The LHCb fixed-target charm

This accreditation to supervise researches focuses on the last six years of the author's professional career. His last post-doctoral position at the Laboratoire Leprince-Ringuet introduced him to the experimental study of QCD, with the exciting challenge of exploiting the unique potential of the LHCb detector and its gas injection system, SMOG. This unusual combination transforms the LHCb experiment into the LHC's only fixed-target experiment. This fixed-target configuration is particularly relevant for constraining quantum chromodynamics via the production and hadronization of the charm quark. Although studied for fifty years, no theoretical model is currently able to describe the production and hadronization of cc-bar pairs in a comprehensive way. However, the LHCb experiment in fixed-target configuration provides access to a hitherto unexplored kinematic region: it is now possible to study the hadronization of the c quark in the valence region of the target nucleons, giving rise to unprecedented constraints in QCD. In addition to constraining the mechanisms involved in the hadronization of these hadrons, measurements of the production of charmonia (cc-bar bound states) and charmed hadrons (containing a single c quark) in various proton-nucleus collisions enable us to study in detail the impact of the nuclear environment on these mechanisms. This quantification, essential to improving our understanding of QCD, is fundamental to the correct interpretation of measurements made in lead-nucleus collisions. Quark-gluon plasma droplets, the extreme state of QCD where quarks and gluons are free, are likely to be created in these lead-nucleus collisions. The so-called sequential suppression of charmonia (J/ψ, ψ(2S), χ_c) in a quark-gluon plasma, predicted since 1986, is a clear probe of the expected color shielding expected within the plasma. To date, this suppression has not been observed in its entirety. To test this color shielding unambiguously, it is therefore essential to measure the production of the 3 charmonia (J/ψ, ψ(2S), χ_c) in a wide range of proton-nucleus and lead-nucleus collisions. The first chapter of this accreditation is dedicated to a brief theoretical presentation of charm production and suppression. In the second chapter, the LHCb detector is introduced, including a description of the fixed-target system and associated data acquisition. The first results of the physics program dedicated to the production of charm in a fixed target configuration are presented. The results obtained by studying pAr and pHe collisions are presented in chapter 3. Although statistically limited, they have made it possible to acquire longer data sets, leading to the first physics results relevant to both charm hadronization and charmonia suppression in nucleus-nucleus collisions. Chapter 4 focuses on these new physics results, obtained with the longest SMOG data capture of LHC run 2, namely pNe collisions. These results were the subject of two papers, respectively dedicated to the study of J/ψ and D⁰ meson production. In 2018, the LHCb experiment collected the first statistically interesting batch of Pb-nucleus collisions in a fixed target configuration (PbNe data). Chapter 5 is dedicated to measuring J/ψ and D⁰ production in these PbNe collisions. All fixed-target data from the LHC Run 2 collisions, which were large enough to produce cc-bar pairs, are presented here. The final chapter presents the new gas system, set up for run 3, which will drastically increase future data acquisition. A brief overview of run 3, in terms of both statistics and analysis, is given, illustrating the entry of the study of charm in a fixed target configuration into the era of precision