J/ψ near-threshold photoproduction off the proton and neutron with CLAS12

In recent years, J/ψ photoproduction in the near-threshold region has seen a renewed theoretical interest due to the wealth of information it has to offer. Near-threshold J/ψ photoproduction proceeds through the exchange of gluons in the t-channel and is expected to provide unique insight about the nucleon gluonic gravitational form factors (GFFs). Previous studies at the Thomas Jefferson National Accelerator Facility (JLab) in Virginia, USA, have already measured the total and differential cross section of J/ψ near-threshold photoproduction on the free proton. This thesis presents the first measurements of J/ψ near-threshold photoproduction on the bound proton and bound neutron in a deuteron target. The CEBAF Large Acceptance Spectrometer at 12 GeV (CLAS12) based at JLab's Hall B uses a 11 GeV electron beam impinging on a fixed liquid deuteron target. J/ψ is then produced via the exchange of a quasi-real photon and decays to a lepton pair which is detected alongside the recoil nucleon by CLAS12. Analysis procedures and in particular machine learning based techniques were developed in order to correctly identify final state particles and select J/ψ photoproduction events. The total and differential cross section of J/ψ near-threshold photoproduction on the bound proton and bound neutron were then measured from the detected J/ψ photoproduction events. The measurements of the cross sections on the bound proton and bound neutron agree well within their statistical uncertainty. This is consistent with the assumed two-gluon exchange production mechanism which is isospin invariant. Overall a first measurement of the J/ψ near-threshold photoproduction cross sections on the bound proton and bound neutron was achieved. A better understanding of the mechanical properties of the nucleon, such as pressure and mass distributions, can be obtained by relating J/ψ production to the nucleon GFFs. This opens the way for exciting new insights into the internal structure of the nucleon, and in particular of the nucleon's gluonic content. An upcoming overhaul of the CLAS12 reconstruction will increase the reconstruction efficiency and the statistical precision of the preliminary measurements described in this thesis, with an expected gain in statistics of at least 50%. Future upgrades at JLab will allow to test some of the theoretical assumptions made in relating J/ψ near-threshold photoproduction to the nucleon GFFs.