Monte Carlo Generator Research Articles

Prompt Photon Production at the Tevatron and LHC

Prompt photon production has been studied by the CDF and DØ experiments at the Fermilab Tevatron collider in pp̅ collisions at the centre of mass energy of √s = 1.96 TeV and by the ATLAS experiment at LHC in pp collisions at √s = 7 TeV.Measurements of the inclusive photon, inclusive photon plus jet, photon plus heavy flavor jet, and diphoton production cross sections are discussed. The results are compared to the perturbative QCD calculations and predictions from Monte Carlo generators.

Connecting accelerator experiments and cosmic ray showers

Currently the uncertainty in the prediction of shower observables for different primary particles and energies is dominated by differences between hadronic interaction models. The LHC data on minimum bias measurements can be used to test Monte Carlo generators and these new constrains will help to reduce the uncertainties in air shower predictions. In this article, after a short introduction on air showers we will show the results of the comparison between the updated version of high energy hadronic interaction models with LHC data. Results for air shower simulations and their consequence on the comparison with air shower data will be discussed. Most of the Astronomy and Astrophysics is done using electromagnetic signals from radio to gamma rays. It gives precious informations on the various objects observed in the Universe and their history. In fact a part of these signals is produced by elementary charged particles like electrons or nuclei which can escape the source and reach the Earth after a long propagation through the (extra)galactic medium. Eventually these charged particles may cross the path of the Earth and enter our field of view: they are cosmic rays. Due to the steeply falling energy spectrum of cosmic rays, direct detection by satellite- or balloon-borne instruments is only possible up to about ∼10 14 eV. Fortunately, at such high energies, the cascades of secondary particles produced by cosmic rays reach the ground and can be detected in coincidence experiments. The cascades are called extensive air showers (EAS) and are routinely used to make indirect measurements of high energy cosmic rays. The upper limit of the detectable energy is given by the area and exposure time of the detector. For instance, the Pierre Auger Observatory (PAO) (1), which is currently taking data in Argentina, is designed to detect particles of ∼10 20 eV for which the flux is less than one particle per km 2 and century. Air showers can be observed using different detection techniques. The most frequently employed technique is the measurement of secondary particles reaching ground. Using an array of particle detectors (for example, sensitive to e ± and ± ), the arrival direction and information on mass and energy of the primary cosmic ray can be reconstructed. The main observables are the number and the lateral (Fig. 2 left-hand side) and temporal distributions of the different secondary particles. At energies above ∼10 17 eV, the longitudinal development of a shower can be directly observed by measuring the fluorescence light induced by the charged particles traversing the atmosphere. Two main observables can be extracted from the longitudinal shower profile: the energy deposit or the number of particles, Nmax, at the shower maximum and Xmax, the atmospheric depth of the maximum (see Fig. 2 right-hand side). Again, these quantities can be used to estimate the energy and mass of the primary particles. Shower-to-shower fluctuations of all observables also provide very useful composition information.

LHC data and extensive air showers

Currently the uncertainty in the prediction of shower observables for different primary particles and energies is dominated by differences between hadronic interaction models. The LHC data on minimum bias measurements can be used to test Monte Carlo generators and these new constraints will help to reduce the uncertainties in air shower predictions. In this article, after a short introduction on air showers we will show the results of the comparison between the updated version of high energy hadronic interaction models EPOS LHC and QGSJETII-04 with LHC data. Results for air shower simulations and their consequences on comparisons with air shower data will be discussed.

On the spin and parity of a single-produced resonance at the LHC

Both LHC experiments, ATLAS and CMS, observed a new narrow resonance near 125 GeV.We demonstrate that the spin, parity, and, more generally, the tensor structure of the boson couplings of this resonance can be obtained by studying mass and angular distributions of events where the resonance decays to pairs of gauge bosons, ZZ, WW and γγ To account for all spin correlations we calculate kinematic distributions analytically and develop a Monte Carlo generator. We also discuss how to use angular and mass distributions of the resonance decay products for optimal background rejection and for distinguishing different signal hypotheses. By the end of the 8 TeV run of the LHC, it might be possible to separate extreme hypotheses of the spin and parity of the new resonance with a confidence level of 99% or better for a wide range of models.

A Hybrid Monte Carlo Generator for Ultra-High Energy Cosmic Rays from their Sources to the Observer

To understand in detail cosmic magnetic fields and sources of Ultra-High Energy Cosmic Rays (UHECRs) we have developed a Monte Carlo simulation for galactic and extragalactic propagation. In our approach we identify three different propagation regimes for UHECRs, the Milky Way, the local universe out to 110 Mpc, and the distant universe. For deflections caused by the galactic magnetic field a lensing technique based on matrices is applied which are created from backtracking of antiparticles through galactic field models. Propagation in the local universe uses forward tracking through structured magnetic fields extracted from simulations of the large scale structure of the universe. UHECRs from distant sources are simulated using parameterized models. In this contribution we present the combination of all three simulation techniques by means of probability maps. The combined probability maps are used to generate a large number of UHECRs, and to create distributions from approximately realistic universe scenarios. Comparisons with physics analyses of UHECR measurements enable the development of new analysis techniques and help to constrain parameters of the underlying physics models like the source density and the magnetic field strength in the universe.

Next-to-leading order QCD corrections toW+W+jjproduction in vector-boson fusion at the LHC

We present a next-to-leading-order QCD calculation for e+\nu e\mu+\nu\mu jj production in vector-boson fusion, i.e. the scattering of two positively charged W bosons at the LHC. We include the complete set of electroweak leading-order diagrams for the six-particle final state and quantitatively assess the size of the s-channel and interference contributions in VBF kinematics. The calculation uses the complex-mass scheme to describe the W-boson resonances and is implemented into a flexible Monte Carlo generator. Using a dynamical scale based on the transverse momenta of the jets, the QCD corrections stay below about 10% for all considered observables, while the residual scale dependence is at the level of 1%.

Antideuterons from dark matter annihilations and hadronization model dependence

We calculate the antideuteron yield in dark matter annihilations on an event-by-event basis using the HERWIG++ Monte Carlo generator. We present the resulting antideuteron fluxes for quark and gauge boson final states. As deuteron production in the coalescence model depends on momentum differences between nucleons that are small compared to \Lambda_{QCD}, it is potentially very sensitive to the hadronization model employed. We therefore compare our antideuteron yields to earlier results based on PYTHIA, thereby estimating their uncertainties. We also briefly discuss the importance of n>2 final states for annihilations of heavy DM particles.

The Monte Carlo event generator AcerMC versions 2.0 to 3.8 with interfaces to PYTHIA 6.4, HERWIG 6.5 and ARIADNE 4.1

The AcerMC Monte Carlo generator is dedicated to the generation of Standard Model background processes which were recognised as critical for the searches at LHC, and generation of which was either unavailable or not straightforward so far. The program itself provides a library of the massive matrix elements (coded by MADGRAPH) and native phase space modules for generation of a set of selected processes. The hard process event can be completed by the initial and the final state radiation, hadronisation and decays through the existing interface with either PYTHIA, HERWIG or ARIADNE event generators and (optionally) TAUOLA and PHOTOS. Interfaces to all these packages are provided in the distribution version. The phase-space generation is based on the multi-channel self-optimising approach using the modified Kajantie–Byckling formalism for phase space construction and further smoothing of the phase space was obtained by using a modified ac-VEGAS algorithm. An additional improvement in the recent versions is the inclusion of the consistent prescription for matching the matrix element calculations with parton showering for a select list of processes. Program summaryProgram title: AcerMC version 3.8Catalogue identifier: ADQQ_v2_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADQQ_v2_0.htmlProgram obtainable from: CPC Program Library, Queen’s 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.: 3853309No. of bytes in distributed program, including test data, etc.: 68045728Distribution format: tar.gzProgramming language: FORTRAN 77 with popular extensions (g77, gfortran).Computer: All running Linux.Operating system: Linux.Classification: 11.2, 11.6.External routines: CERNLIB (http://cernlib.web.cern.ch/cernlib/), LHAPDF (http://lhapdf.hepforge.org/)Catalogue identifier of previous version: ADQQ_v1_0Journal reference of previous version: Comput. Phys. Comm. 149(2003)142Does the new version supersede the previous version?: YesNature of problem: Despite a large repertoire of processes implemented for generation in event generators like PYTHIA [1] or HERWIG [2] a number of background processes, crucial for studying the expected physics of the LHC experiments, is missing. For some of these processes the matrix element expressions are rather lengthy and/or to achieve a reasonable generation efficiency it is necessary to tailor the phase space selection procedure to the dynamics of the process. That is why it is not practical to imagine that any of the above general purpose generators will contain every, or even only observable, processes which will occur at LHC collisions. A more practical solution can be found in a library of dedicated matrix-element-based generators, with the standardised interfaces like that proposed in [3], to the more universal one which is used to complete the event generation.Solution method: The AcerMC EventGenerator provides a library of the matrix-element-based generators for several processes. The initial- and final-state showers, beam remnants and underlying events, fragmentation and remaining decays are supposed to be performed by the other universal generator to which this one is interfaced. We will call it a supervising generator. The interfaces to PYTHIA 6.4, ARIADNE 4.1 and HERWIG 6.5, as such generators, are provided. Provided is also an interface to TAUOLA [4] and PHOTOS [5] packages for τ-lepton decays (including spin correlations treatment) and QED radiations in decays of particles. At present, the following matrix-element-based processes have been implemented: gg,qq̄→tt̄bb̄, qq̄→W(→ℓν)bb̄; qq̄→W(→ℓν)tt̄; gg,qq̄→Z/γ∗(→ℓℓ)bb̄; gg,qq̄→Z/γ∗(→ℓℓ,νν,bb̄)tt̄; complete EW gg,qq̄→(Z/W/γ∗→)tt̄bb̄; gg,qq̄→tt̄tt̄; gg,qq̄→(tt̄→)ff̄bff̄b̄; gg,qq̄→(WWbb→)ff̄ff̄bb̄. Both interfaces allow the use of the LHAPDF/LHAGLUE library of parton density functions. Provided is also a set of control processes: qq̄→W→ℓν; qq̄→Z/γ∗→ℓℓ; gg,qq̄→tt̄ and gg→(tt̄→)WbWb̄;Reasons for new version: Implementation of several new processes and methods.Summary of revisions: Each version added new processes or functionalities, a detailed list is given in the section “Changes since AcerMC 1.0”.Restrictions: The package is optimised for the 14 TeV pp collision simulated in the LHC environment and also works at the achieved LHC energies of 7 TeV and 8 TeV. The consistency between results of the complete generation using PYTHIA 6.4 or HERWIG 6.5 interfaces is technically limited by the different approaches taken in both these generators for evaluating αQCD and αQED couplings and by the different models for fragmentation/hadronisation. For the consistency check, in the AcerMC library contains native coded definitions of the QCD and αQED. Using these native definitions leads to the same total cross-sections both with PYTHIA 6.4 or HERWIG 6.5 interfaces.Additional comments: !!!!! The distribution file for this program is over 67 Mbytes and therefore is not delivered directly when download or Email is requested. Instead an html file giving details of how the program can be obtained is sent. !!!!!Running time: On an PIII 800 MHz PC it amounts to ∼0.05→1.1 events/sec, depending on the choice of process.

ATLAS measurements of the properties of jets for boosted particle searches

Measurements are presented of the properties of high transverse momentum jets, produced in proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV. The data correspond to an integrated luminosity of 35 pb^-1 and were collected with the ATLAS detector in 2010. Jet mass, width, eccentricity, planar flow and angularity are measured for jets reconstructed using the anti-kt algorithm with distance parameters R = 0.6 and 1.0, with transverse momentum pT > 300 GeV and pseudorapidity |eta| < 2. The measurements are compared to the expectations of Monte Carlo generators that match leading-logarithmic parton showers to leading-order, or next-to-leading-order, matrix elements. The generators describe the general features of the jets, although discrepancies are observed in some distributions.

Measurement of prompt hadron production ratios in pp collisions at $\sqrt{s} = 0.9\mbox{ and }7~\mathrm{TeV}$

The charged-particle production ratios $\bar{p}/p$ , K −/K +, π −/π +, $(p + \bar{p})/(\pi^{+} + \pi^{-})$ , (K ++K −)/(π ++π −) and $(p + \bar{p})/(K^{+} + K^{-})$ are measured with the LHCb detector using 0.3 nb−1 of pp collisions delivered by the LHC at $\sqrt{s} = 0.9~\mathrm{TeV}$ and 1.8 nb−1 at $\sqrt{s} = 7~\mathrm{TeV}$ . The measurements are performed as a function of transverse momentum p T and pseudorapidity η. The production ratios are compared to the predictions of several Monte Carlo generator settings, none of which are able to describe adequately all observables. The ratio $\bar{p}/p$ is also considered as a function of rapidity loss, Δy≡y beam−y, and is used to constrain models of baryon transport.

Standard SANC modules for NLO QCD radiative corrections to single top-quark production

In this paper we present the results obtained with the newly created standard SANC modules for calculation of the NLO QCD corrections to single top-quark-production processes in s and t channels at the partonic level, as well as to top-decays. The main aim of these results is to prove the correct work of modules. A comprehensive comparison with the results of the CompHEP system is given, where possible. These modules are intended to be used in Monte Carlo generators for single top-quark-production processes at the LHC. As in our recent paper, devoted to the electroweak corrections to these processes, we study the regularization of the top-legs associated infrared divergences with the aid of the complex mass of the top quark. A comparison of QCD corrections with those computed by the conventional method is presented both for top production and for decays. For s-channel production we give an analytic proof of equivalence of the two methods in the limit of low top width.

Measurement of the underlying event in the Drell–Yan process in proton–proton collisions at $\sqrt{s} = 7~\mbox{TeV}$

A measurement of the underlying event (UE) activity in proton-proton collisions at a center-of-mass energy of 7 TeV is performed using Drell--Yan events in a data sample corresponding to an integrated luminosity of 2.2 inverse femtobarns, collected by the CMS experiment at the LHC. The activity measured in the muonic final state (q q-bar to opposite-sign muons) is corrected to the particle level and compared with the predictions of various Monte Carlo generators and hadronization models. The dependence of the UE activity on the dimuon invariant mass is well described by PYTHIA and HERWIG++ tunes derived from the leading jet/track approach, illustrating the universality of the UE activity. The UE activity is observed to be independent of the dimuon invariant mass in the region above 40 GeV, while a slow increase is observed with increasing transverse momentum of the dimuon system. The dependence of the UE activity on the transverse momentum of the dimuon system is accurately described by MADGRAPH, which simulates multiple hard emissions.

Hadron production in lepton-nucleus interactions at high energies: Monte carlo generator HARDPING 2.0

Hadron production in lepton-nucleus interactions at high-energies is considered in framework of developing Monte Carlo event generator HARDPING (HARD Probe INteraction Generator). Such effects as formation length, energy loss and multiple rescattering for produced hadrons and their constituents are implemented into the HARDPING 2.0. Available data from HERMES collaboration on hadron production in lepton-nucleus collisions are described by the present version of the HARDPING generator in a reasonable agreement.

NuWro: the Wrocław Monte Carlo Generator of Neutrino Interactions

Recent developments in NuWro Monte Carlo generator of neutrino interactions are presented.

Forward-backward correlations and charged-particle azimuthal distributions in pp interactions using the ATLAS detector

Abstract Using inelastic proton-proton interactions at $ \sqrt {s} = 900 $ GeV and 7 TeV, recorded by the ATLAS detector at the LHC, measurements have been made of the correlations between forward and backward charged-particle multiplicities and, for the first time, between forward and backward charged-particle summed transverse momentum. In addition, jet-like structure in the events is studied by means of azimuthal distributions of charged particles relative to the charged particle with highest transverse momentum in a selected kinematic region of the event. The results are compared with predictions from tunes of the pythia and herwig++ Monte Carlo generators, which in most cases are found to provide a reasonable description of the data.

Can Go address the multicore issues of today and the manycore problems of tomorrow?

Current High Energy and Nuclear Physics (HENP) libraries and frameworks were written before multicore systems became widely deployed and used. From this environment, a 'single-thread' processing model naturally emerged but the implicit assumptions it encouraged are greatly impairing our abilities to scale in a multicore/manycore world. While parallel programming - still in an intensive phase of R&D despite the 30+ years of literature on the subject - is an obvious topic to consider, other issues (build scalability, code clarity, code deployment and ease of coding) are worth investigating when preparing for the manycore era. Moreover, if one wants to use another language than C++, a language better prepared and tailored for expressing concurrency, one also needs to ensure a good and easy reuse of already field-proven libraries. We present the work resulting from such investigations applied to the Go programming language. We first introduce the concurrent programming facilities Go is providing and how its module system addresses the build scalability and dependency hell issues. We then describe the process of leveraging the many (wo)man-years put into scientific Fortran/C/C++ libraries and making them available to the Go ecosystem. The ROOT data analysis framework, the C-BLAS library and the Herwig-6 MonteCarlo generator will be taken as examples. Finally, performances of the tools involved in a small analysis written in Go and using ROOT I/O library will be presented.

Shape, transverse size, and charged-hadron multiplicity of jets in pp collisions at $ \sqrt{s}=7\;TeV $

Abstract Measurements of jet characteristics from inclusive jet production in protonproton collisions at a centre-of-mass energy of 7 TeV are presented. The data sample was collected with the CMS detector at the LHC during 2010 and corresponds to an integrated luminosity of 36 pb−1. The mean charged-hadron multiplicity, the differential and integral jet shape distributions, and two independent moments of the shape distributions are measured as functions of the jet transverse momentum for jets reconstructed with the anti-k T algorithm. The measured observables are corrected to the particle level and compared with predictions from various QCD Monte Carlo generators.

Measurement of the charge asymmetry in top quark pair production in pp collisions at [Formula: see text] using the ATLAS detector.

A measurement of the top-antitop production charge asymmetry AC is presented using data corresponding to an integrated luminosity of 1.04 fb−1 of pp collisions at sqrt{s} = 7 TeV collected by the ATLAS detector at the LHC. Events are selected with a single lepton (electron or muon), missing transverse momentum and at least four jets of which at least one jet is identified as coming from a b-quark. A kinematic fit is used to reconstruct the tbar{t} event topology. After background subtraction, a Bayesian unfolding procedure is performed to correct for acceptance and detector effects. The measured value of AC is A_{C}= -0.019 pm0.028 (mathrm{stat}.) pm0.024 (mathrm{syst.}), consistent with the prediction from the MC@NLO Monte Carlo generator of AC=0.006±0.002. Measurements of AC in two ranges of invariant mass of the top-antitop pair are also shown.

EKHARA — new gamma gamma generator

The Monte Carlo generator EKHARA allows to simulate the process (e+e- to e+e-pi0) at energies of meson factories. We review the motivation, describe the current status of the generator, and discuss features, which will appear in the forthcoming releases: the two-photon production of eta and eta-prime mesons.

New hadronic currents in TAUOLA: for confrontation with the experimental data

The status of implementation of new hadronic currents into the Monte Carlo system for simulation of τ -lepton production and decay in high-energy accelerator experiments is reviewed. Since the τ -lepton conference in 2010 substantial progress was achieved: (i) For the TAUOLA Monte Carlo generator of τ -lepton decays, automated and simultaneous use of many versions of form factors for the calculation of optional weights for fits was developed and checked to work in the Belle and BaBar software environment. Alternative parameterizations of hadronic currents based on the Resonance Chiral approach are available now. This was achieved for more than 88% of the total τ hadronic width. (ii) the TAUOLA universal interface based on HepMC (the C++ event record) is available. This is the case for C++ users of PHOTOS Monte Carlo for radiative corrections in decays, as well. An algorithm for weighted events to explore spin effects in analysis of hard processes was prepared. (iii) Kernels featuring a complete first-order matrix element are available now for PHOTOS users interested in decays of Z and W bosons. New tests with different options of matrix elements for those and for K l 3 decays are available as well. Presented results illustrate the status of the projects performed in collaboration with Zofia Czyczula, Nadia Davidson, Tomasz Przedziński, Olga Shekhovtsova, Elżbieta Richter-Wąs, Pablo Roig, Qingjun Xu and others.