Fixed-target physics with the LHCb experiment at CERN

Owing to its detector geometry covering the forward direction and its excellent track reconstruction and particle identification performance, the LHCb experiment at CERN started in 2015 a pioneering study of beam-gas collisions by injecting noble gases in the LHC accelerator. In this thesis, my contributions to this physics programme, addressing the demand for extending the physics reach of the LHC complex stressed in the 2020 European Strategy for Particle Physics Update, are presented. Using a sample of proton-helium collisions collected in 2016, I performed the measurement of the non-prompt antiproton production at $\sqrt{s_{\textrm{NN}}}$ = 110 GeV, complementing a previous analysis only considering the prompt component. The results provide a key ingredient to a more accurate description of the antiproton production in collisions between primary cosmic rays and the interstellar medium, representing the dominant limitation for the interpretation of data on the antiproton flux in space. This is being measured by experiments, notably the AMS-02 spectrometer, searching for an excess over the expected flux that could provide an indirect evidence for a hypothetical Dark Matter particle annihilation or decay process. Being the particle identification performance in fixed-target data limited by the size of calibration samples, I conceived and developed a machine-learning and data-based approach to its parametrization and demonstrated that it achieves a better performance with respect to models based on the LHCb detailed simulation. An upgrade of the fixed-target device is ongoing, mainly consisting in the installation of a 20-cm-long cell confining the gas target in a region upstream the nominal LHCb interaction point. During my Ph.D. programme, I have been responsible for the software activities aiming to allow beam-beam and beam-gas data to be simultaneously acquired. I tested and adapted the full event reconstruction sequence on simulated beam-gas data and implemented a selection strategy within the challenging real-time framework being deployed for the LHCb upgrade. In view of the significant increase of the luminosity and of the wider choice of gas species offered by the target upgrade, this work is paving the way to the implementation of a unique full-scale fixed-target experiment at the LHC.