Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarise new developments, essential features, and ongoing improvements within the Sherpa 3 release series. Physics improvements include higher-order electroweak corrections, simulations of photoproduction and hard diffraction at NLO QCD, heavy-flavour matching in NLO multijet merging, spin-polarised cross section calculations, and a new model of colour reconnections. In addition, the modelling of hadronisation, the underlying event and QED effects in both production and decay has been improved, and the overall event generation efficiency has been enhanced.

Germany

Enrico Bothmann

United Kingdom

Lois Flower

Christian Gütschow

United States

Stefan Höche

Mareen Hoppe

Joshua Isaacson

Max Knobbe

Frank Krauss

Switzerland

Peter Meinzinger

Italy

Davide Napoletano

Poland

Alan Price

CERN

Daniel Reichelt

Marek Schönherr

Steffen Schumann

Frank Siegert

Event generation with Sherpa 3

Top-quark pair production in association with two b-jets is computed at next-to-leading order QCD precision, including effects of the b-quark mass, and matched to a   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$ +jets simulation in a variable flavor number scheme. The Monte Carlo realization of this method, called fusing, consistently embeds the four-flavor calculation in a particle-level event generator. As a first phenomenological application, we present observables relevant to the data-driven estimation of irreducible backgrounds to   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>H</mi> </math>  $$ t\overline{t}H $$ -production.

Lars Ferencz

Judith Katzy

<math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> <mi>b</mi> <mover> <mi>b</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t}b\overline{b} $$  at NLO precision in a variable flavor number scheme

Measurements of vector boson polarisation in vector boson production processes offer a powerful probe of the electroweak symmetry breaking mechanism, scrutinising the Standard Model and new physics scenarios alike. Since massive vector bosons can only be observed as intermediate particles, polarised cross section templates from simulation are necessary to extract their polarisation from measurable unpolarised distributions. In this work we present an extension of the Sherpa Monte-Carlo event generator allowing the simulation of polarised cross sections for vector boson production processes. Based on the narrow-width approximation, polarised cross sections of all possible polarisation combinations for an arbitrary number of intermediate vector bosons can be simulated in a single simulation run. In addition, it is possible to directly predict the interference between different intermediate polarisation states, and various differing polarisation definitions can be studied simultaneously. Besides the simulation of polarised cross sections at fixed LO and LO+PS accuracy as well as in multijet-merged calculations, we also present parton-shower-matched polarised cross sections with approximate NLO QCD corrections in the vector boson production processes. We demonstrate that the differences of this approximation to full NLO QCD predictions are small and it thus opens up the possibility for fully-simulated calculations at the hadron level including polarisation information and higher-order QCD effects for the first time.

Polarised cross sections for vector boson production with Sherpa

We present the first fully differential predictions for tau neutrino scattering in the energy region relevant to the DUNE experiment, including all spin correlations and all tau lepton decay channels. The calculation is performed using a generic interface between the neutrino event generator achilles and the publicly available, general-purpose collider event simulation framework sherpa.

Sherry Wang

Tau polarization and correlated decays in neutrino experiments

We present an update of the Universal FeynRules Output model format, commonly known as the UFO format, that is used by several automated matrix-element generators and high-energy physics software. We detail different features that have been proposed as extensions of the initial format during the last ten years, and collect them in the current second version of the model format that we coin the Universal Feynman Output format. Following the initial philosophy of the UFO, they consist of flexible and modular additions to address particle decays, custom propagators, form factors, the renormalisation group running of parameters and masses, and higher-order quantum corrections.

France

Luc Darmé

Belgium

Céline Degrande

Claude Duhr

Benjamin Fuks

Mark Goodsell

Gudrun Heinrich

Valentin Hirschi

Marius Höfer

Olivier Mattelaer

Thorsten Ohl

Davide Pagani

Jürgen Reuter

Peter Richardson

Hua-Sheng Shao

Marco Zaro

UFO 2.0: the ‘Universal Feynman Output’ format

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 $$e^+e^- \rightarrow W^+W^-b{\bar{b}}$$ <math><mrow><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>-</mo></msup><mo>→</mo><msup><mi>W</mi><mo>+</mo></msup><msup><mi>W</mi><mo>-</mo></msup><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow></math>  and $$pp \rightarrow W^+W^-j_bj_b$$ <math><mrow><mi>p</mi><mi>p</mi><mo>→</mo><msup><mi>W</mi><mo>+</mo></msup><msup><mi>W</mi><mo>-</mo></msup><msub><mi>j</mi><mi>b</mi></msub><msub><mi>j</mi><mi>b</mi></msub></mrow></math>  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.

Sebastian Liebschner

Resonance-aware subtraction in the dipole method

We propose a novel technique for the combination of multijet merged simulations in the five-flavor scheme with calculations for the production of <math><mi>b</mi></math>-quark associated final states in the four-flavor scheme. We show the equivalence of our algorithm to the FONLL method at the fixed-order and logarithmic accuracy inherent to the matrix-element and parton-shower simulation employed in the multijet merging. As a first application, we discuss <math><mi>Z</mi><mi>b</mi><mover><mi>b</mi><mo>¯</mo></mover></math> production at the Large Hadron Collider.

Johannes Krause

Multijet merging in a variable flavor number scheme

We present precise predictions for prompt photon production in association with a Z boson and jets. They are obtained within the Sherpa framework as a consistently merged inclusive sample. Leptonic decays of the Z boson are fully included in the calculation with all off-shell effects. Virtual matrix elements are provided by OpenLoops and parton-shower effects are simulated with a dipole parton shower. Thanks to the NLO QCD corrections included not only for inclusive Zγ production but also for the Zγ + 1-jet process we find significantly reduced systematic uncertainties and very good agreement with experimental measurements at s=8TeV . Predictions at s=13TeV are displayed including a study of theoretical uncertainties. In view of an application of these simulations within LHC experiments, we discuss in detail the necessary combination with a simulation of the Z + jets final state. In addition to a corresponding prescription we introduce recommended cross checks to avoid common pitfalls during the overlap removal between the two samples.

NLO QCD predictions for Z+γ + jets production with Sherpa

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.

Momentum conservation and unitarity in parton showers and NLL resummation

We present differential cross sections for the production of top-quark pairs in conjunction with up to two jets, computed at next-to-leading order in perturbative QCD and consistently merged with a parton shower in the S herpa +O pen L oops framework. Top quark decays including spin correlation effects are taken into account at leading order accuracy. The calculation yields a unified description of top-pair plus multi-jet production, and detailed results are presented for various key observables at the Large Hadron Collider. A large improvement with respect to the multi-jet merging approach at leading order is found for the total transverse energy spectrum, which plays a prominent role in searches for physics beyond the Standard Model.

Philipp Maierhöfer

Stefano Pozzorini

Next-to-leading order QCD predictions for top-quark pair production with up to two jets merged with a parton shower