General-purpose Event Generators Research Articles

Hadronization and decay of excited heavy hadrons in Herwig 7

We revisit the hadronization and decay of excited heavy mesons and heavy baryons in Herwig 7 general-purpose event-generator, following four distinct steps: (i) Passing through the polarisation of heavy hadrons at the end of parton shower through the application of heavy quark effective theory (HQET), where the emergence of a spin-flavour symmetry allows for the determination of the polarisations of the excited heavy mesons and heavy baryons from the helicity states of the light and heavy quarks. (ii) Improving the strong and radiative decay modes of the excited heavy mesons, where in the absence of conclusive experimental data on many of the decays, one needs to rely on HQET symmetries to determine the favoured decay modes, widths and branching ratios. (iii) Re-examination of the production rates of heavy hadrons using all available experimental data sources and (iv) performing a general tune for Herwig’s free parameters to reflect the implemented changes. We compare our predictions against existing experimental data in the presence/absence of the newly implemented updates. These improvements will be available with Herwig-7.3.0 public release.

Accelerating LHC event generation with simplified pilot runs and fast PDFs

Poor computing efficiency of precision event generators for LHC physics has become a bottleneck for Monte-Carlo event simulation campaigns. We provide solutions to this problem by focusing on two major components of general-purpose event generators: The PDF evaluator and the matrix-element generator. For a typical production setup in the ATLAS experiment, we show that the two can consume about 80% of the total runtime. Using NLO simulations of pprightarrow ell ^+ell ^-+text {jets} and pprightarrow tbar{t}+text {jets} as an example, we demonstrate that the computing footprint of Lhapdf and Sherpa can be reduced by factors of order 10, while maintaining the formal accuracy of the event sample. The improved codes are made publicly available.

NLO multijet merging for Higgs production beyond the VBF approximation

We present results of the simulation of electroweak Higgs boson production at the Large Hadron Collider using both the NLO multijet merging and NLO matching frameworks provided by the general purpose event generator Herwig 7. For the simulation of the hard scattering processes, we use the HJets library for the full calculation and VBFNLO for the approximate calculation to compute the 2rightarrow h+n amplitudes at tree-level with n=2,3,4 and at one-loop with n=2,3.

Comparison of selected general-purpose event generators for particle fluence simulation in LHC silicon trackers

At the LHC era, the detector systems are operating at the harsh hadronic environment with the unprecedentedly high particle flux. Position-sensitive silicon devices are usually positioned at the innermost regions of the experimental setups and must cope with highly non-uniform radiation fields. At the end of LHC Run 2, fluence in silicon trackers reached 10^{15} n_{mathrm{eq}}/cm^2. Initial simulation studies predict that the maximal fluence for the HL-LHC may be up to two orders of magnitude higher than the one seen in LHC Run 1 and Run 2. In this paper, two general-purpose physics event generators used for simulation of proton–proton collisions for the radiation damage studies at LHC energies: PYTHIA 8.2 and Dpmjet-III are compared. Fluences obtained using these models, with the latest tuning to the LHC data, in detectors situated close to the proton–proton interaction point are determined as well. We also indicate a potential new method for actual fluence estimation using experiment real-time data monitoring system.

Short-Lived Resonances as Probes of the Medium Produced in Heavy-Ion Collisions

Hadronic resonances play an important role in the study of the physics of heavy-ion collisions. In these proceedings, we discuss how the resonances can probe the reaction dynamics, the strangeness production and the properties of the hadronic phase in heavy-ion collisions at center-of-mass energies of sNN = 4–11 GeV. The resonance properties predicted by the general-purpose event generators are found to be very sensitive to the properties and space-time evolution of the medium produced in heavy-ion collisions.

The Angantyr model for heavy ions in Pythia8

Pythia8 is the only general purpose event generator able to produce complete fully exclusive hadronic final states in collisions involving heavy ions. The model that allows this is called Angantyr and will be discussed in this contribution. The contribution also discusses models based on interactions between strings in an extention of the Lund model that are able to describe some collective effects in proton–proton collisions. In particular, a reformulation of the so-called shoving will be described, which also works for heavy ion collisions as generated by the Angantyr model. Preliminary results for the ridge effects are presented for proton–proton and proton–lead collisions.

A Positive Resampler for Monte Carlo events with negative weights

We propose the Positive Resampler to solve the problem associated with event samples from state-of-the-art predictions for scattering processes at hadron colliders typically involving a sizeable number of events contributing with negative weight. The proposed method guarantees positive weights for all physical distributions, and a correct description of all observables. A desirable side product of the method is the possibility to reduce the size of event samples produced by General Purpose Event Generators, thus lowering the resource demands for subsequent computing-intensive event processing steps. We demonstrate the viability and efficiency of our approach by considering its application to a next-to-leading order + parton shower merged prediction for the production of a W boson in association with multiple jets.

Hard diffraction in photoproduction with Pythia 8

We present a new framework for modeling hard diffractive events in photoproduction, implemented in the general purpose event generator Pythia 8. The model is an extension of the model for hard diffraction with dynamical gap survival in mathrm {p}mathrm {p} and mathrm {p}overline{mathrm {p}} collisions proposed in 2015, now also allowing for other beam types. It thus relies on several existing ideas: the Ingelman–Schlein approach, the framework for multiparton interactions and the recently developed framework for photoproduction in gamma mathrm {p}, gamma gamma , mathrm {e}mathrm {p} and mathrm {e}^+mathrm {e}^- collisions. The model proposes an explanation for the observed factorization breaking in photoproduced diffractive dijet events at HERA, showing an overall good agreement with data. The model is also applicable to ultraperipheral collisions with mathrm {p}mathrm {p} and mathrm {p}{mathrm {Pb}} beams, and predictions are made for such events at the LHC.

LHC collider phenomenology of minimal universal extra dimensions

We discuss the collider phenomenology of the model of Minimal Universal Extra Dimensions (MUED) at the Large hadron Collider (LHC). We derive analytical results for all relevant strong pair-production processes of two level 1 Kaluza–Klein partners and use them to validate and correct the existing MUED implementation in the fortran version of the Pythia event generator. We also develop a new implementation of the model in the C++ version of Pythia. We use our implementations in conjunction with the Checkmate package to derive the LHC bounds on MUED from a large number of published experimental analyses from Run 1 at the LHC. Program summaryProgram title:Pythia6 MUED and Pythia8 MUEDProgram Files doi:http://dx.doi.org/10.17632/fkvcgdw9j6.1Licensing provisions: CC0 1.0Programming language: Fortran and C++Journal reference of previous version: Comput. Phys. Comm. 181 (2010) 122–127Does the new version supersede the previous version?: YesReasons for new version: The Minimal Universal Extra Dimension (MUED) model was implemented in the Pythia6 event generator from version 6.4.18 onwards. The new version now allows for different masses for various level 1 KK excitations, modified and corrected subroutines to handle the kinematics and squared matrix elements. A new production process has been added. Additionally the model is now made available in the Pythia8 event generator.Summary of revisions: The Minimal Universal Extra Dimension (MUED) model implementation in the Pythia6 event generator now allows for different masses for various level 1 KK excitations. These may appear both in the final states and in the propagators of t∕u channel contributions. Routines handling the kinematics and squared matrix elements are suitably modified to incorporate correct kinematics and ensure proper interferences of diagrams. For some processes, corrections in the expressions for the existing squared matrix elements have been made. A new subprocess qq̄→g1∗g1∗ has been added. A couple of production modes (which originally had some subprocesses lumped together) are now split to handle different masses of doublet and singlet KK excitations.Nature of problem: The ongoing run of the Large Hadron Collider (LHC) at CERN continues to look for new physics Beyond the Standard Model (BSM). For many years, the only complete models which were seriously investigated by the LHC collaborations, were different versions of low energy supersymmetry: mSUGRA, gauge-mediated SUSY, anomaly-mediated SUSY, etc. The MUED model provides an interesting and motivated alternative to supersymmetry, with unique and challenging collider phenomenology. In order to study the MUED model, it was implemented in the fortran version Pythia6 general purpose event generator from version 6.4.18 onwards. The MUED model is not available in the C++ version of Pythia8.Solution method: The new implementation validates and fixes the backward compatibility and some issues in the current Pythia6 version. The model has now been implemented in the Pythia8 event generator.

Consistent simulation of direct-photon production in hadron collisions including associated two-jet production

We have developed an event generator for direct-photon production in hadron collisions, including associated 2-jet production in the framework of the GR@PPA event generator. The event generator consistently combines [Formula: see text] + 2-jet production processes with the lowest-order [Formula: see text] + jet and photon-radiation (fragmentation) processes from quantum chromodynamics (QCD) 2-jet production using a subtraction method. The generated events can be fed to general-purpose event generators to facilitate the addition of hadronization and decay simulations. Using the obtained event information, we can simulate photon isolation and hadron-jet reconstruction at the particle (hadron) level. The simulation reasonably reproduces measurement data obtained at the large hadron collider (LHC) concerning not only the inclusive photon spectrum, but also the correlation between the photon and jet. The simulation implies that the contribution of the [Formula: see text] + 2-jet is very large, especially in low photon-[Formula: see text] ([Formula: see text] 50 GeV) regions. Discrepancies observed at low [Formula: see text], although marginal, may indicate the necessity for the consideration of further higher-order processes. Unambiguous particle-level definition of the photon-isolation condition for the signal events is desired to be given explicitly in future measurements.

Hard diffraction in Pythia 8

We present an overview of the options for diffraction implemented in the general-purpose event generator Pythia 8 [1]. We review the existing model for soft diffraction and present a new model for hard diffraction. Both models use the Pomeron approach pioneered by Ingelman and Schlein, factorising the diffractive cross section into a Pomeron flux and a Pomeron PDF, with several choices for both implemented in Pythia 8. The model of hard diffraction is implemented as a part of the multiparton interactions (MPI) framework, thus introducing a dynamical gap survival probability that explicitly breaks factorisation.

Hard Diffraction in \textsf {PYTHIA8}

We present an overview of the options for diffraction implemented in the general-purpose event generator PYTHIA8. We review the existing model for low- and high-mass soft diffraction and present a new model for hard diffraction in pp and pp¯ collisions. Both models use the Pomeron approach pioneered by Ingelman and Schlein, factorising the single diffractive cross section into a Pomeron flux and a Pomeron PDF. The model for hard diffraction is implemented as a part of the multiparton interactions framework, thereby introducing a dynamical rapidity gap survival probability that explicitly breaks factorisation.

Higgs boson pair production merged to one jet

We develop a Monte Carlo event generator for Higgs Boson pair production merged to exact one-jet matrix elements. The matrix elements are generated with OpenLoops and event generation is performed with the HERWIG++ general-purpose event generator. This allows us to simulate fully-exclusive hadronic final states with accurate description of the kinematics of the leading jet in conjunction with a parton shower. We use the implementation to examine in detail the systematic uncertainties which result from the merging procedure. We assess the magnitude of the impact of the merging on experimental searches of Standard Model di-Higgs production that aim to constrain the Higgs boson self-coupling. We find that the use of a merged sample can reduce theoretical systematic uncertainties in the efficiencies of cuts on certain observables. This constitutes the most accurate simulation of the process available to date. The Monte Carlo event generator developed for this project is available as an add-on to the HERWIG++ event generator at http://www.itp.uzh.ch/~andreasp/hh

Consistent simulation of nonresonant diphoton production at hadron collisions with a custom-made parton shower

We have developed a Monte Carlo event generator for non-resonant diphoton ($\gamma\gamma$) production at hadron collisions in the framework of GR@PPA, which consistently includes additional one-jet production. The jet-matching method developed for initial-state jet production has been extended to the final state in order to regularize the final-state QED divergence in the $qg \rightarrow \gamma\gamma + q$ process. A QCD/QED-mixed parton shower (PS) has been developed to complete the matching. The PS has the capability of enforcing hard-photon radiation, and small-$Q^{2}$ photon radiations that are not covered by the PS are supplemented by using a fragmentation function. The generated events can be passed to general-purpose event generators in order to perform the simulations down to the hadron level. Thus, we can simulate the isolation requirements that must be applied in experiments at the hadron level. The simulation results are in reasonable agreement with the predictions from RESBOS and DIPHOX. The simulated hadron-level events can be further fed to detector simulations in order to investigate the detailed performance of experiments.

GR@PPA 2.8: Initial-state jet matching for weak-boson production processes at hadron collisions

The initial-state jet matching method introduced in our previous studies has been applied to the event generation of single W and Z production processes and diboson (W+W−, WZ and ZZ) production processes at hadron collisions in the framework of the GR@PPA event generator. The generated events reproduce the transverse momentum spectra of weak bosons continuously in the entire kinematical region. The matrix elements (ME) for hard interactions are still at the tree level. As in previous versions, the decays of weak bosons are included in the matrix elements. Therefore, spin correlations and phase-space effects in the decay of weak bosons are exact at the tree level. The program package includes custom-made parton shower programs as well as ME-based hard interaction generators in order to achieve self-consistent jet matching. The generated events can be passed to general-purpose event generators to make the simulation proceed down to the hadron level. Program summaryProgram title: GR@PPA 2.8Catalogue identifier: ADRH_v3_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADRH_v3_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 112 146No. of bytes in distributed program, including test data, etc.: 596 667Distribution format: tar.gzProgramming language: Fortran; with some included libraries coded in C and C++Computer: AllOperating system: Any UNIX-like systemRAM: 1.6 Mega bytes at minimumClassification: 11.2Catalogue identifier of previous version: ADRH_v2_0Journal reference of previous version: Comput. Phys. Comm. 175 (2006) 665External routines: Bash and Perl for the setup, and CERNLIB, ROOT, LHAPDF, PYTHIA according to the userʼs choice.Does the new version supersede the previous version?: No, this version supports only a part of the processes included in the previous versions.Nature of problem: We need to combine those processes including 0 jet and 1 jet in the matrix elements using an appropriate matching method, in order to simulate weak-boson production processes in the entire kinematical region.Solution method: The leading logarithmic components to be included in parton distribution functions and parton showers are subtracted from 1-jet matrix elements. Custom-made parton shower programs are provided to ensure satisfactory performance of the matching method.Reasons for new version: An initial-state jet matching method has been implemented.Summary of revisions: Weak-boson production processes associated with 0 jet and 1 jet can be consistently merged using the matching method.Restrictions: The built-in parton showers are not compatible with the PYTHIA new PS and the HERWIG PS.Unusual features: A large number of particles may be produced by the parton showers and passed to general-purpose event generators.Running time: About 10 min for initialization plus 25 s for every 1k-event generation for W production in the LHC condition, on a 3.0-GHz Intel Xeon processor with the default setting.

Inclusive and exclusive observables from dipoles in high energy collisions

We present a new model for simulating exclusive final states in minimum-bias collisions between hadrons. In a series of papers we have developed a Monte Carlo model based on Mueller's dipole picture of BFKL-evolution, supplemented with non-leading corrections, which has shown to be very successful in describing inclusive and semi-inclusive observables in hadron collisions. In this paper we present a further extension of this model to also describe exclusive final states. This is a highly non-trivial extension, and we have encountered many details that influence the description, and for which no guidance from perturbative QCD could be found. Hence we have had to make many choices based on semi-classical and phenomenological arguments. The end result is a new event generator called DIPSY which can be used to simulate complete minimum-bias non-diffractive hadronic collision events. Although the description of data from the Tevatron and LHC is not quite as good as for PYTHIA, the most advanced of the general purpose event generator programs for these processes, our results are clearly competitive, and can be expected to improve with careful tuning. In addition, as our model is very different from conventional multiple scattering scenaria, the DIPSY program can be used to gain deeper insight in the soft and semi-hard processes involved both in hadronic and heavy ion collisions.

APACIC++ 2.0. A PArton Cascade In C++

The new version of the parton shower module APACIC++ for the SHERPA event generator framework is presented. It incorporates some features, that are specific for the consistent merging with multi-particle matrix elements at tree-level. This publication also includes some example results and a short description of the upgraded class structure of APACIC++, version 2.0. Program summary Title of program: APACIC++, version 2.0 Catalogue identifier:ADNE_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADNE_v2_0 Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Catalogue identifier of previous version: ADNE Authors of previous version: R. Kuhn, F. Krauss, B. Iványi, G. Soff Journal reference of previous version: Comput. Phys. Commun. 134 (2001) 223 Licensing provisions: none Operating system under which the program has been tested: Linux, Unix, Darwin Programming language: C++ No. of lines in distributed program, including test data, etc.: 78 352 No. of bytes in distributed program, including test data, etc.: 2 561 276 External routines/libraries used: The makeinstall requires the autotools package Distribution format: tar gzip file Nature of the physical problem: Modern collider-based experiments in particle physics are characterized by increasing energies that are released and, thus, by a growing number of particles that are produced. The large numbers of involved particles renders it impossible to derive fully analytical predictions for a detailed description of the final state. In addition, the transformation of the partons of perturbative quantum field theory into the experimentally observable hadrons is until now not understood on a quantitative level. These obstacles prevent a direct comparison of analytical predictions with experimental data, and simulation tools need to be employed to bridge this gap. Method of solution: The event generator SHERPA with a new version of the parton shower module APACIC++ is able to simulate e + e − -annihilation experiments as well as hadron–hadron collision. The generated events are suitable for direct comparison with experiment. This is achieved by dividing the simulation into well-separated steps. First, the signal process is selected by employing multi-particle matrix elements at tree-level. Then the strong interacting particles experience additional radiation of soft or collinear partons described by means of the parton shower. Finally, the partons are translated into observable hadrons using phenomenological models. The module APACIC++ concentrates on the parton shower evolution of jets, both in the initial and in the final state of the signal process. Suitable interfaces to other modules of the event generator SHERPA are provided. Reasons for the new version: This new version is able to perform not only final state shower but also initial state shower evolutions. Thus the program gives now also a realistic description of proton–proton and proton–anti-proton collisions. It is particularly designed to simulate events at the Tevatron or the LHC. Summary of revisions: The package has been extended by a number of classes for the description of the initial state shower. In order to give optimal support for these new routines, all existing classes of the final state shower have been revised, but the basic structure and concept of the program has been maintained. In addition a new dicing strategy has been introduced in the time-like evolution routine, which substantially improved the performance of the final state shower. Additional comments: The package APACIC++ is used as the parton shower module of the general purpose event generator SHERPA. There it takes full advantage of its capabilities to merge multi-jet matrix element and parton shower evolution. Running time: The example programs take a matter of seconds to run.

GR@PPA_4b: A four bottom quark production event generator for [formula omitted] collisions

We have developed an event generator, named GR@PPA_4b, for the four bottom quark ( b b ̄ b b ̄ ) production processes at pp and p p ̄ collisions. The program implements all of the possible processes at the tree level within the framework of the Standard Model. Users can generate events from the Higgs boson and γ/ Z mediated processes, as well as those from pure QCD interactions. The integration and the event generation are performed within the newly developed GR@PPA framework, an extension of the GRACE automatic event-generator generation system to hadron collisions. This program is so designed that it can be embedded in a general-purpose event generator PYTHIA version 6.1. PYTHIA adds the initial- and final-state parton showers and simulates the hadronization and decays to make generated events realistic. It should be emphasized that a huge number of diagrams and complicated four-body kinematics are dealt with strictly in GR@PPA_4b. This program will be useful for studies of Higgs boson productions, especially those in extended models where the Yukawa coupling to b quarks is greatly enhanced.

Monte Carlo simulation of general hard processes with coherent QCD radiation

In this paper we extend our previous work on the simulation of coherent soft-gluon radiation to hard collisions that involve incoming as well as outgoing coloured partons. Existing simulations correctly sum the leading collinear singularities for initial- and final-state radiation, and in some cases the leading infrared contributions from outgoing partons, but not those for incoming (or the interference between incoming and outgoing). Asymptotically, however, the leading infrared and collinear contributions are comparable, the bulk of gluon emission occuring in the soft region. Furthermore, a correct treatment of leading infrared terms is necessary for the inclusive cancellation of singularities in the Sudakov form factor. We show how such a treatment ,ay be formulated in terms of an angular ordering procedure applicable to all hard processes. We then describe a new Monte Carlo program which incorporates this procedure, together with other new features such as azimuthal correlations due to gluon polarization and interference. The program is designed as a general-purpose event generator, simulating hard lepton-lepton, lepton-hadron and hadron-hadron scattering in a single package. Simulation of soft hadronic collisions and underlying events is also included. We present the predictions of the program for a wide variety of processes, and compare them with analytical results and experimental data.