W+jets as a background to top physics: the quest for many jets

The latest progress in calculating electroweak gauge boson production in association with QCD jets at hadron colliders is summarized. Particular emphasis is given to the recently completed QCD one-loop calculations of W+3jets and Wb final states. Furthermore recent developments in improving Monte Carlo event generators by means of combining tree-level matrix elements with parton showers is reviewed.Comment: Talk given at Top 2010, Bruges, Belgium, May 31 - June 4, 201

Hard photon production and matrix-element parton-shower merging

We present a Monte-Carlo approach to prompt-photon production, where photons and QCD partons are treated democratically. The photon fragmentation function is modelled by an interleaved QCD+QED parton shower. This known technique is improved by including higher-order real-emission matrix elements. To this end, we extend a recently proposed algorithm for merging matrix elements and truncated parton showers. We exemplify the quality of the Monte-Carlo predictions by comparing them to measurements of the photon fragmentation function at LEP and to measurements of prompt photon and diphoton production from the Tevatron experiments.Comment: 18 pages, 5 figures, revised version with minor update

Bottom-quark mass effects in associated production with ZZ and HH bosons

In this study, predictions obtained in the four and in the five flavour schemes are compared for two important processes involving heavy flavours at the LHC: the production of a $Z$ or a Higgs boson in association with $b$ quarks. In particular we obtain predictions with \Sherpa's \MCatNLO implementation for the four--flavour scheme, treating the $b$'s as massive, and with multijet merging at leading and next-to leading order for the five--flavour scheme. While differences between the two schemes, at the inclusive level, are well understood from resummation of possibly large logs into the $b$-PDFs, differences in shape present a major problem for experimental measurements. We make use of data for $Z+b(\bar{b})$ production at the $7$ TeV LHC to exhibit strengths and weaknesses of the different approaches and we use these results to validate predictions for $b$-associated Higgs-boson production at the 13 TeV Run II.Comment: Proceedings of XXV International Workshop on Deep-Inelastic Scattering and Related Subjects, 3-7 April 2017, University of Birmingham, UK. 10 pages, 6 figure

Studies of Quantum Chromodynamics at the LHC

A successful description of hadron-hadron collision data demands a profound understanding of quantum chromodynamics. Inevitably, the complexity of strong-interaction phenomena requires the use of a large variety of theoretical techniques -- from perturbative cross-section calculations up to the modelling of exclusive hadronic final states. Together with the unprecedented precision of the data provided by the experiments in the first running period of the LHC, a solid foundation of hadron-hadron collision physics at the TeV scale could be established that allowed the discovery of the Higgs boson and that is vital for estimating the background in searches for new phenomena. This chapter on studies of quantum chromodynamics at the LHC is part of a recent book on the results of LHC Run 1 and presents the advances in theoretical methods side-by-side with related key measurements in an integrated approach.Comment: 49 pages, 24 figures, To appear in "The Large Hadron Collider -- Harvest of Run 1", Thomas Sch\"orner-Sadenius (ed.), Springer, 2015 (532 pages, 253 figures; ISBN 978-3-319-15001-7, for more details, see http://www.springer.com/de/book/9783319150000

Constraining scalar resonances with top-quark pair production at the LHC

Constraints on models which predict resonant top-quark pair production at the LHC are provided via a reinterpretation of the Standard Model (SM) particle level measurement of the top-anti-top invariant mass distribution, $m(t\bar{t})$. We make use of state-of-the-art Monte Carlo event simulation to perform a direct comparison with measurements of $m(t\bar{t})$ in the semi-leptonic channels, considering both the boosted and the resolved regime of the hadronic top decays. A simplified model to describe various scalar resonances decaying into top-quarks is considered, including CP-even and CP-odd, color-singlet and color-octet states, and the excluded regions in the respective parameter spaces are provided.Comment: 34 pages, 17 figure

Simulation of signal and background processes for collider experiments

This thesis is concerned with the theoretical modelling of scattering processes as they are studied at high-energy collider experiments. Special attention is thereby given to an accurate simulation of multi-particle final states that are expected to constitute the key signatures for the existence of physics beyond the Standard Model of particle physics

Establishing jet scaling patterns with a photon

Staircase and Poisson scaling are two typical patterns we observe for the exclusive number of jets at high energy hadron colliders. We examine these scaling properties for photon plus jets production at the LHC and find that this channel is well suited to study these features. We illustrate and discuss when to expect each of the two patterns, how to induce a transition through kinematic cuts, and how photons are related to heavy gauge bosons. Measurements of photon+jets production is therefore providing valuable information on exclusive jet scaling, which is going to help to eventually understand the theoretical origin of exclusive jet scaling properties in more detail