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

The midpoint between dipole and parton showers

We present a new parton-shower algorithm. Borrowing from the basic ideas of dipole cascades, the evolution variable is judiciously chosen as the transverse momentum in the soft limit. This leads to a very simple analytic structure of the evolution. A weighting algorithm is implemented, that allows to consistently treat potentially negative values of the splitting functions and the parton distributions. We provide two independent, publicly available implementations for the two event generators Pythia and Sherpa.Comment: 23 pages, 9 figure

Higgs-boson production through gluon fusion at NNLO QCD with parton showers

We discuss how the UN2LOPS scheme for matching NNLO calculations to parton showers can be applied to processes with large higher-order perturbative QCD corrections. We focus on Higgs-boson production through gluon fusion as an example. We also present an NNLO fixed-order event generator for this reaction.Comment: 9 pages, 3 figures, 1 tabl

Simulation of vector boson plus many jet final states at the high luminosity LHC

We present a novel event generation framework for the efficient simulation of vector boson plus multi-jet backgrounds at the high-luminosity LHC and at possible future hadron colliders. MPI parallelization of parton-level and particle-level event generation and storage of parton-level event information using the HDF5 data format allow us to obtain leading-order merged Monte-Carlo predictions with up to nine jets in the final state. The parton-level event samples generated in this manner correspond to an integrated luminosity of 3ab-1 and are made publicly available for future phenomenological studies.Comment: 10 pages, 8 figures, 5 table

Perturbative QCD in event generation

This thesis is concerned with the simulation of particle physics processes involving strong interactions in modern event generators. New algorithms to reinstate colour in colour- ordered amplitudes through colour dressing are presented and their analytical and numerical properties are discussed in detail. The colour-dressed Berends-Giele recursive relations are extended to the full Standard Model Lagrangian and implemented into the numerical program Comix for large multiplicity matrix element computation. New algorithms for phase space integration are proposed, whereof one is capable to effectively couple colour and momentum sampling. Comparisons to other high-multiplicity generators are shown. QCD parton evolution and the CKKW algorithm to correctly include real next-to-leading order corrections are revisited. New types of jet measures are proposed for the merging of matrix elements and parton showers and their analytical and numerical properties are discussed. The implementation into the event generator Sherpa is presented using two different types of matrix element generators. Corresponding results and comparisons are shown. A further comparison between different types of merging algorithms is presented, including various numerical codes, which implement different merging approaches. Finally, the implementation of BFKL evolution in a Markovian approach is introduced and corresponding results from a numerical simulation are presented. Implications on event generation for current and future colliders are discussed throughout

Comix, a new matrix element generator

Abstract: We present a new tree-level matrix element generator, based on the recently introduced colour dressed Berends-Giele recursive relations. We discuss two new algorithms for phase space integration, dedicated to be used with large multiplicities and colour sampling.

Next-to-leading order QCD predictions for top-quark pair production with up to three jets

We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with Sherpa and OpenLoops. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale HT/2 at fixed order and the MINLO procedure. Analyzing various cross sections and distributions for tt+0,1,2,3 jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and MINLO results. The differences are typically below the respective factor-two scale variations, suggesting that for all considered jet multiplicities, missing higher-order effects should not exceed the ten percent level.Comment: 10 pages, 6 figures, 2 table