The center-of-mass energies available at modern accelerators, such as the Large Hadron Collider (LHC), and at forthcoming generation accelerators, such as the Electron-Ion Collider (EIC), offer us a unique opportunity to investigate hadronic matter under the most extreme conditions ever reached. In particular, they allow access to the so-called Regge-Gribov (or semi-hard) limit of QCD, characterized by the scale hierarchy $s \gg \{Q^2 \} \gg \Lambda_{{ \rm{QCD}}}^2$, where $\sqrt{s}$ is the center-of-mass energy, $\{ Q \}$ a set of hard scales characterizing the process and $\Lambda_{{\rm{QCD}}}$ is the QCD mass scale. This kinematic limit is the stage where some of the most intriguing phenomena of strong interactions manifest themselves. One of these is the growth of gluon density in protons, explained in the context of the Balitsky-Fadin-Kuraev-Lipatov (BFKL) approach in terms of the gluon Reggeization in QCD. Furthermore, the ultimate saturation of this parton density within protons or nuclei leads to the formation of a state of hadronic matter, characterized by a high particle density and a disordered field distribution, known as the Color Glass Condensate (CGC).
In this talk, I will discuss various aspects of high-energy QCD, specifically within the BFKL and saturation regimes, with a focus on the precision frontier.