Semihard QCD Processes beyond the Collinear Approximation

The main features and characteristics of the k_T -factorization QCD approach, which is based on BFKL-like noncollinear gluon evolution equations, are outlined. One of the advantages of this approach is connected with a convenience of including a significant part of higher order perturbative QCD corrections. The explicit expression and solutions of some parton distribution evolution equations in the leading logarithmic approximation are presented. Frequently used in calculations gluon and quark distribution functions, which depend on the transverse momenta of the initial interacting particles, are shown. Various techniques to calculate the off-shell amplitudes of parton subprocesses are discussed. The application of the k_T -factorization approach to some QCD processes that are intensively studied in modern experiments at the LHC is considered, in particular, to processes of the single and double production of heavy quarks, as well as the inclusive and associated (correlated with one or two hadronic jets) production of direct photons (or heavy gauge boson) in proton collisions at high energies. A comparison of the obtained results with predictions made using the standard (collinear) approach in the next-to-leading order of perturbative QCD is presented. Basic capabilities and features of modern Monte-Carlo event generators PEGASUS, CASCADE and KATIE are also discussed.