The LHCb GPU high level trigger and measurements of neutral pion and photon production with the LHCb detector

In Run 3 of the LHC, the LHCb experiment will collect data without a hardware trigger. As a result, the LHCb software trigger must process the full LHC collision rate. This thesis presents Allen, an implementation of the LHCb high level trigger on GPUs. The Allen framework and reconstruction algorithms are described and the physics and computing performance are presented. This thesis also presents studies of neutral pion and photon production in proton-lead collisions. The LHCb detector's forward geometry allows it to study these processes in a previously unexplored kinematic regime. Measurements of the $\pi^0$ nuclear modification factor at forward and backward rapidities are presented. These results provide constraints on the cold nuclear matter effects that modify the partonic structure of nuclei. Additionally, measurements of the direct photon excess ratio in proton-proton and proton-lead collisions are presented.