In comparisons of precision collider data to the most accurate highest-order calculations in perturbative quantum chromodynamics (QCD), it is required to correct for nonperturbative effects. Such effects are typically studied using Monte Carlo event generators that complement fixed-order predictions with perturbative parton showers and models for the nonperturbative effects of the Underlying Event and hadronisation. Thereby, the final state of collision events can be predicted at the level of stable particles, which serve as input for full detector simulations. This article investigates the impact of nonperturbative effects on two processes that may be used for precision determinations of the strong coupling constant and the proton structure: the triple-differential dijet and Z+jet production. While nonperturbative effects impact both processes, significant differences among them are observed and further investigated. Indications are found that the Underlying Event and hadronisation cannot fully explain these differences and the perturbative modelling may play a significant role as well.

We investigate the decay modes of a CP-even scalar boson cent that mixes with the Standard Model Higgs boson, focusing on the mass range between 2 GeV and 2m. Starting from a higher-order perturbative calculation of the inclusive decays cent- gg and cent- ss, we employ a hadronisation model to obtain predictions for individual hadronic final states. Our hadronisation model is based on the Herwig cluster model, but incorporates various conservation laws to determine the allowed final states and their respective weights. The model includes two tunable parameters, which we determine using dispersion relation results at m cent = 2 GeV, enabling extrapolation to higher masses. Our predictions show that two-particle hadronic final states like 'r+'r-and K+K-dominate over & micro;+& micro;- for m cent near 2 GeV, suggesting promising targets for future experimental searches.

We introduce building blocks for the cluster hadronization model in light of a new structure, focusing on cluster fission and cluster decay. We propose theoretically motivated matrix elements for cluster fission and decay as building blocks and study some first phenomenological implications at different energies. In particular we develop a set of observables which can be used to dissect the hadronization history and have constraining power on the individual building blocks. Our analysis will be completed by including colour reconnection in a follow-up work.

We provide an overview of the status of Monte-Carlo event generators for high-energy particle physics. Guided by the experimental needs and requirements, we highlight areas of active development, and opportunities for future improvements. Particular emphasis is given to physics models and algorithms that are employed across a variety of experiments. These common themes in event generator development lead to a more comprehensive understanding of physics at the highest energies and intensities, and allow models to be tested against a wealth of data that have been accumulated over the past decades. A cohesive approach to event generator development will allow these models to be further improved and systematic uncertainties to be reduced, directly contributing to future experimental success. Event generators are part of a much larger ecosystem of computational tools. They typically involve a number of unknown model parameters that must be tuned to experimental data, while maintaining the integrity of the underlying physics models. Making both these data, and the analyses with which they have been obtained accessible to future users is an essential aspect of open science and data preservation. It ensures the consistency of physics models across a variety of experiments.

A new release of the Monte Carlo event generator Herwig (version 7.3) has been launched. This iteration encompasses several enhancements over its predecessor, version 7.2. Noteworthy upgrades include: the implementation of a process-independent electroweak angular-ordered parton shower integrated with QCD and QED radiation; a new recoil scheme for initial-state radiation improving the behaviour of the angular-ordered parton shower; the incorporation of the heavy quark effective theory to refine the hadronization and decay of excited heavy mesons and heavy baryons; a dynamic strategy to regulate the kinematic threshold of cluster splittings within the cluster hadronization model; several improvements to the structure of the cluster hadronization model allowing for refined models; the possibility to extract event-by-event hadronization corrections in a well-defined way; the possibility of using the string model, with a dedicated tune. Additionally, a new tuning of the parton shower and hadronization parameters has been executed. This article discusses the novel features introduced in version 7.3.0.

Herwig 7 is a general-purpose Monte Carlo generator of particle collisions comprising both hard perturbative as well as soft phenomenological physics. Herwig is therefore capable to describe the entire final state of hadronized particles in a collision event. A spacetime topology of a parton system entering hadronization is fully described and tested for the first time. A combination of information from particles momenta and spacetime positions is utilized to minimize a boost-invariant distance measure of the parton system. We present a reasonable agreement of the model with a selection of experimental data and conclude that spacetime event topology can be meaningfully used in the further development.

A new model of color reconnection in the Monte Carlo generator Herwig 7 is presented. It is based on the minimization of a boost-invariant distance of the parton system, where all partons have momentum as well as spacetime position assigned. We test the influence of both types of variables, namely the rapidity span and transverse distance, on the actual need to reconnect the system to better describe soft physics measurements. We find reasonable agreement with the data and conclude that spacetime topology of the event can be useful for hadron collision modeling.

A new release of the Monte Carlo event generator Herwig++ (version 2.7) is now available. This version comes with a number of improvements including: an interface to the Universal FeynRules Output (UFO) format allowing the simulation of a wide range of new-physics models; developments of the Matchbox framework for next-to-leading order (NLO) simulations; better treatment of QCD radiation in heavy particle decays in new-physics models; a new tune of underlying event and colour connection parameters that allows a good simultaneous description of both Tevatron and LHC underlying event data and the effective cross-section parameter for double-parton scattering.

The modeling of multiple parton interactions in Monte Carlo event generators is a crucial part not only for the dressing of signal processes but also to describe data with a minimum bias on the event selection. Much work has and will be put into the theoretical framework and the numerical implementation of these models. In this contribution, we document various improvements of the multiple parton interaction model of Herwig 7 (Bellm et al. in Eur Phys J C76(4):196, 2016), that lead to an improved description of minimum bias and underlying event data.

We present an idea of how to use a continuous extrapolation of the perturbative results of colour evolution at the amplitude level to the nonperturbative regime. Then we apply it as a guiding principle for a colour reconnection built on top of cluster hadronization model.

Monte Carlo event generators (MCEGs) are the indispensable workhorses of particle physics, bridging the gap between theoretical ideas and first-principles calculations on the one hand, and the complex detector signatures and data of the experimental community on the other hand. All collider physics experiments are dependent on simulated events by MCEG codes such as Herwig, Pythia, Sherpa, POWHEG, and MG5_aMC@NLO to design and tune their detectors and analysis strategies. The development of MCEGs is overwhelmingly driven by a vibrant community of academics at European Universities, who also train the next generations of particle phenomenologists. The new challenges posed by possible future collider-based experiments and the fact that the first analyses at Run II of the LHC are now frequently limited by theory uncertainties urge the community to invest into further theoretical and technical improvements of these essential tools. In this short contribution to the European Strategy Update, we briefly review the state of the art, and the further developments that will be needed to meet the challenges of the next generation.

We present a model for generating spacetime coordinates in the Monte Carlo event generator Herwig 7, and perform colour reconnection by minimizing a boost-invariant distance measure of the system. We compare the model to a series of soft physics observables. We find reasonable agreement with the data, suggesting that pp -collider colour reconnection may be able to be applied in larger systems.

A new release of the Monte Carlo event generator Herwig (version 7.2) is now available. This version introduces a number of improvements over the major version 7.0, notably: multi-jet merging with the dipole shower at LO and NLO QCD; spin correlations in both the dipole and angular-ordered parton showers; an improved choice of evolution variable in the angular-ordered parton shower; improvements to mass effects and top decays in the dipole shower, improvements to the simulation of multiple-parton interactions, including diffractive processes; a new model for baryonic colour reconnection; improvements to strangeness production; as well as a new tune of the hadronisation parameters and support for generic Lorentz structures in BSM models. This article illustrates new features of versions 7.1 and 7.2.

We present an algorithm to combine multiple matrix elements at LO and NLO with a parton shower. We build on the unitarized merging paradigm. The inclusion of higher orders and multiplicities reduce the scale uncertainties for observables sensitive to hard emissions, while preserving the features of inclusive quantities. The combination allows further soft and collinear emissions to be predicted by the all-order parton-shower approximation. We inspect the impact of terms that are formally but not parametrically negligible. We present results for a number of collider observables where multiple jets are observed, either on their own or in the presence of additional uncoloured particles. The algorithm is implemented in the event generator Herwig.

We consider soft gluon evolution at the amplitude level to expose the structure of colour reconnection from a perturbative point of view. Considering the cluster hadronization model and an universal Ansatz for the soft anomalous dimension we find strong support for geometric models considered earlier. We also show how reconnection into baryonic systems arises, and how larger cluster systems evolve. Our results provide the dynamic basis for a new class of colour reconnection models for cluster hadronization.

This Report summarizes the proceedings of the 2017 Les Houches workshop on Physics at TeV Colliders. Session 1 dealt with (I) new developments relevant for high precision Standard Model calculations, (II) theoretical uncertainties and dataset dependence of parton distribution functions, (III) new developments in jet substructure techniques, (IV) issues in the theoretical description of the production of Standard Model Higgs bosons and how to relate experimental measurements, (V) phenomenological studies essential for comparing LHC data from Run II with theoretical predictions and projections for future measurements, and (VI) new developments in Monte Carlo event generators.

We present an extension to the colour reconnection model in the Monte Carlo event generator Herwig to account for the production of baryons and compare it to a series of observables for soft physics. The new model is able to improve the description of charged-particle multiplicities and hadron flavour observables in pp collisions.