Exceptionally empty charm - Central exclusive production of charmonia at LHCb

Protons, together with neutrons and electrons, make up all matter. Once thought as a fundamental particle, protons are made up of quarks and gluons. These are elementary particles, and are bound within the proton by the strong force, one of the four fundamental forces of nature. Quarks and gluons, known as partons, carry different fractions of the total proton momentum. The probability of a given parton to have a specific fraction of the total proton momentum is given by the parton distribution functions (PDFs). The aforementioned momentum fraction is known as Bjorken-$x$. These PDFs are based on experimental data, and the content of the proton in terms of PDFs is better documented for high Bjorken-$x$ values than for low ones. At low Bjorken-$x$ values, the gluon PDF is expected to be dominant. However, barely any data is available and the gluon PDF remains largely unconstrained in that region. The LHCb experiment allows to access that region via the central exclusive production of charmonia mesons. This diffractive process involves two protons interacting via the exchange of a pomeron and a photon. The thesis presents valuable experimental results that can be used to determine the gluon PDF more precisely at low Bjorken-$x$ values. In addition, this thesis provides some insight on the nature of the pomeron.