Qcd at High Energy

After more than 25 years of considerable theoretical and experimental efforts, it appears that QCD is the theory of strong interactions. Ideally, in high-energy QCD one needs one single piece of information from the experiments: the value of αs. Starting from that measured value, every observable can be computed from first principles. In practice this is not feasible, since we don’t know how to perform calculations in terms of the hadrons that experiments measure in their detectors. Perturbation theory offers a viable way out, since it allows to prove, at least formally, the socalled factorization theorems. These give explicit prescriptions to write physical observables as the convolution of shortand long-distance parts, up to terms suppressed by the power of some large scale. We can imagine this factorization to occur at an arbitrary scale μ; with a suitable choice of μ the short-distance pieces, which are entirely expressed in terms of quarks and gluons, are perturbatively calculable. The long-distance pieces (such as parton densities) cannot be computed in perturbation theory, but their dependence on μ can. Furthermore, they are universal, which means that they don’t depend on short-distance physics, but solely on the nature of the hadrons involved, which is a key factor for perturbative QCD to have predictive power. Pending a general solution of QCD, the computing framework based on perturbation theory