions at 0.9 TeV

The Large Hadron Collider (LHC) at CERN near Geneva is an accelerator designed to collide protons at a centre-of-mass energy of $\sqrt{s}$ = 14 TeV. It is operational since November 2009 and has delivered collisions up to an energy of $\sqrt{s}$ = 7 TeV in 2010. The LHCb experiment is one of the four major LHC experiments, together with ATLAS, CMS and ALICE. It is a single-arm forward spectrometer designed to study CP-violation and rare decays in the $b$-quark sector. The tracking system of the LHCb experiment is composed of the silicon Vertex Locator (VELO), the silicon Tracker Turicensis (TT), the dipole magnet, the silicon Inner Tracker (IT) and the straw-tube Outer Tracker (OT). The IT is covering the innermost acceptance region close to the beam-pipe, where the track multiplicity is highest (20% of the tracks for 1.5% of the acceptance). Two topics have been addressed in this doctoral thesis. The first part is dedicated to the IT commissioning phase. In particular the first version of the data-quality online monitoring is described. Several histograms and cluster maps have been developed, in order to watch the detector operation and ensure fast problem identification. The online monitoring allowed the identification of cosmic ray tracks which were used for a first spatial alignment. In September 2008 and June 2009, LHC injection tests from the SPS to the LHC ring were performed. Bunches of 2-5x10$^9$ protons were dumped onto a beam stopper (TED), producing a shower of secondary particles heading towards LHCb. These events were recorded by the LHCb detector and allowed a time-alignment of the IT up to a precision of 1 ns. The second part describes an analysis of a data sample corresponding to an integrated luminosity of 6.8 $\pm$ 1.0 $\mu b^{-1}$ and recorded in 2009 from proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 0.9 TeV. The double-differential $V^0 (K_S, \Lambda$ and $\bar{\Lambda}$ production cross-sections as a function of transverse momentum $p_T$ and rapidity $y$ have been measured using only the tracking system of the experiment. A simple selection using an $ad hoc$ geometrical variable based on the impact parameters of the daughters and mother particles has been developed. The results are compared with a few Monte Carlo (MC) generator tunings. They indicate that the $K_S$ production cross-section is mostly overestimated for $p_T$ < 0.4 GeV/$c$ and underestimated for $p_T$ < 1 GeV/$c$ for all considered models. For $\Lambda$ and $\bar{\Lambda}$, the production cross-section is mostly underestimated by all models for $p_T$ < 0.8 GeV/$c$. In addition, the data seem to indicate that the $\bar{\Lambda}/\Lambda$ ratio is smaller than predicted by the considered models.