16,261 research outputs found
Introduction to parton-shower event generators
This lecture discusses the physics implemented by Monte Carlo event
generators for hadron colliders. It details the construction of parton showers
and the matching of parton showers to fixed-order calculations at higher orders
in perturbative QCD. It also discusses approaches to merge calculations for a
varying number of jets, the interface to the underlying event and
hadronization.Comment: 40 pages, 12 figures. Lectures presented at TASI 201
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
The categorical limit of a sequence of dynamical systems
Modeling a sequence of design steps, or a sequence of parameter settings,
yields a sequence of dynamical systems. In many cases, such a sequence is
intended to approximate a certain limit case. However, formally defining that
limit turns out to be subject to ambiguity. Depending on the interpretation of
the sequence, i.e. depending on how the behaviors of the systems in the
sequence are related, it may vary what the limit should be. Topologies, and in
particular metrics, define limits uniquely, if they exist. Thus they select one
interpretation implicitly and leave no room for other interpretations. In this
paper, we define limits using category theory, and use the mentioned relations
between system behaviors explicitly. This resolves the problem of ambiguity in
a more controlled way. We introduce a category of prefix orders on executions
and partial history preserving maps between them to describe both discrete and
continuous branching time dynamics. We prove that in this category all
projective limits exist, and illustrate how ambiguity in the definition of
limits is resolved using an example. Moreover, we show how various problems
with known topological approaches are now resolved, and how the construction of
projective limits enables us to approximate continuous time dynamics as a
sequence of discrete time systems.Comment: In Proceedings EXPRESS/SOS 2013, arXiv:1307.690
Multiple Interactions and Beam Remnants
Open issues on the structure of multiple interactions are outlined. An
improved model is summarized, with a new approach to correlated parton
densities in flavour, colour, longitudinal and transverse momenta, for both
hard-scattering partons and beam-remnant ones.Comment: LaTeX, 5 pages, submitted to the proceedings of the Workshop on
Physics at TeV Colliders, Les Houches, France, 26 May - 6 June 200
Gaining analytic control of parton showers
Parton showers are widely used to generate fully exclusive final states
needed to compare theoretical models to experimental observations. While, in
general, parton showers give a good description of the experimental data, the
precise functional form of the probability distribution underlying the event
generation is generally not known. The reason is that realistic parton showers
are required to conserve four-momentum at each vertex. In this paper we
investigate in detail how four-momentum conservation is enforced in a standard
parton shower and why this destroys the analytic control of the probability
distribution. We show how to modify a parton shower algorithm such that it
conserves four-momentum at each vertex, but for which the full analytic form of
the probability distribution is known. We then comment how this analytic
control can be used to match matrix element calculations with parton showers,
and to estimate effects of power corrections and other uncertainties in parton
showers.Comment: 12 pages, 6 figures, v2: final journal versio
QCD Radiation off Heavy Particles
We study QCD radiation in decay processes involving heavy particles. As
input, the first-order gluon emission rate is calculated in a number of
reactions, and comparisons of the energy flow patterns show a non-negligible
process dependence. To proceed further, the QCD parton shower language offers a
convenient approach to include multi-gluon emission effects, and to describe
exclusive event properties. An existing shower algorithm is extended to take
into account the process-dependent mass, spin and parity effects, as given by
the matrix element calculations. This allows an improved description of
multiple gluon emission effects off b and t quarks, and also off nonstandard
particles like squarks and gluinos. Phenomenological applications are presented
for bottom production at LEP, Higgs particle decay to heavy flavours, top
production and decay at linear colliders, and some simple supersymmetric
processes.Comment: 44 pages, 15 pages, 4 table
Progress on Multiple Interactions
We report on the development of a new model for the underlying event in
hadron-hadron collisions. The model includes parton showers for all
interactions, as well as non-trivial flavour, momentum, and colour correlations
between interaction initiators and beam remnant partons.Comment: To appear in the proceedings of HEP 2003, 3p
A simple shower and matching algorithm
We present a simple formalism for parton-shower Markov chains. As a first
step towards more complete uncertainty bands, we incorporate a comprehensive
exploration of the ambiguities inherent in such calculations. To reduce this
uncertainty, we then introduce a matching formalism which allows a generated
event sample to simultaneously reproduce any infrared safe distribution
calculated at leading or next-to-leading order in perturbation theory, up to
sub-leading corrections. To enable a more universal definition of perturbative
calculations, we also propose a more general definition of the hadronization
cutoff. Finally, we present an implementation of some of these ideas for
final-state gluon showers, in a code dubbed VINCIA.Comment: 32 pages, 6 figure
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