2023

Penttala, Jani

Diffractive processes are very sensitive to the target’s gluon distribution in the highenergy limit, making them a good candidate for probing the target in the nonlinear region of quantum chromodynamics. The nonlinear effects are expected to eventually lead to gluon saturation which is naturally described in the color-glass condensate (CGC) effective field theory. While there are strong hints of gluon saturation in the currently available data, no unambiguous signal has been observed. It is then important to improve the theoretical understanding of processes sensitive to saturation to find a clear difference between predictions from the linear and nonlinear regions of QCD. This includes calculating diffractive processes beyond the leading order in perturbation theory. In this thesis, we calculate diffractive processes at next-to-leading order (NLO) in the high-energy limit, with an emphasis on exclusive vector meson production and inclusive diffraction in deep inelastic scattering (DIS). Calculations in the highenergy limit can be done using the dipole picture, the basics of which are briefly reviewed. This includes using the CGC effective field theory to describe the nonperturbative dipole-target scattering amplitude which appears in practically all calculations in the dipole picture. The universality of the dipole-target scattering amplitude at NLO is shown numerically, in the sense that the same dipole-target scattering amplitude can be used to describe the data in both massless and massive quark production in inclusive DIS, and also in diffractive processes where exclusive vector meson production is considered. The analytical NLO calculations of exclusive vector meson production and inclusive diffraction in DIS are also explained. Exclusive vector meson production is calculated in the nonrelativistic limit for heavy mesons and the limit of large photon virtuality for light mesons. Also, the importance of including relativistic corrections to the heavy vector meson wave function in exclusive vector meson production is considered. For inclusive diffraction in DIS, we focus on the NLO corrections to the final state and show how the divergences cancel.