Heavy Quark Fragmentation

I present the main aspects of open charm production in \pi^-\p collisions in the context of the Lund String Fragmentation Model as implemented in the Monte Carlo program PYTHIA. The emphasis is on the transition from large to small strings and the dependence on model parameters. A modified version is presented and compared with experimental results both on asymmetries, single-charm spectra and correlations.Comment: 10 pages, 6 figures, to appear in the proceedings of the Workshop on Heavy Quarks at Fixed Target, editors H.W.K. Cheung and J.N. Butle

High-Energy-Physics Event Generation with PYTHIA 6.1

PYTHIA version 6 represents a merger of the PYTHIA 5, JETSET 7 and SPYTHIA programs, with many improvements. It can be used to generate high-energy-physics `events', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions. The underlying physics is not understood well enough to give an exact description; the programs therefore contain a combination of analytical results and various models. The emphasis in this article is on new aspects, but a few words of general introduction are included. Further documentation is available on the web.Comment: 1 + 27 pages, submitted to Computer Physics Communication

QCD Phenomenology of Heavy Particle Dynamics

In this thesis the phenomenology of Quantum Chromo Dynamics (QCD) is studied in a variety of circumstances from both perturbative and non-perturbative perspectives. The main emphasis is on the production and hadronization of heavy particles, specifically heavy quarks, gauge bosons, leptoquarks and supersymmetric particles. In the hadroproduction of long-lived heavy quarks (i.e. charm and bottom) in the context of string fragmentation, the pull of a beam remnant at the other end of a string may give a heavy hadron more energy than the perturbatively produced quark. The collapse of a low-mass string to a single hadron is the extreme case of this effect, and gives rise to asymmetries between hadron and anti-hadron spectra. The Lund string fragmentation model is studied and improved by considering fixed-target data and the model is then used to predict the outcome of future experiments. The describtion of QCD radiation off heavy quarks and supersymmetric coloured particles is improved by using matrix element corrections to the massless parton shower in the event generator PYTHIA. In the process e+e- -> W+W- -> q qbar q' qbar', the colour flow can be ambiguous and a transverse momentum based measure of the colour flow is investigated as a tool to determine string topologies. The method is used to study different models of colour reconnection. QCD aspects of leptoquark production at HERA are discussed with focus on initial- and final-state QCD radiation. By taking into account corrections to this in mass reconstructions of a leptoquark, a narrower mass peak and a better average mass value is obtained

Effects of colour reconnection in W

We have investigated some observable consequences of colour reconnection in the reaction e + e \Gamma ! W + W \Gamma ! q 1 q 2 q 3 q 4 . We use a measure based on the momentum structure of the event, comparing reconnected events with `standard' ones. Some different recoupling models are described and studied as well as simpler toy models to show where the effects are present and what makes most of them disappear in realistic models. Some attempts are made to introduce suitable cuts in order to select the interesting events. 1 [email protected] 2 [email protected] 1 Introduction The reaction e + e \Gamma ! fl =Z 0 ! qq is well known and has been studied for some time, for example at LEP at CERN, where the Z 0 has been produced at roughly 90 GeV energy. This reaction is well described by QCD in the perturbative (shower) region and the Lund string model [1] in the non perturbative (fragmentation) one. In the Lund model strings are drawn between the quarks, poss..