Track reconstruction and prompt $K^{0}_{S}$ production at the LHCb experiment

This work presents the implementation and performance of a track reconstruction algorithm for the main tracking system of LHCb. This algorithm has a high reconstruction efficiency and a low fraction of wrongly reconstructed tracks. It is fast enough to be used in the first stage of the LHCb software trigger to quickly confirm or reject the decision made by the hardware trigger. Moreover, it is the only algorithm in LHCb to reconstruct cosmic muons in the main tracking system, an important track sample for alignment and calibration purposes. Due to its good performance and speed, the algorithm reconstructs tracks in the standalone main tracking system at LHCb both in the software trigger and for offline reconstruction. Furthermore, the first measurement of the cross section from prompt $K^0_S$ production in $pp$ collisions at $\sqrt{s}$ = 900 GeV with the LHCb detector is presented. The $K^0_S$ is reconstructed via its decay to two charged hadrons. The measurement thus relies heavily on the understanding of the track reconstruction. Covering a range in rapidity of 2.5 $\leq$ y < 4 and $p_T$ < 1.6 GeV in $p_T$ (excluding $p_T$ < 200 MeV for 2.5 $\leq$ y < 3)$\sigma(pp \to K^0_{S}X)$ = (6410.3 $\pm$ 169.6 $\pm$ 463.19) $\mu b$ has been measured. The measurement can serve as input to validate and optimise Monte Carlo event generators. The correct description of fragmentation and the structure of the underlying event at hadron colliders by the Monte Carlo generators is crucial for many future LHCb measurements. Being the first measurement performed by LHCb, it has served as an important test for the analysis procedures and the understanding of the detector, before more complicated analyses have been performed.