The PYTHIA program is a standard tool for the generation of events in high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multiparticle final state. It contains a library of hard processes, models for initial- and final-state parton showers, matching and merging methods between hard processes and parton showers, multiparton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and several interfaces to external programs. PYTHIA 8.2 is the second main release after the complete rewrite from Fortran to C++, and now has reached such a maturity that it offers a complete replacement for most applications, notably for LHC physics studies. The many new features should allow an improved description of data.

This thesis involves phenomenological models for describing high energy particle collisions. One class of these models, multiparton interactions (MPI), deals with the fact that when colliding two hadrons, it is possible for more than one pair of their constituents to interact. The other deals with parton showers, which are used to fill enhanced regions of phase space and give fully exclusive final states. These two components, when part of an event generation framework, help in the simulation of complete high energy collision events. Paper I presents a study of an extension to the MPI model, enhanced screening, where the amount of colour screening in an incoming hadron is increased in those events with a large amount of activity. Paper II presents another extension to the MPI model, rescattering. Commonly, in MPI models, interactions are between pairs of partons which originate from the incoming hadrons. With rescattering, an MPI can instead involve partons which come from previous interactions or shower branchings. Paper III studies the first emission of the parton shower. First, the interface to a next-to-leading generator, POWHEG-hvq, is examined. Second, a dampening of the first shower emission, based on general matrix element arguments is studied. Paper IV again deals with parton showers. Minor changes to the framework are outlined, before a kinematic comparison of the shower is made against 2 to 3 QCD real emission matrix elements. Finally, tunes of the generator are made to both Tevatron and LHC data. Paper V examines an extension to the impact parameter formalism of the MPI model, where the size of an incoming proton is varied depending on the x value of a parton being taken from it.

General-purpose Monte Carlo event generators have become important tools in particle physics, allowing the simulation of exclusive hadronic final states. In this article we examine the Pythia 8 generator, in particular focusing on its parton-shower algorithms. Some relevant new additions to the code are introduced, that should allow for a better description of data. We also implement and compare with 2 → 3 real-emission QCD matrix elements, to check how well the shower algorithm fills the phase space away from the soft and collinear regions. A tuning of the generator to Tevatron data is performed for two PDF sets and the impact of first new LHC data is examined.

Theoretical arguments, supported by other indirect evidence, suggest that the wave function of high-x partons should be narrower than that of low-x ones. In this article, we present a modification to the variable impact parameter framework of Pythia 8 to model this effect. In particular, a Gaussian hadronic matter profile is introduced, with a width dependent on the x value of the constituent being probed. Results are compared against the default single-and double-Gaussian profiles, as well as an intermediate overlap function.

The concept of multiple partonic interactions in hadronic events is vital for the understanding of both minimum-bias and underlying-event physics. The area is rather little studied, however, and current models offer a far from complete coverage, even of the effects we know ought to be there. In this article we address one such topic, namely that of rescattering, where an already scattered parton is allowed to take part in another subsequent scattering. A framework for rescattering is introduced for the Pythia 8 event generator and fully integrated with normal multiparton interactions and initial- and final-state radiation. Using this model, the effects on event structure are studied, and distributions are shown both for minimum-bias and jet events.

Several methods to improve the parton-shower description of hard processes by an injection of matrix-element-based information have been presented over the years. In this article we study (re)weighting schemes for the first/hardest emission. One objective is to provide a consistent matching of the POWHEG next-to-leading order generator to the Pythia shower algorithms. Another is to correct the default behaviour of these showers at large transverse momenta, based on a comparison with real-emission matrix elements.

Modelling multiple partonic interactions in hadronic events is vital for understanding minimum-bias physics, as well as the underlying event of hard processes. A brief overview of the current Pythia 8 multiple interactions (MI) model is given, before looking at two additional effects which can be included in the MI framework. With rescattering, a previously scattered parton is allowed to take part in another subsequent scattering, while with enhanced screening, the effects of varying initial-state fluctuations are modelled.