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Matched calculations combining perturbative QCD with parton showers are an indispensable tool for LHC physics. Two methods for NLO matching are in widespread use: MC@NLO and POWHEG. We describe an alternative, KrkNLO, reformulated to be easily applicable to any colour-singlet process. The primary distinguishing characteristic of KrkNLO is its use of an alternative factorisation scheme, the 'Krk' scheme, to achieve NLO accuracy. We describe the general implementation of KrkNLO in Herwig 7, using diphoton production as a test process. We systematically compare its predictions to those produced by MC@NLO with several different choices of shower scale, both truncated to one-emission and with the shower running to completion, and to ATLAS data from LHC Run 2., Comment: 40 pages, 11 figures

Hadronization models used in event generators are physics-inspired functions with many tunable parameters. Since we do not understand hadronization from first principles, there have been multiple proposals to improve the accuracy of hadronization models by utilizing more flexible parameterizations based on neural networks. These recent proposals have focused on the kinematic properties of hadrons, but a full model must also include particle flavor. In this paper, we show how to build a deep learning-based hadronization model that includes both kinematic (continuous) and flavor (discrete) degrees of freedom. Our approach is based on Generative Adversarial Networks and we show the performance within the context of the cluster hadronization model within the Herwig event generator., Comment: 9 pages, 4 figures

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., Comment: 10 pages, 3 figures. Herwig is available from https://herwig.hepforge.org

In this paper, we present a new proposal on how to measure quark/gluon jet properties at the LHC. The measurement strategy takes advantage of the fact that the LHC has collected data at different energies. Measurements at two or more energies can be combined to yield distributions of any jet property separated into quark and gluon jet samples on a statistical basis, without the need for an independent event-by-event tag. We illustrate our method with a variety of different angularity observables, and discuss how to narrow down the search for the most useful observables., Comment: 20 pages, 30 figures

Hadronization is a critical step in the simulation of high-energy particle and nuclear physics experiments. As there is no first principles understanding of this process, physically-inspired hadronization models have a large number of parameters that are fit to data. Deep generative models are a natural replacement for classical techniques, since they are more flexible and may be able to improve the overall precision. Proof of principle studies have shown how to use neural networks to emulate specific hadronization when trained using the inputs and outputs of classical methods. However, these approaches will not work with data, where we do not have a matching between observed hadrons and partons. In this paper, we develop a protocol for fitting a deep generative hadronization model in a realistic setting, where we only have access to a set of hadrons in data. Our approach uses a variation of a Generative Adversarial Network with a permutation invariant discriminator. We find that this setup is able to match the hadronization model in Herwig with multiple sets of parameters. This work represents a significant step forward in a longer term program to develop, train, and integrate machine learning-based hadronization models into parton shower Monte Carlo programs., Comment: 14 pages, 4 figures

Hadronization is a complex quantum process whereby quarks and gluons become hadrons. The widely-used models of hadronization in event generators are based on physically-inspired phenomenological models with many free parameters. We propose an alternative approach whereby neural networks are used instead. Deep generative models are highly flexible, differentiable, and compatible with Graphical Processing Unit (GPUs). We make the first step towards a data-driven machine learning-based hadronization model by replacing a compont of the hadronization model within the Herwig event generator (cluster model) with a Generative Adversarial Network (GAN). We show that a GAN is capable of reproducing the kinematic properties of cluster decays. Furthermore, we integrate this model into Herwig to generate entire events that can be compared with the output of the public Herwig simulator as well as with $e^+e^-$ data., Comment: 18 pages, 6 figures

Streams of event weights in particle-level Monte Carlo event generators are a convenient and immensely CPU-efficient approach to express systematic uncertainties in phenomenology calculations, providing systematic variations on the nominal prediction within a single event sample. But the lack of a common standard for labelling these variation streams across different tools has proven to be a major limitation for event-processing tools and analysers alike. Here we propose a well-defined, extensible community standard for the naming, ordering, and interpretation of weight streams that will serve as the basis for semantically correct parsing and combination of such variations in both theoretical and experimental studies., Comment: 11 pages

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., Comment: 34 pages; editors Efe Yazgan, Josh McFayden and Andrea Valassi

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., Comment: Contribution to: ICHEP 2020

Common and community software packages, such as ROOT, Geant4 and event generators have been a key part of the LHC's success so far and continued development and optimisation will be critical in the future. The challenges are driven by an ambitious physics programme, notably the LHC accelerator upgrade to high-luminosity, HL-LHC, and the corresponding detector upgrades of ATLAS and CMS. In this document we address the issues for software that is used in multiple experiments (usually even more widely than ATLAS and CMS) and maintained by teams of developers who are either not linked to a particular experiment or who contribute to common software within the context of their experiment activity. We also give space to general considerations for future software and projects that tackle upcoming challenges, no matter who writes it, which is an area where community convergence on best practice is extremely useful., Comment: 40 pages contribution to Snowmass 2021

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., Comment: 20 pages; editors Andrea Valassi, Efe Yazgan and Josh McFayden; addressed additional comments by journal reviewers

A new release of the Monte Carlo event generator Herwig (version 7.2) is now available. This version introduces a number of improvements, notably: improvements to the simulation of multiple-parton interactions, including diffractive processes; a new model for baryonic colour re-connection; spin correlations in both the dipole and angular-ordered parton showers; improvements to strangeness production; an improved choice of evolution variable in the angular-ordered parton shower; support for generic Lorentz structures in BSM models., Comment: 7 pages, 5 figures. Herwig is available from https://herwig.hepforge.org. arXiv admin note: text overlap with arXiv:1705.06919

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., Comment: 14 pages, 12 figures, updated to final accepted version (EPJ C)

This document reports the first year of activity of the VBSCan COST Action network, as summarised by the talks and discussions happened during the VBSCan Thessaloniki 2018 workshop. The VBSCan COST action is aiming at a consistent and coordinated study of vector-boson scattering from the phenomenological and experimental point of view, for the best exploitation of the data that will be delivered by existing and future particle colliders., Comment: Editors: Lucrezia Stella Bruni, Roberto Covarelli, Pietro Govoni, Piergiulio Lenzi, Narei Lorenzo-Martinez, Joany Manjarres, Matthias Ulrich Mozer, Giacomo Ortona, Mathieu Pellen, Daniela Rebuzzi, Magdalena Slawinska, Marco Zaro. Proceedings for the second annual meeting of the VBSCan COST action

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., Comment: Monte Carlo Community input to European Strategy Update

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., Comment: 24 pages, 15 figures

We propose and carry a detailed study of an observable sensitive to different mechanisms of minijet production. The class of observables measures how the transverse momenta of hadrons produced in association with various trigger objects are balanced as a function of rapidity. It is shown that the observables are sensitive to the model parameters relevant to the minijet production mechanisms: low-$p_{\rm T}$ cut-off regulating jet cross-section, transverse distribution of partons in protons, and parton distribution functions. We perform our test at different charge-particle multiplicities and collision energies. The MC models, which describe many features of the LHC data, are found to predict quite different results demonstrating high discriminating power of the proposed observables. We also review mechanisms and components of \sc{Herwig}, \sc{Pythia}, and \sc{Sherpa} Monte Carlo models relevant to the minijet production., Comment: 27 pages, 16 figures

The properties of quark and gluon jets, and the differences between them, are increasingly important at the LHC. However, Monte Carlo event generators are normally tuned to data from $e^+e^-$ collisions which are primarily sensitive to quark-initiated jets. In order to improve the description of gluon jets we make improvements to the perturbative and the non-perturbative modelling of gluon jets and include data with gluon-initiated jets in the tuning for the first time. The resultant tunes significantly improve the description of gluon jets and are now the default in Herwig 7.1.

A new release of the Monte Carlo event generator Herwig (version 7.1) is now available. This version introduces a number of improvements, notably: multi-jet merging with the dipole shower at LO and NLO QCD; a new model for soft interactions and diffraction; improvements to mass effects and top decays in the dipole shower, as well as a new tune of the hadronisation parameters., Comment: 7 pages, 7 figures. Herwig is available from https://herwig.hepforge.org/

By measuring the substructure of a jet, one can assign it a "quark" or "gluon" tag. In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (C_F versus C_A). In this paper, we confront the challenges faced when going beyond this leading-order understanding, using both parton-shower generators and first-principles calculations to assess the impact of higher-order perturbative and nonperturbative physics. Working in the idealized context of electron-positron collisions, where one can define a proxy for quark and gluon jets based on the Lorentz structure of the production vertex, we find a fascinating interplay between perturbative shower effects and nonperturbative hadronization effects. Turning to proton-proton collisions, we highlight a core set of measurements that would constrain current uncertainties in quark/gluon tagging and improve the overall modeling of jets at the Large Hadron Collider., Comment: 50 pages, 20 figures, extended version of the Les Houches 2015 study from arXiv:1605.04692; v2: updated to approximately match journal version, including improved discussion of nonperturbative Casimir scaling and more umlauts

We report on the possibility of reweighting parton-shower Monte Carlo predictions for scale variations in the parton-shower algorithm. The method is based on a generalization of the Sudakov veto algorithm. We demonstrate the feasibility of this approach using example physical distributions. Implementations are available for both the parton-shower modules in the Herwig 7 event generator., Comment: 8 pages, 5 figures

We perform a detailed study of the sources of perturbative uncertainty in parton shower predictions within the Herwig 7 event generator. We benchmark two rather different parton shower algorithms, based on angular-ordered and dipole-type evolution, against each other. We deliberately choose leading order plus parton shower as the benchmark setting to identify a controllable set of uncertainties. This will enable us to reliably assess improvements by higher-order contributions in a follow-up work., Comment: 18 pages, 26 figures

With the advent and recent extension of the BLHA standard to interface Monte Carlo event generators and one-loop matrix element providers, the Herwig++ event generator has expanded its range of applicability to a multitude of underlying hard processes at NLO QCD. The new NLO development is centered around the Matchbox framework, which turns fixed NLO QCD calculations into parton shower matched calculations - to be matched to the two parton shower variants of Herwig++. Matchbox provides thereby for the automated setup of the underlying fixed NLO QCD calculations and the interface to the one-loop matrix element providers, as well as for an efficient and automated multi-channel phase space sampling, and forms the basis for the NLO capabilities of the new release of Herwig++. Along with several other new features and developments, the new release marks the end of distinguishing Herwig++ and (Fortran) HERWIG, and constitutes the first major release of version 7 of the Herwig event generator., Comment: 11 pages, 12th International Symposium on Radiative Corrections (Radcor 2015) and LoopFest XIV (Radiative Corrections for the LHC and Future Colliders), 15-19 June 2015, UCLA Department of Physics & Astronomy Los Angeles, CA, USA

A major new release of the Monte Carlo event generator Herwig++ (version 3.0) is now available. This release marks the end of distinguishing Herwig++ and HERWIG development and therefore constitutes the first major release of version 7 of the Herwig event generator family. The new version features a number of significant improvements to the event simulation, including: built-in NLO hard process calculation for all Standard Model processes, with matching to both angular-ordered and dipole shower modules via both subtractive (MC@NLO-type) and multiplicative (Powheg-type) algorithms; QED radiation and spin correlations in the angular ordered shower; a consistent treatment of perturbative uncertainties within the hard process and parton showering. Several of the new features will be covered in detail in accompanying publications, and an update of the manual will follow in due course., Comment: 8 pages. Herwig is available from https://herwig.hepforge.org

We carry out simple analytical calculations and Monte Carlo studies to better understand the impact of QCD radiation on some well-known jet substructure methods for jets arising from the decay of boosted Higgs bosons. Understanding differences between taggers for these signal jets assumes particular significance in situations where they perform similarly on QCD background jets. As an explicit example of this we compare the Y-splitter method to the more recently proposed Y-pruning technique. We demonstrate how the insight we gain can be used to significantly improve the performance of Y-splitter by combining it with trimming and show that this combination outperforms the other taggers studied here, at high $p_T$. We also make analytical estimates for optimal parameter values, for a range of methods and compare to results from Monte Carlo studies., Comment: 53 pages, 21 figures. Version published in JHEP: addressed several points raised by the referee

An overview of theoretical and experimental progress in double parton scattering (DPS) is presented. The theoretical topics cover factorization in DPS, models for double parton distributions and DPS in charm production and nuclear collisions. On the experimental side, CMS results for dijet and double J/\psi\ production, in light of DPS, as well as first results for the 4-jet channel are presented. ALICE reports on a study of open charm and J/\psi\ multiplicity dependence., Comment: 7 pages, 3 figures, contribution of the Double Parton Scattering track to the proceedings of MPI@LHC 2013

We discuss various issues related to the definition of single and double open charm cross sections. We conclude that LHCb's extraction of sigma_effective, the effective cross section for double-parton scattering, is too large by a factor of two. This correction brings the data from open-charm pairs closer to that from J/psi plus open charm and jet production., Comment: 21 pages, 1 figure

We review the modelling of multiple interactions in the event generator Herwig++ and study implications of recent tuning efforts to Tevatron and LHC data. It is often said that measurements of the effective cross section for double-parton scattering, sigma_effective, are in contradiction with models of the final state of multi-parton interactions, but we show that the Herwig++ model is consistent with both and gives stable predictions for underlying event observables at 14 TeV.

We review the implementation of a model for multiple partonic interactions in Herwig++. Moreover, we show how recent studies on the colour structure of events in Herwig++ led to a significant improvement in the description of soft inclusive observables in pp interactions at the LHC., Comment: 6 pages, 4 figures; Talk presented at the 42. International Symposium on Multiparticle Dynamics, 16-21 September 2012, Kielce, Poland

In their 1997 paper, CDF measured sigma_effective, the normalization factor that relates the cross section for double parton scattering to the product of the inclusive cross sections of the two individual scatters, in a model in which they are assumed to be independent. In his 2007 paper, Treleani pointed out that CDF used a non-standard definition, in which the double parton scattering cross section corresponds to exactly two scatters, rather than the more conventional one in which it is the inclusive two-scatter cross section. He also estimated the correction from one definition to the other, to give a corrected value of sigma_effective. Treleani's form would be correct under the assumption that CDF were able to uniquely identify and count the number of scatters in an event, which is certainly not the case. In this publication we consider CDF's event definition in more detail to provide an improved correction., Comment: 19 pages

We investigate the breakdown of collinear factorization for non-inclusive observables in hadron-hadron collisions. For pure QCD processes, factorization is violated at the three-loop level and it has a structure identical to that encountered previously in the case of super-leading logarithms. In particular, it is driven by the non-commutation of Coulomb/Glauber gluon exchanges with other soft exchanges. Beyond QCD, factorization may be violated at the two-loop level provided that the hard subprocess contains matrix element contributions with phase differences between different colour topologies., Comment: Version 2: minor improvements for journal publication

We review the modelling of multiple interactions in the event generator Herwig++ and study implications of recent tuning efforts to LHC data. A crucial ingredient to a successful description of minimum-bias and underlying-event observables is a model for colour reconnection. Improvements to this model, inspired by statistical physics, are presented., Comment: 4 pages, 3 figures. Contribution to the DIS 2012 proceedings

We describe the implementation details of the colour reconnection model in the event generator Herwig++. We study the impact on final-state observables in detail and confirm the model idea from colour preconfinement on the basis of studies within the cluster hadronization model. Moreover, we show that the description of minimum bias and underlying event data at the LHC is improved with this model and present results of a tune to available data., Comment: 19 pages, 21 figures, 2 tables. Matches with published version

The claim that the W boson mass might be measured at the LHC with a precision of O(10 MeV) is critically reviewed. It is argued that such a precision cannot be achieved, unless a dedicated measurement program, specific to the LHC is pursued. We propose such a program. Its main target is to significantly improve the experimental control of the relative polarisation of the W+, W- and Z bosons. We propose to achieve this goal either by running dedicated isoscalar beams at the LHC or by running, in parallel to the standard p-p collision program, a dedicated muon scattering "LHC-support-experiment" at the CERN SPS. One of these auxiliary measurements is necessary for the "precision measurement program" at the LHC, but not sufficient. It must be followed by dedicated measurement strategies which are robust with respect to both the systematic measurement uncertainties and to the perturbative and non-perturbative QCD effects. We propose such strategies and evaluate their precision. At the LHC, contrary to the Tevatron case, both the masses of the W+ and of the W- bosons must be measured with high precision. In this context, we propose and evaluate LHC dedicated strategies to measure the difference of the masses of the W+ and W- bosons and of the absolute mass of the W boson assuming both masses are equal. We show how one can overcome the obstacles in measuring the masses of W+ and W- to a precision of 10 MeV. We present a detailed evaluation of the precision of the proposed methods based on the studies of a large, O(10^11), sample of simulated W and Z production events., Comment: 4 pages, to appear in the proceedings of the 2009 Europhysics Conference on High Energy Physics

The transverse momentum of electroweak bosons in a Drell-Yan process is an important quantity for the experimental program at the LHC. The new model of non-perturbative gluon emission in an initial state parton shower presented in this note gives a good description of this quantity for the data taken in previous experiments over a wide range of CM energy. The model's prediction for the transverse momentum distribution of Z bosons for the LHC is presented and used for a comparison with other approaches., Comment: 9 pages, 3 figures; based on a talk by Andrzej Siodmok at the Cracow Epiphany Conference on "Hadron interactions at the dawn of the LHC", 5-7 January 2009; dedicated to the memory of Jan Kwiecinski, to be published in Acta Physica Polonica

We consider a model of transverse momentum production in which non-perturbative smearing takes place throughout the perturbative evolution, by a simple modification to an initial state parton shower algorithm. Using this as the important non-perturbative ingredient, we get a good fit to data over a wide range of energy. Combining it with the non-perturbative masses and cutoffs that are a feature of conventional parton showers also leads to a reasonable fit. We discuss the extrapolation to the LHC., Comment: 14 pages, 6 figures; version accepted by JHEP