Tackling the Uncertainties of Event Generators

This thesis is composed of four papers, which concern Monte Carlo event generators for high energy particle physics. Precise predictions of the outcome of particle collisions are important for the exploration of the Standard Model of particle physics at collider experiments like the LHC at CERN. The papers address the uncertainties of these predictions in different contexts.Paper I presents an algorithm for tuning Monte Carlo event generators with high dimensional parameter spaces by splitting the parameter space algorithmically. The algorithm is tested in ideal conditions and real-life examples of tuning Herwig and Pythia for LEP.Paper II concerns the perturbative uncertainty of unitarized next-to-leading order multi-jet merging prescriptions by discussing scale variations and scheme variations. The uncertainties are addressed for collisions at LEP and LHC.Paper III deals with QCD/QED interference effects in parton showers. For this purpose, we implement a shower including QED and QCD emissions and tree-level matrix element corrections for fixed color configurations.Paper IV proposes a method for the consistent removal of overlapping singularities in QCD parton showers at next-to-leading order in the strong coupling

Scale and Scheme Variations in Unitarized NLO Merging

Precision background predictions with well-defined uncertainty estimates are important for interpreting collider-physics measurements and for planning future high-energy collider experiments. It is especially important to estimate the perturbative uncertainties in predictions of inclusive measurements of jet observables, that are designed to be largely insensitive to non-perturbative effects such as the structure of beam-remnants, multi-parton scattering or hadronization. In this study, we discuss possible pit-falls in defining the perturbative uncertainty of unitarized next-to-leading order multi-jet merged predictions, using the PYTHIA event generator as our vehicle. For this purpose, we consider different choices of unitarized NLO merging schemes as well as consistent variations of renormalization scales in different parts of the calculation. Such a combined discussion allows to rank the contribution of scale variations to the error budget in comparison to other contributions due to algorithmic choices that are often assumed fixed. The scale uncertainty bands of different merging schemes largely overlap, but differences between the "central" predictions in different schemes can remain comparable to scale uncertainties even for very well-separated jets, or be larger than scale uncertainties in transition regions between calculations of different jet multiplicity. The availability of these variations within PYTHIA will enable more systematic studies of perturbative uncertainties in precision background calculations in the future.Comment: 15 pages, 4 figures with example result

Coloring mixed QCD/QED evolution

Parton showers are crucial components of high-energy physics calculations. Improving their modelling of QCD is an active research area since shower approximations are stumbling blocks for precision event generators. Naively, the interference between sub-dominant Standard-Model interactions and QCD can be of similar size to subleading QCD corrections. This article assesses the impact of QCD/QED interference effects in parton showers, by developing a sophisticated shower including QED, QCD at fixed color, and employing complete tree-level matrix element corrections for individual NC=3 color configurations to embed interference. The resulting simulation indicates that QCD/QED interference effects are small for a simple test case and dwarfed by electro-weak resonance effects

Tuning Geneva+Pythia 8 Using Professor 2

We study the tuning of the Geneva Monte Carlo framework to LHC data. Geneva improves the predictions for Drell-Yan production by including NNLO QCD corrections and extending the resummation accuracy to NNLL$'$ for 0-jettiness and NLL for 1-jettiness. The partonic results provided by Geneva are interfaced to Pythia 8 for showering including its multiple parton interaction (MPI) model. This allows us to obtain sensible predictions for Underlying Event (UE) sensitive observables too. Retuning Geneva + Pythia 8 to LHC data with the Professor 2 package shows an improved agreement for both UE sensitive and more inclusive observables

Disentangling soft and collinear effects in QCD parton showers

We introduce a method for the separation of soft and collinear logarithms in QCD parton evolution at O(αs2) and at leading color. Using an implementation of the technique in the dire parton shower, we analyze the numerical impact of genuine triple-collinear corrections from quark pair emission in e+e-→ hadrons

A comprehensive guide to the physics and usage of PYTHIA 8.3

This manual describes the PYTHIA 8.3 event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision "events", i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and reproduce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the PYTHIA web page: https://www.pythia.org/Comment: 315 pages, 19 figures. The program code is available at https://www.pythia.org

A comprehensive guide to the physics and usage of PYTHIA 8.3

This manual describes the Pythia event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision "events", i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and re-produce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the Pythia web page: https://www.pythia.org/.peerReviewe

A standard convention for particle-level Monte Carlo event-variation weights

Streams of event weights in particle-level Monte Carlo event generators are a convenient and immensely CPU-efficient approach to express systematic uncertainties in phenomenology calculations, providing systematic variations on the nominal prediction within a single event sample. But the lack of a common standard for labelling these variation streams across different tools has proven to be a major limitation for event-processing tools and analysers alike. Here we propose a well-defined, extensible community standard for the naming, ordering, and interpretation of weight streams that will serve as the basis for semantically correct parsing and combination of such variations in both theoretical and experimental studies