General-purpose Monte Carlo Event Research Articles

Particle Production as a Function of Underlying-event Activity and Search for Jet-like Modifications in \(pp\), \(p\)–Pb, and Pb–Pb Collisions at \(\sqrt {s_{NN}}=5.02\)\thinspace TeV with ALICE

The similarity between small- and large-collision systems is explored using the charged-particle multiplicity in the transverse region, $N^{\rm T}_{\rm ch}$, which is sensitive to the underlying event. Measurements of charged-particle production as a function of $N^{\rm T}_{\rm ch}$ in pp, p-Pb and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV in the toward, away and transverse regions are discussed. These three regions are defined relative to the track with the largest transverse momentum in the event ($p_{\rm T}^{\rm trig}$). The activity in the transverse region is subtracted from the activity in the toward and the away regions to search for jet-like modifications in small-collision systems. The jet-like signals are studied both as a function of $N^{\rm T}_{\rm ch}$ and $p_{\rm T}^{\rm trig}$. Results are compared with two general purpose Monte Carlo event generators: PYTHIA~8 and EPOS~LHC.

Hadronic rescattering in pA and AA collisions

In a recent article we presented a model for hadronic rescattering, and some results were shown for mathrm {p}mathrm {p} collisions at LHC energies. In order to extend the studies to mathrm {p}mathrm {A} and mathrm {A}mathrm {A} collisions, the Angantyr model for heavy-ion collisions is taken as the starting point. Both these models are implemented within the general-purpose Monte Carlo event generator Pythia, which makes the matching reasonably straightforward, and allows for detailed studies of the full space–time evolution. The rescattering rate is significantly higher than in mathrm {p}mathrm {p}, especially for central mathrm {A}mathrm {A} collisions, where the typical primary hadron rescatters several times. We study the impact of rescattering on a number of distributions, such as p_{perp } and eta spectra, and the space–time evolution of the whole collision process. Notably rescattering is shown to give a significant contribution to elliptic flow in mathrm {XeXe} and mathrm {PbPb}, and to give a nontrivial impact on charm production.

CIMBA: Fast Monte Carlo generation using cubic interpolation

Monte Carlo generation of high energy particle collisions is a critical tool for both theoretical and experimental particle physics, connecting perturbative calculations to phenomenological models, and theory predictions to full detector simulation. The generation of minimum bias events can be particularly computationally expensive, where non-perturbative effects play an important role and specific processes and fiducial regions can no longer be well defined. In particular scenarios, particle guns can be used to quickly sample kinematics for single particles produced in minimum bias events. CIMBA (Cubic Interpolation for Minimum Bias Approximation) provides a comprehensive package to smoothly sample predefined kinematic grids, from any general purpose Monte Carlo generator, for all particles produced in minimum bias events. These grids are provided for a number of beam configurations including those of the Large Hadron Collider. Program summaryProgram title: CIMBA (Cubic Interpolation for Minimum Bias Approximation)CPC Library link to program files:http://dx.doi.org/10.17632/49m44md4ph.1Licensing provisions: GPL version 2 or laterProgramming language: Python, C++Nature of problem: generation of simulated events in high energy particle physics is quickly becoming a bottleneck in analysis development for collaborations on the Large Hadron Collider (LHC). With the expected long-term continuation of the high luminosity LHC, this problem must be solved in the near future. Significant progress has been made in designing new ways to perform detector simulation, including parametric detector models and machine learning techniques, e.g. calorimeter shower evolution with generative adversarial networks. Consequently, the efficiency of generating physics events using general purpose Monte Carlo event generators, rather than just detector simulation, needs to be improved.Solution method: in many cases, single particle generation from pre-sampled phase-space distributions can be used as a fast alternative to full event generation. Phase-space distributions sampled in particle pseudorapidity and transverse momentum are sampled from large, once-off, minimum bias samples generated with Pythia 8. A novel smooth sampling of these distributions is performed using piecewise cubic Hermite interpolating polynomials. Distributions are created for all generated particles, as well as particles produced directly from hadronisation. Interpolation grid libraries are provided for a number of common collider configurations, and code is provided which can produce custom interpolation grid libraries.Restrictions: Single particle generation

Event generation with Sherpa 2.2

Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarise 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.

Interleaved parton showers and tuning prospects

General-purpose Monte Carlo event generators have become important tools in particle physics, allowing the simulation of exclusive hadronic final states. In this article we examine the Pythia 8 generator, in particular focusing on its parton-shower algorithms. Some relevant new additions to the code are introduced, that should allow for a better description of data. We also implement and compare with 2 to 3 real-emission QCD matrix elements, to check how well the shower algorithm fills the phase space away from the soft and collinear regions. A tuning of the generator to Tevatron data is performed for two PDF sets and the impact of first new LHC data is examined.

Herwig++ physics and manual

In this paper we describe \(\mathsf{Herwig++}\) version 2.2, a general-purpose Monte Carlo event generator for the simulation of hard lepton-lepton and hadron-hadron collisions. A number of important hard scattering processes are available, together with an interface via the Les Houches Accord to specialized matrix element generators for additional processes. The simulation of Beyond the Standard Model (BSM) physics includes a range of models and allows new models to be added by encoding the Feynman rules of the model. The parton-shower approach is used to simulate initial- and final-state QCD radiation, including colour coherence effects, with special emphasis on the correct description of radiation from heavy particles. The underlying event is simulated using an eikonal multiple parton-parton scattering model. The formation of hadrons from the quarks and gluons produced in the parton shower is described using the cluster hadronization model. Hadron decays are simulated using matrix elements, where possible including spin correlations and off-shell effects.

HERWIG 6: an event generator for hadron emission reactions with interfering gluons (including supersymmetric processes)

HERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program. This version of the manual (version 3) is updated to HERWIG version 6.5, which is expected to be the last major release of Fortran HERWIG. Future developments will be implemented in a new C++ event generator, HERWIG++.

Foam: Multi-dimensional general purpose Monte Carlo generator with self-adapting simplical grid

A new general purpose Monte Carlo event generator with self-adapting grid consisting of simplices is described. In the process of initialization, the simplex-shaped cells divide into daughter subcells in such a way that: (a) cell density is biggest in areas where integrand is peaked, (b) cells elongate themselves along hyperspaces where integrand is enhanced/singular. The grid is anisotropic, i.e. memory of the axes directions of the primary reference frame is lost. In particular, the algorithm is capable of dealing with distributions featuring strong correlation among variables (like ridge along diagonal). The presented algorithm is complementary to others known and commonly used in the Monte Carlo event generators. It is, in principle, more effective than any other one for distributions with very complicated patterns of singularities – the price to pay is that it is memory-hungry. It is therefore aimed at a small number of integration dimensions ( <10 ). It should be combined with other methods for higher dimension. The source code in Fortran 77 is available from http://home.cern.ch/ hadach.

VNI 3.1 MC-simulation program to study high-energy particle collisions in QCD by space-time evolution of parton-cascades and parton-hadron conversion

VNI is a general-purpose Monte Carlo event generator, which includes the simulation of lepton-lepton, lepton-hadron, lepton-nucleus, hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions. On the basis of renormalization-group improved parton description and quantum-kinetic theory, it uses the real-time evolution of parton cascades in conjunction with a self-consistent hadronization scheme that is governed by the dynamics itself. The causal evolution from a specific initial state (determined by the colliding beam particles) is followed by the time development of the phase-space densities of partons, pre-hadronic parton clusters, and final-state hadrons, in position space, momentum space and color space. The parton evolution is described in terms of a space-time generalization of the familiar momentum-space description of multiple (semi) hard interactions in QCD, involving 2 → 2 parton collisions, 2 → 1 parton fusion processes, and 1 → 2 radiation processes. The formation of color-singlet pre-hadronic clusters and their decays into hadrons, on the other hand, is treated by using a spatial criterion motivated by confinement and a non-perturbative model for hadronization. This article gives a brief review of the physics underlying VNI, which is followed by a detailed description of the program itself. The latter program description emphasizes easy-to-use pragmatism and explains how to use the program (including a simple example), annotates input and control parameters, and discusses output data provided by it.