The Inclusion of Higher-order Qcd Corrections into Parton Shower Monte Carlos

Event Generators (denoted as EvG’s henceforth) have been the workhorses of all modern experiments in high-energy physics. For good reasons: in spite of being conceptually simple, they provide fairly good descriptions of the real events occurring in detectors, allowing experimenters to perform a variety of tasks, from computing efficiencies to design strategies for achieving given measurements or searches. On the other hand, EvG’s may not be the ideal tools for predicting the physical observables with high accuracy, something that is needed in order to – say – extracting the non-computable parameters of the theory from data; traditionally, this task is performed by a class of codes that can be called cross section integrators (CSI’s). In a loose sense, CSI’s can also output events; however, such events can be used only to predict a limited number of observables (for example, the transverse momentum of single-inclusive jets) and are not a faithful description of actual events taking place in real detectors. Although complementary in nature, EvG’s and CSI’s are based on the same simple description of an elementary process (the hard subprocess), which doesn’t even need to be a physically-observable one. To clarify this point, let us consider a gedanken experiment which, at an imaginary accelerator that collides 45 GeV u-quarks with 45 GeV ū-quarks, observes a dd̄ quark pair produced through the decay of a Z. The process of interest is therefore uū → Z → dd̄ at 90 GeV. Any theoretical model describing this process must start from the knowledge of its cross section