Monte Carlo Generator Research Articles

Forward production of charged pions with incident protons on nuclear targets at the CERN Proton Synchrotron

Measurements of the double-differential charged pion production cross-section in the range of momentum 0.5 GeV/c < p < 8.0 GeV/c and angle 0.025 rad < theta <0.25 rad in collisions of protons on beryllium, carbon, nitrogen, oxygen, aluminium, copper, tin, tantalum and lead are presented. The data were taken with the large acceptance HARP detector in the T9 beam line of the CERN PS. Incident particles were identified by an elaborate system of beam detectors. The data were taken with thin targets of 5% of a nuclear interaction length. The tracking and identification of the produced particles was performed using the forward system of the HARP experiment. Results are obtained for the double-differential cross section mainly at four incident proton beam momenta (3 GeV/c, 5 GeV/c, 8 GeV/c and 12 GeV/c). Measurements are compared with the GEANT4 and MARS Monte Carlo generators. A global parametrization is provided as an approximation of all the collected datasets which can serve as a tool for quick yields estimates.

A Monte Carlo generator of nucleon configurations in complex nuclei including nucleon–nucleon correlations

We developed a Monte Carlo event generator for production of nucleon configurations in complex nuclei consistently including effects of nucleon–nucleon (NN) correlations. Our approach is based on the Metropolis search for configurations satisfying essential constraints imposed by short- and long-range NN correlations, guided by the findings of realistic calculations of one- and two-body densities for medium-heavy nuclei. The produced event generator can be used for Monte Carlo (MC) studies of pA and AA collisions. We perform several tests of consistency of the code and comparison with previous models, in the case of high energy proton–nucleus scattering on an event-by-event basis, using nucleus configurations produced by our code and Glauber multiple scattering theory both for the uncorrelated and the correlated configurations; fluctuations of the average number of collisions are shown to be affected considerably by the introduction of NN correlations in the target nucleus. We also use the generator to estimate maximal possible gluon nuclear shadowing in a simple geometric model.

Determination of the strong coupling constant using matched NNLO+NLLA predictions for hadronic event shapes in e+e−annihilations

We present a determination of the strong coupling constant from a fit of QCD predictions for six event-shape variables, calculated at next-to-next-to-leading order (NNLO) and matched to resummation in the next-to-leading-logarithmic approximation (NLLA). These event shapes have been measured in e+e- annihilations at LEP, where the data we use have been collected by the ALEPH detector at centre-of-mass energies between 91 and 206 GeV. Compared to purely fixed order NNLO fits, we observe that the central fit values are hardly affected, but the systematic uncertainty is larger because the NLLA part re-introduces relatively large uncertainties from scale variations. By combining the results for six event-shape variables and eight centre-of-mass energies, we find alpha_s(MZ)= 0.1224 +/- 0.0009 (stat) +/- 0.0009 (exp) +/- 0.0012 (had) +/- 0.0035 (theo), which improves previously published measurements at NLO+NLLA. We also carry out a detailed investigation of hadronisation corrections, using a large set of Monte Carlo generator predictions.

Measurement of particle production and inclusive differential cross sections inpp¯collisions ats=1.96 TeV

We report a set of measurements of particle production in inelastic pbar{p} collisions collected with a minimum-bias trigger at the Tevatron Collider with the CDF II experiment. The inclusive charged particle transverse momentum differential cross section is measured, with improved precision, over a range about ten times wider than in previous measurements. The former modeling of the spectrum appears to be incompatible with the high particle momenta observed. The dependence of the charged particle transverse momentum on the event particle multiplicity is analyzed to study the various components of hadron interactions. This is one of the observable variables most poorly reproduced by the available Monte Carlo generators. A first measurement of the event transverse energy sum differential cross section is also reported. A comparison with a Pythia prediction at the hadron level is performed. The inclusive charged particle differential production cross section is fairly well reproduced only in the transverse momentum range available from previous measurements. At higher momentum the agreement is poor. The transverse energy sum is poorly reproduced over the whole spectrum. The dependence of the charged particle transverse momentum on the particle multiplicity needs the introduction of more sophisticated particle production mechanisms, such as multiple parton interactions, in order to be better explained.

Chapter 3 Analysis Tools

The following sections are included: Monte Carlo Generators Luminosity Measurements at BES-III Particle Identification Kinematic Fitting Partial Wave Analysis Dalitz-Plot Analysis Formalism

Substructure of high-pTjets at the LHC

We study high-pt jets from QCD and from highly-boosted massive particles such as tops, W, Z and Higgs, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly-boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities. We emphasize that for massive jets, these and other observables can be analyzed perturbatively.

TAUOLA, TAUOLA universal interface PHOTOS and MC-TESTER: Status Report

The status of Monte Carlo programs for the simulation of τ -lepton production and decay in high-energy accelerator experiments is reviewed. No basic changes in the organization of the programs were necessary since the previous τ -lepton conference in 2006. Minor in size, but practical, extensions for the use of the programs in high precision mixed languages enviromens are being introduced: (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 discussed. A pilot example of its use was presented. (ii) New tests for PHOTOS Monte Carlo for QED bremsstrahlung in W decays were shown. (iii) Prototype version of the TAUOLA universal interface based on HepMC (the C++ event record) was mentioned. Its tests with the help of MC-TESTER were discussed. Presented results illustrate the status of the projects performed in collaboration with Vladimir Cherepanov, Nadia Davidson, Piotr Golonka, Gizo Nanava, Tomasz Przedziński, Elżbieta Richter-Wa̧s and others.

Measurement of cross sections forbjet production in events with aZboson inpp¯collisions ats=1.96 TeV

A measurement of the $\bjet$ production cross section is presented for events containing a $Z$ boson produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV, using data corresponding to an integrated luminosity of 2 fb$^{-1}$ collected by the CDF II detector at the Tevatron. $Z$ bosons are selected in the electron and muon decay modes. Jets are considered with transverse energy $E_T>20$ GeV and pseudorapidity $|\eta|<1.5$ and are identified as $\bjets$ using a secondary vertex algorithm. The ratio of the integrated $Z+\bjet$ cross section to the inclusive $Z$ production cross section is measured to be $3.32 \pm 0.53 {\rm (stat.)} \pm 0.42 {\rm (syst.)}\times 10^{-3}$. This ratio is also measured differentially in jet $E_T$, jet $\eta$, $Z$-boson transverse momentum, number of jets, and number of $\bjets$. The predictions from leading order Monte Carlo generators and next-to-leading-order QCD calculations are found to be consistent with the measurements within experimental and theoretical uncertainties.

Carlomat: A program for automatic computation of lowest order cross sections

The current version of carlomat, a program for automatic computation of the lowest order cross sections of multiparticle reactions, is described. The program can be used as the Monte Carlo generator of unweighted events as well. Program summary Program title: carlomat Catalogue identifier: AEDQ_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEDQ_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 101 613 No. of bytes in distributed program, including test data, etc.: 1 092 251 Distribution format: tar.gz Programming language: Fortran 90/95 Computer: all Operating system: Linux Classification: 4.4, 11.2 Nature of problem: Description of two particle scattering reactions with possibly up to 10 particles in the final state with a complete set of the Feynman diagrams in the lowest order of the Standard Model. Solution method: The matrix element for a user specified process and phase space parametrizations, which are necessary for the multichannel Monte Carlo integration of the lowest order cross sections and event generation, are generated automatically. Both the electroweak and quantum chromodynamics lowest order contributions are taken into account. Particle masses are not neglected in the program. Matrix elements are calculated numerically with the helicity amplitude method. Constant widths of unstable particles are implemented by modifying mass parameters in corresponding propagators. Restrictions: The number of external particles is limited to 12. Only the Standard Model is implemented at the moment in the program. No higher order effects are taken into account, except for assuming the fine structure constant and the strong coupling at appropriate scale and partial summation of the one particle irreducible loop corrections by introducing fixed widths of unstable particles. Running time: Generation of the Fortran code with carlomat on a PC with the Pentium 4 3.0 GHz processor for reactions with 8 particles in the final state relevant for the associated top quark pair and Higgs boson production and decay takes about 10 minutes CPU time. This relatively long time of the code generation is determined by a lot of write to and read from a disk commands which have to be introduced in order to circumvent limitations of the Fortran compilers concerning possible array sizes. The compilation time of generated routines depends strongly on a compiler used and an optimization option chosen. Typically, for the reactions mentioned, it takes about one hour to compile all the routines generated. Most of the time is used for the compilation of the kinematical routines. The execution time of the Monte Carlo (MC) integration with about 2 million calls to the integrand amounts typically to a few hours and, if the MC summing over polarizations is employed, it is dominated by computation of the phase space normalization.

DRAGON: Monte Carlo generator of particle production from a fragmented fireball in ultrarelativistic nuclear collisions

A Monte Carlo generator of the final state of hadrons emitted from an ultrarelativistic nuclear collision is introduced. An important feature of the generator is a possible fragmentation of the fireball and emission of the hadrons from fragments. Phase space distribution of the fragments is based on the blast wave model extended to azimuthally non-symmetric fireballs. Parameters of the model can be tuned and this allows to generate final states from various kinds of fireballs. A facultative output in the OSCAR1999A format allows for a comprehensive analysis of phase-space distributions and/or use as an input for an afterburner. Program summary Program title: DRAGON Catalogue identifier: AEDK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEDK_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 6383 No. of bytes in distributed program, including test data, etc.: 32 756 Distribution format: tar.gz Programming language: C++ Computer: PC Pentium 4, though no particular tuning for this machine was performed Operating system: Linux; the program has been successfully run on Gentoo Linux 2.6, RedHat Linux 9, Debian Linux 4.0, all with g++ compiler. It also ran successfully on MS Windows under Microsoft Visual C++ 2008 Express Edition as well as under cygwin/g++ RAM: 100 Mbytes Supplementary material: Sample output files from the test run, provided in the distribution, are available. Classification: 11.2 Nature of problem: Deconfined matter produced in ultrarelativistic nuclear collisions expands and cools down and eventually returns into the confined phase. If the expansion is fast, the fireball could fragment either due to spinodal decomposition or due to suddenly arising bulk viscous force. Particle abundances are reasonably well described with just a few parameters within the statistical approach. Momentum spectra integrated over many events can be interpreted as produced from an expanding and locally thermalised fireball. The present Monte Carlo model unifies these approaches: fireball decays into fragments of some characteristic size. The fragments recede from each other as given by the pre-existing expansion of the fireball. They subsequently emit stable and unstable hadrons with momenta generated according to thermal distribution. Resonances then decay and their daughters acquire momenta as dictated by decay kinematics. Solution method: The Monte Carlo generator repeats a loop in which it generates individual events. First, sizes of fragments are generated. Then the fragments are placed within the decaying fireball and their velocities are determined from the one-to-one correspondence between the position and the expansion velocity in the blast wave model. Since hadrons may be emitted from fragments as well as from the remaining bulk fireball, first those from the bulk are generated according to the blast wave model. Then, hadron production from the fragments is treated. Each hadron is generated in the rest frame of the fragment and then boosted to the global frame. Finally, after all directly produced hadrons are generated, resonance decay channels are chosen and the momenta and positions of final state hadrons are determined. Running time: Generation of 100 events can take anything between 2 hours to a couple of days. This depends mainly on the size and density of fragments. Simulations with small fragments may be very slow. At the beginning of a run there is a period of up to 1 hour in which the program calculates thermal weights due to statistical model. This period is long if many species are included in the simulation.

Higgs boson production in association with multiple hard jets

We elucidate a new technique for estimating the production of multiple (at least two) hard jets in Higgs production via gluon-gluon fusion. The approach is based upon high energy factorisation, with the region of applicability extended by constraints on the analytic behaviour of the scattering amplitudes stemming from known all-order results. The method approximates both real and virtual corrections, and allows for the resummation in an n-parton inclusive event sample of the terms dominant in the high energy limit. The resulting approximation is matched to the known tree level matrix elements for the production of a Higgs boson in association with 2 and 3 jets, and implemented in a Monte Carlo generator. Example results are presented and characteristic radiation patterns discussed.

Production of Jets in Association with W and Z at ATLAS

The study of final states with hadronic jets accompanying a W or Z boson is of high importance at hadronic colliders both to understand Standard Model processes and measure background contribution to Beyond the Standard Model searches. In view of the imminent start up of the LHC, we present the ATLAS prospects for the cross-section measurement of W / Z + jets events in proton-proton collisions at 14 TeV centre of mass energy. The statistical and systematic limitations are discussed in terms of probing perturbative QCD predictions and Monte Carlo generators.

MUons from PArametric formulas: A fast GEnerator of atmospheric [formula omitted]-bundles for neutrino telescopes (MUPAGE)

Atmospheric muons play an important role for neutrino telescopes, because they provide the most abundant source of events for real time monitoring, calibration and tests. On the other side, they also represent the major background source. A fast Monte Carlo generator (called MUPAGE) of atmospheric muon bundles for underwater/ice neutrino telescopes is presented here. MUPAGE is based on parametric formulas [Y. Becherini, A. Margiotta, M. Sioli, M. Spurio, Astrop. Phys. 25 (2006) 1–13; M. Spurio, Nucl. Instr. and Meth. A 567 (2006) 492] obtained from a full Monte Carlo simulation of cosmic ray showers generating muons in bundles, which are propagated down to 5 km w.e. It produces the event kinematics on the surface of a user-defined virtual cylinder, surrounding the detector. The multiplicity of the muons in the bundle, the muon lateral distribution and energy spectrum are simulated according to a specific model of the primary cosmic ray flux, with constraints from measurements of the muon flux obtained in underground experiments. Some examples of application are presented.

A Monte Carlo study on the production scale and internal structure of jets in high energy collisions

The production scale and internal structure of jets produced in high energy collisions were studied using Jetset and Herwig Monte Carlo generators. Two scales were found. One is the jet-development scale, which determines the size of the jet developed from a mother-parton. The other is the jet-production scale with which the jets produced are the most consistent with QCD jet-production dynamics and will provide the most reliable dynamical information about their mother-partons.

Four fermion two pair production from γγ collisions : from PLC to LHC

A new analytical formula for the asymptotic QED two pair production cross-section is given. A fully ROOT integrated Monte-Carlo generator is presented and used for LHC cross-section computations. An explicit orthogonal color basis for γg → q q ¯ Q Q ¯ and gg → q q ¯ Q Q ¯ is provided.

Prospects for the discovery of 4th family quarks with the ATLAS detector

The pair production of heavy fourth-generation quarks, which are predicted under the hypothesis of flavor democracy, is studied using tree-level Monte Carlo generators and fast detector simulation. Two heavy-quark mass values, 500 and 750$\gev$, are considered with the assumption that the fourth family mixes primarily with the two light families. It is shown that a clear signature will be observed in the data collected by the ATLAS detector, after the first year of low-luminosity running at the Large Hadron Collider.

Mini–review on Monte Carlo programs for Bhabha scattering

We review the status of Monte Carlo generators presently used for simulations of the large-angle Bhabha process at electron-positron colliders of moderately high energy (flavour factories), operating at centre-of-mass energies between about 1 GeV and 10 GeV. It is shown how the theoretical accuracy reached by present Bhabha programs for physics at flavour factories is at the level of 0.1% and, therefore, comparable with that reached about a decade ago for luminosity monitoring through small-angle Bhabha scattering at LEP.

ZandW±production associated with quark-antiquark pair inkT-factorization at the LHC

We calculate and analyze Z and W production in association with quark-antiquark pair in k_T-factorization. Numerical calculations are performed using the Monte Carlo generator CASCADE for proton proton collisions at LHC energy. We compare total and differential cross sections calculated in k_T-factorization approach with total differential cross sections obtained in LO and NLO calculations in collinear factorization approach. We provide strong evidence that some of the effects of the NLO and even higher order collinear calculation are already included in the LO k_T-factorization calculation.

Initial State Radiation: A success story

The investigation of events with Initial State Radiation(ISR) and subsequent Radiative Return has become an impressively successful and guiding tool in low and intermediate energy hadron physics with electron positron colliders: it allows to measure hadronic cross sections and the ratio R from threshold up to the maximum energy of the colliders running at fixed energy, to clarify reaction mechanisms and reveal substructures (intermediate states and their decay mechanisms) and to search for new highly excited mesonic states with J P C = 1 − − . While being discussed since the sixties-seventies ISR became a powerful tool for experimentalists only with the development of EVA-PHOKHARA , a Monte Carlo generator developed over almost 10 years, while increasing its complexity, which is user friendly, flexible and easy to implement into the software of existing detectors.

STUDY ON THE GEOMETRICAL CHARACTERISTICS OF JETS WITH THE HBT CORRELATION METHOD IN e+e- COLLISIONS AT $\sqrt{s} = 91.2\, {\rm GeV}$

The geometrical characters of quark and gluon jets are carefully studied with the HBT correlation method using Monte Carlo generator produced e + e - collision events at [Formula: see text]. It turns out that the size of the jets can be measured by the HBT correlation method with the indistinguishability of identical final state pions. The size of quark jets and gluon jets are independent of the flavor of initial quarks that produced these jets but they are dependent on their energy. The average radius of quark and gluon jets in three-jet events of e + e - collisions at [Formula: see text] are obtained at the end of parton evolvement.