High-energy QCD: semi-hard processes and unintegrated gluon densities

The high energies reachable at the LHC and in possible future hadron, lepton and lepton-hadron colliders offer us a great chance in the search for long-waited signals of New Physics. They offer, however, also a unique opportunity to test the Standard Model in unprecedented kinematic ranges. The study of semi- hard reactions still represent a challenge in the high-energy limit of QCD. Here, the fixed-order perturbative description misses the effect of large energy loga- rithms, which compensate the smallness of the QCD coupling constant and must therefore be resummed to all orders. The Balitsky-Fadin-Kuraev-Lipatov (BFKL) approach is the most powerful tool to resum to all orders these large logarithms both in the leading (LLA) and the next-to-leading (NLA) approximation. With the aim to improve our understanding of QCD dynamics, in the first part of this seminar we present our recent phenomenological analyses of different processes in the NLA BFKL. Then, unveiling the connection between the small-x world and unintegrated parton densities, we introduce and focus on the unintegrated gluon distribution (UGD), describing the probability that a gluon can be emitted by a colliding proton, with definite longitudinal fraction and transverse momentum. Through the leptoproduction of the ρ-meson at HERA, we investigate existent models for the UGD and compare them with experimental data. Some novel studies on differential cross sections for the forward Drell–Yan reaction will be also presented.