Stefan Hoeche, Frank Krauss, Marek Schonherr, Frank Siegert
In this publication, uncertainties in and differences between the MC@NLO and POWHEG methods for matching next-to-leading order QCD calculations with parton showers are discussed. Implementations of both algorithms within the event generator Sherpa and based on Catani-Seymour subtraction are employed to assess the impact on a representative selection of observables. In the case of MC@NLO a substantial simplification is achieved by using dipole subtraction terms to generate the first emission. A phase space restriction is employed, which allows to vary in a transparent way the amount of non-singular radiative corrections that are exponentiated. Effects on various observables are investigated, using the production of a Higgs boson in gluon fusion, with or without an associated jet, as a benchmark process. The case of H+jet production is presented for the first time in an NLO+PS matched simulation. Uncertainties due to scale choices and non-perturbative effects are explored in the production of W and Z bosons in association with a jet. Corresponding results are compared to data from the Tevatron and LHC experiments.
Frank Siegert, Stefan Hoeche, Frank Krauss, Marek Schonherr
The MC@NLO method as implemented in the Sherpa MC generator is presented using the production of W-bosons in conjunction with up to three jets as an example. Corresponding results computed at next-to leading order in QCD and including parton shower corrections are compared to recent experimental data from the Large Hadron Collider.
Stefan Hoeche, Frank Krauss, Steffen Schumann, Frank Siegert
We derive an improved prescription for the merging of matrix elements with parton showers, extending the CKKW approach. A flavour-dependent phase space separation criterion is proposed. We show that this new method preserves the logarithmic accuracy of the shower, and that the original proposal can be derived from it. One of the main requirements for the method is a truncated shower algorithm. We outline the corresponding Monte Carlo procedures and apply the new prescription to QCD jet production in e+e- collisions and Drell-Yan lepton pair production. Explicit colour information from matrix elements obtained through colour sampling is incorporated in the merging and the influence of different prescriptions to assign colours in the large N_C limit is studied. We assess the systematic uncertainties of the new method.
Stefan Hoeche, Steffen Schumann, Frank Siegert
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.
Stefan Hoeche, Frank Krauss, Peter Meinzinger
We present the first complete simulation framework, in the Sherpa event generator, for resolved photon interactions at next-to leading order accuracy. It includes photon spectra obtained through the equivalent-photon approximation, parton distribution functions to parametrize the hadronic structure of quasi-real photons, the matching of the parton shower to next-to leading order QCD calculations for resolved photon cross sections, and the modelling of multiple-parton interactions. We validate our framework against a wide range of photo-production data from LEP and HERA experiments, observing good overall agreement. We identify important future steps relevant for high-quality simulations at the planned Electron-Ion Collider.
Stefan Hoeche, Junwu Huang, Gionata Luisoni, Marek Schoenherr, Jan Winter
We present an analysis of the forward-backward asymmetry in the production of top quark pairs at the Tevatron collider. We use novel Monte Carlo methods for merging matrix elements and parton showers to combine NLO QCD predictions for tt and tt+jet production. Theoretical uncertainties are quantified in detail. We find agreement with experimental data on the transverse momentum dependence of the asymmetry.
Enrico Bothmann, Gurpreet Singh Chahal, Stefan Höche, Johannes Krause, Frank Krauss, Silvan Kuttimalai, Sebastian Liebschner, Davide Napoletano, Marek Schönherr, Holger Schulz, Steffen Schumann, Frank Siegert
Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarize essential features and improvements of the Sherpa 2.2 release series, which is heavily used for event generation in the analysis and interpretation of LHC Run 1 and Run 2 data. We highlight a decade of developments towards ever higher precision in the simulation of particle-collision events.
John M Campbell, Stefan Höche, Hai Tao Li, Christian T Preuss, Peter Skands
We outline a new technique for the fully-differential matching of final-state parton showers to NNLO calculations, focussing here on the simplest case of leptonic collisions with two final-state jets. The strategy is facilitated by working in the antenna formalism, making use of NNLO antenna subtraction on the fixed-order side and the sector-antenna framework on the shower side. As long as the combined real-virtual and double-real corrections do not overcompensate the real-emission term in the three-jet region, negative weights can be eliminated from the matching scheme. We describe the implementation of all necessary components in the VINCIA antenna shower in PYTHIA 8.3.
The HSF Physics Event Generator WG, :, Andrea Valassi, Efe Yazgan, Josh McFayden, Simone Amoroso, Joshua Bendavid, Andy Buckley, Matteo Cacciari, Taylor Childers, Vitaliano Ciulli, Rikkert Frederix, Stefano Frixione, Francesco Giuli, Alexander Grohsjean, Christian Gütschow, Stefan Höche, Walter Hopkins, Philip Ilten, Dmitri Konstantinov, Frank Krauss, Qiang Li, Leif Lönnblad, Fabio Maltoni, Michelangelo Mangano, Zach Marshall, Olivier Mattelaer, Javier Fernandez Menendez, Stephen Mrenna, Servesh Muralidharan, Tobias Neumann, Simon Plätzer, Stefan Prestel, Stefan Roiser, Marek Schönherr, Holger Schulz, Markus Schulz, Elizabeth Sexton-Kennedy, Frank Siegert, Andrzej Siódmok, Graeme A. Stewart
We review the main software and computing challenges for the Monte Carlo physics event generators used by the LHC experiments, in view of the High-Luminosity LHC (HL-LHC) physics programme. This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group as an input to the LHCC review of HL-LHC computing, which has started in May 2020.
Stefan Hoeche, Jonathan Kozaczuk, Andrew J. Long, Jessica Turner, Yikun Wang
We analyze Higgs condensate bubble expansion during a first-order electroweak phase transition in the early Universe. The interaction of particles with the bubble wall can be accompanied by the emission of multiple soft gauge bosons. When computed at fixed order in perturbation theory, this process exhibits large logarithmic enhancements which must be resummed to all orders when the wall velocity is large. We perform this resummation both analytically and numerically at leading logarithmic accuracy. The numerical simulation is achieved by means of a particle shower in the broken phase of the electroweak theory. The two approaches agree to the 10\% level. For fast-moving walls, we find the scaling of the thermal pressure exerted against the wall to be $P\sim γ^2T^4$, independent of the particle masses, implying a significantly slower terminal velocity than previously suggested.
The HSF Physics Event Generator WG, :, Efe Yazgan, Josh McFayden, Andrea Valassi, Simone Amoroso, Enrico Bothmann, Andy Buckley, John Campbell, Gurpreet Singh Chahal, Taylor Childers, Gloria Corti, Rikkert Frederix, Stefano Frixione, Francesco Giuli, Alexander Grohsjean, Stefan Hoeche, Phil Ilten, Frank Krauss, Michal Kreps, David Lange, Leif Lonnblad, Zach Marshall, Olivier Mattelaer, Stephen Mrenna, Paolo Nason, Simon Plaetzer, Christian Preuss, Emanuele Re, Stefan Roiser, Marek Schoenherr, Steffen Schumann, Markus Seidel, Elizabeth Sexton-Kennedy, Frank Siegert, Andrzej Siodmok, Graeme A. Stewart, Aravind Thachayath Sugunan, Zbigniew Was
This paper has been prepared by the HEP Software Foundation (HSF) Physics Event Generator Working Group (WG), as an input to the second phase of the LHCC review of High-Luminosity LHC (HL-LHC) computing, which is due to take place in November 2021. It complements previous documents prepared by the WG in the context of the first phase of the LHCC review in 2020, including in particular the WG paper on the specific challenges in Monte Carlo event generator software for HL-LHC, which has since been updated and published, and which we are also submitting to the November 2021 review as an integral part of our contribution.
Nicolas Greiner, Stefan Hoeche, Gionata Luisoni, Marek Schonherr, Jan-Christopher Winter, Valery Yundin
We present a detailed phenomenological analysis of the production of a Standard Model Higgs boson in association with up to three jets. The Higgs is produced via gluon fusion, which is an irreducible background to the vector boson fusion mechanism. We calculate the next-to-leading order corrections in QCD in the limit of an infinitely heavy top quark. Numerical results are presented for a large variety of observables, for different selection cuts, and for different choices of the jet tagging scheme.
Stefan Hoeche, Daniel Reichelt, Frank Siegert
We present a systematic study of differences between NLL resummation and parton showers. We first construct a Markovian Monte-Carlo algorithm for resummation of additive observables in electron-positron annihilation. Approximations intrinsic to the pure NLL result are then removed, in order to obtain a traditional, momentum and probability conserving parton shower based on the coherent branching formalism. The impact of each approximation is studied, and an overall comparison is made between the parton shower and pure NLL resummation. Differences compared to modern parton-shower algorithms formulated in terms of color dipoles are analyzed.
Stefan Höche, Stefan Prestel
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.
Nicolas Greiner, Stefan Hoeche, Gionata Luisoni, Marek Schonherr, Jan-Christopher Winter
The first computation of Higgs production in association with three jets at NLO in QCD has recently been performed using the effective theory, where the top quark is treated as an infinitely heavy particle and integrated out. This approach is restricted to the regions in phase space where the typical scales are not larger than the top quark mass. Here we investigate this statement at a quantitative level by calculating the leading-order contributions to the production of a Standard Model Higgs boson in association with up to three jets taking full top-quark and bottom-quark mass dependence into account. We find that the transverse momentum of the hardest particle or jet plays a key role in the breakdown of the effective theory predictions, and that discrepancies can easily reach an order of magnitude for transverse momenta of about 1 TeV. The impact of bottom-quark loops are found to be visible in the small transverse momentum region, leading to corrections of up to 5 percent. We further study the impact of mass corrections when VBF selection cuts are applied and when the center-of-mass energy is increased to 100 TeV.
Joshua Isaacson, Stefan Höche, Diego Lopez Gutierrez, Noemi Rocco
An event generation framework is presented that enables the automatic simulation of events for next-generation neutrino experiments in the Standard Model or extensions thereof. The new generator combines the calculation of the leptonic current based on an automated matrix element generator, and the computation of the hadronic current based on a state-of-the-art nuclear physics model. The approach is validated in Standard-Model simulations for electron scattering and neutrino scattering. Furthermore, the first fully-differential neutrino trident production results are shown in the quasielastic region.
Stefan Höche, Frank Krauss, Peter Meinzinger, Daniel Reichelt
We present the first next-to-leading order matched and multi-jet merged predictions based on the Alaric parton shower. The components needed for infrared subtraction in the S-MC@NLO algorithm are computed analytically for the case of color singlet decays to hadronic final states and validated against existing approaches for up to e+e- to 5 jets. Phenomenological results for e+e- to hadrons at the Z pole are obtained with up to five jets at next-to-leading order precision, for the first time using an evolution algorithm with NLL-preserving kinematics mapping.
Stefan Höche, Matt LeBlanc, Jennifer Roloff, Grant Whitman
We present a compact form of the massive $1\to 3$ tree-level QCD splitting functions and discuss a decomposition of the results in terms of lower-order expressions, scalar dipole antenna functions and pure higher-order remainders. The two-gluon radiator functions introduced in this context are novel and generalize expressions obtained from the double-soft approximation. Our results are obtained without reference to soft or quasi-collinear limits.
Stefan Höche, Sebastian Liebschner, Frank Siegert
We present a technique for infrared subtraction in next-to-leading order QCD calculations that preserves the virtuality of resonant propagators. The approach is based on the pseudo-dipole subtraction method proposed by Catani and Seymour in the context of identified particle production. As the first applications, we compute the ee > WWbb and pp > WWjbjb cross-section, which are both dominated by top-quark pair production above the threshold. We compare the efficiency of our approach with a calculation performed using the standard dipole subtraction technique.
Katharina Danziger, Stefan Höche, Frank Siegert
An increase in theoretical precision of Monte Carlo event generators is typically accompanied by an increased need for computational resources. One major obstacle are negative weighted events, which appear in Monte Carlo simulations with higher perturbative accuracy. While they can be handled somewhat easily in fixed-order calculations, they are a major concern for particle level event simulations. In this article, the origin of negative weights in the S-MC@NLO method is reviewed and mechanisms to reduce the negative weight fraction in simulations with the Sherpa event generator are presented, with a focus on V+jets and tt+jets simulations.