Introduction to the theory of LHC collisions

This chapter illustrates the basic principles underlying the use of perturbative QCD in predicting the structure of hard processes in high-energy hadronic collisions. It starts with a discussion of the factorization formula, which is the basis for the description of all hard processes in terms of universal functions parametrizing the density of quarks and gluons inside the proton. It then discusses the evolution of the perturbative final states, made of quarks and gluons, toward physical systems made of hadrons. Finally, several applications and examples of comparisons between the theoretical predictions and current data are presented. These provide a picture of the success of this theoretical framework, giving good confidence in the reliability of its future applications to the study of LHC collisions.