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| 2. |
- Bothmann, Enrico, et al.
(författare)
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A standard convention for particle-level Monte Carlo event-variation weights
- 2023
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Ingår i: SciPost Physics Core. - 2666-9366. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Bellm, Johannes, et al.
(författare)
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High dimensional parameter tuning for event generators
- 2020
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 80:1
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Tidskriftsartikel (refereegranskat)abstract
- Monte Carlo Event Generators are important tools for the understanding of physics at particle colliders like the LHC. In order to best predict a wide variety of observables, the optimization of parameters in the Event Generators based on precision data is crucial. However, the simultaneous optimization of many parameters is computationally challenging. We present an algorithm that allows to tune Monte Carlo Event Generators for high dimensional parameter spaces. To achieve this we first split the parameter space algorithmically in subspaces and perform a Professor tuning on the subspaces with binwise weights to enhance the influence of relevant observables. We test the algorithm in ideal conditions and in real life examples including tuning of the event generators Herwig 7 and Pythia 8 for LEP observables. Further, we tune parts of the Herwig 7 event generator with the Lund string model.
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| 4. |
- Frederix, Rikkert, et al.
(författare)
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Matrix element corrections in top quark decays for the tt¯W± process
- 2024
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Ingår i: European Physical Journal C. - 1434-6044. ; 84:4
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Tidskriftsartikel (refereegranskat)abstract
- We present a method that allows enabling matrix element corrections (MECs) in Pythia8 with MC@NLO matching, without incurring double counting. MECs are an interesting feature that may contribute to the accuracy of theoretical predictions, alongside matching and merging. We directly compare our method to a specific choice of settings in Pythia8, which can remove double-counting for MECs in certain processes. We show results by taking the tt¯W process as an example. This choice allows us to study the impact of decay MECs in the 2SSℓ and 3ℓ final states. We find that jet-related observables receive these corrections unevenly throughout the phase space. They can contribute up to ±6% in certain regions.
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| 5. |
- Gellersen, Leif, et al.
(författare)
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Coloring mixed QCD/QED evolution
- 2022
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Ingår i: SciPost Physics. - 2542-4653. ; 13:2
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Tidskriftsartikel (refereegranskat)abstract
- 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 subdominant 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.
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| 6. |
- Gellersen, Leif, et al.
(författare)
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Disentangling soft and collinear effects in QCD parton showers
- 2022
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Ingår i: Physical Review D. - 2470-0010. ; 105:11
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Gellersen, Leif, et al.
(författare)
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Scale and scheme variations in unitarized NLO merging
- 2020
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Ingår i: Physical Review D. - 2470-0010. ; 101:11
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Tidskriftsartikel (refereegranskat)abstract
- 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, multiparton scattering, or hadronization. In this study, we discuss possible pitfalls in defining the perturbative uncertainty of unitarized next-to-leading order (NLO) multijet 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.
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| 8. |
- Gellersen, Leif
(författare)
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Tackling the Uncertainties of Event Generators
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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