arXiv subjects

Steffen Schumann

Publications and source records attributed to Steffen Schumann.

At least 55 records · Page 3Linked to original sources

arXiv2016

Simulating $b$-associated production of $Z$ and Higgs bosons with SHERPA

We compare four-- and five--flavour scheme predictions for $b$-associated production of $Z$ and Higgs bosons. The results are obtained with \Sherpa's MC@NLOimplementation 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 schemes. Comparison with data for $Z+b(\bar{b})$ production at the $7$ TeV LHC exhibit strengths and weaknesses of the different approaches and are used to validate predictions for $b$-associated Higgs-boson production at the 13 TeV Run II.

hep-ph↗

arXiv2016

Reweighting QCD matrix-element and parton-shower calculations

We present the implementation and validation of the techniques used to efficiently evaluate parametric and perturbative theoretical uncertainties in matrix-element plus parton-shower simulations within the Sherpa event-generator framework. By tracing the full $α_s$ and PDF dependences, including the parton-shower component, as well as the fixed-order scale uncertainties, we compute variational event weights on-the-fly, thereby greatly reducing the computational costs to obtain theoretical-uncertainty estimates.

hep-ph↗

arXiv2016

QCD Jet Rates with the Inclusive Generalized kt Algorithms

We derive generating functions, valid to next-to-double logarithmic accuracy, for QCD jet rates according to the inclusive forms of the kt, Cambridge/Aachen and anti-kt algorithms, which are equivalent at this level of accuracy. We compare the analytical results with jet rates and average jet multiplicities from the SHERPA event generator, and study the transition between Poisson-like and staircase-like behaviour of jet ratios.

hep-ph↗

arXiv2016

Aspects of pQCD at a 100 TeV future hadron collider

In this publication we consider particle production at a future circular hadron collider with 100 TeV centre of mass energy within the Standard Model, and in particular their QCD aspects. Accurate predictions for these processes pose severe theoretical challenges related to large hierarchies of scales and possible large multiplicities of final state particles. We investigate scaling patterns in multijet-production rates allowing to extrapolate predictions to very high final-state multiplicities. Furthermore, we consider large-area QCD jets and study the expectation for the mean number of subjets to be reconstructed from their constituents and confront these with analytical resummed predictions and with the expectation for boosted hadronic decays of top-quarks and W-bosons. We also discuss the validity of Higgs-Effective-Field-Theory in making predictions for Higgs-boson production in association with jets. Finally, we consider the case of New Physics searches at such a 100 TeV hadron-collider machine and discuss the expectations for corresponding Standard-Model background processes.

hep-ph↗

arXiv2015

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.

hep-ex↗

arXiv2015

Fast evaluation of theoretical uncertainties with Sherpa and MCgrid

The determination of theoretical error estimates and PDF/$α_S$-fits require fast evaluations of differential cross sections for varied QCD input parameters. These include PDFs, the strong coupling constant $α_S$ and the renormalisation and factorisation scales. Beyond leading order QCD, a full dedicated calculation for each set of parameters is often too time-consuming, certainly when performing PDF-fits. We report on two methods to overcome this issue for any QCD NLO calculation: The novel event-reweighting feature in Sherpa and the automated generation of interpolation grids using the recently introduced MCgrid interface. For MCgrid we present the newly added support for fastNLO tables and highlight some future developments.

hep-ph↗

arXiv2015

Beyond Standard Model calculations with Sherpa

We present a fully automated framework as part of the Sherpa event generator for the computation of tree-level cross sections in beyond Standard Model scenarios, making use of model information given in the Universal FeynRules Output format. Elementary vertices are implemented into C++ code automatically and provided to the matrix-element generator Comix at runtime. Widths and branching ratios for unstable particles are computed from the same building blocks. The corresponding decays are simulated with spin correlations. Parton showers, QED radiation and hadronization are added by Sherpa, providing a full simulation of arbitrary BSM processes at the hadron level.

hep-ph↗

arXiv2015

Soft evolution of multi-jet final states

We present a new framework for computing resummed and matched distributions in processes with many hard QCD jets. The intricate color structure of soft gluon emission at large angles renders resummed calculations highly non-trivial in this case. We automate all ingredients necessary for the color evolution of the soft function at next-to-leading-logarithmic accuracy, namely the selection of the color bases and the projections of color operators and Born amplitudes onto those bases. Explicit results for all QCD processes with up to $2\to 5$ partons are given. We also devise a new tree-level matching scheme for resummed calculations which exploits a quasi-local subtraction based on the Catani-Seymour dipole formalism. We implement both resummation and matching in the Sherpa event generator. As a proof of concept, we compute the resummed and matched transverse-thrust distribution for hadronic collisions.

hep-ph↗

arXiv2014

(MC)**3 -- a Multi-Channel Markov Chain Monte Carlo algorithm for phase-space sampling

A new Monte Carlo algorithm for phase-space sampling, named (MC)**3, is presented. It is based on Markov Chain Monte Carlo techniques but at the same time incorporates prior knowledge about the target distribution in the form of suitable phase-space mappings from a corresponding Multi-Channel importance sampling Monte Carlo. The combined approach inherits the benefits of both techniques while typical drawbacks of either solution get ameliorated.

hep-ph↗

arXiv2013

MCgrid: projecting cross section calculations on grids

MCgrid is a software package that provides access to the APPLgrid interpolation tool for Monte Carlo event generator codes, allowing for fast and flexible variations of scales, coupling parameters and PDFs in cutting edge leading and next-to-leading-order QCD calculations. This is achieved by providing additional tools to the Rivet analysis system for the construction of MCgrid enhanced Rivet analyses. The interface is based around a one-to-one correspondence between a Rivet histogram class and a wrapper for an APPLgrid interpolation grid. The Rivet system provides all of the analysis tools required to project a Monte Carlo weight upon an observable bin, and the MCgrid package provides the correct conversion of the event weight to an APPLgrid fill call. MCgrid has been tested and designed for use with the SHERPA event generator, however as with Rivet the package is suitable for use with any code which can produce events in the HepMC event record format.

hep-ph↗

arXiv2013

Understanding Jet Scaling and Jet Vetos in Higgs Searches

Jet counting and jet vetos are crucial analysis tools for many LHC searches. We can understand their properties from the distribution of the exclusive number of jets. LHC processes tend to show either a distinct staircase scaling or a Poisson scaling, depending on kinematic cuts. We illustrate our approach in a detailed study of jets in weak boson fusion Higgs production.

hep-ph↗

arXiv2012

Scaling Patterns for QCD Jets

Jet emission at hadron colliders follows simple scaling patterns. Based on perturbative QCD we derive Poisson and staircase scaling for final state as well as initial state radiation. Parton density effects enhance staircase scaling at low multiplicities. We propose experimental tests of our theoretical findings in Z+jets and QCD gap jets production based on minor additions to current LHC analyses.

hep-ph↗

arXiv2012

From Jet Counting to Jet Vetoes

The properties of multi-jet events impact many LHC analysis. The exclusive number of jets at hadron colliders can be described in terms of two simple patterns: staircase scaling and Poisson scaling. In photon plus jets production we can interpolate between the two patterns using simple kinematic cuts. The associated theoretical errors are well under control. Understanding such exclusive jet multiplicities significantly impacts Higgs searches and searches for supersymmetry at the LHC.

hep-ph↗

arXiv2012

Systematic improvement of QCD parton showers

In this contribution, we will give a brief overview of the progress that has been achieved in the field of combining matrix elements and parton showers. We exemplify this by focusing on the case of electron--positron collisions and by reporting on recent developments as accomplished within the Sherpa event generation framework.

hep-ph↗

arXiv2012

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.

hep-ph↗

arXiv2011

W+jets, Z+jets, multijets and new physics searches

W+jets, Z+jets and QCD multijet production processes at hadron colliders are backgrounds to many searches for physics beyond the Standard Model which involve leptons and missing energy in the final state. We review the current theoretical and experimental status of these processes at the LHC. Furthermore, we discuss several methods that allow for reliable predictions for these processes in the context of new-physics searches.

hep-ph↗

arXiv2011

Hadronically decaying color-adjoint scalars at the LHC

We study the phenomenology of the pair-production of scalar color-octet electroweak singlet states at the LHC. Such states appear in many extensions of the Standard Model. They can be pair-produced copiously at the LHC and will signal themselves as resonances in multijet final states. Beyond the QCD pair-production process we consider a vectorlike confinement scenario with an additional color-octet vector state. These vector particles can be produced in the s-channel and through their decay contribute to the scalar pair production. We point out the differences between the two hypotheses and device a strategy to distinguish them.

hep-ph↗

arXiv2011

Jets plus Missing Energy with an Autofocus

Jets plus missing transverse energy is one of the main search channels for new physics at the LHC. A major limitation lies in our understanding of QCD backgrounds. Using jet merging we can describe the number of jets in typical background channels in terms of a staircase scaling, including theory uncertainties. The scaling parameter depends on the particles in the final state and on cuts applied. Measuring the staircase scaling will allow us to also predict the effective mass for Standard Model backgrounds. Based on both observables we propose an analysis strategy avoiding model specific cuts which returns information about the color charge and the mass scale of the underlying new physics.

hep-ph↗