Hadronic Generators Research Articles

Fast hadron freeze-out generator. II. Noncentral collisions

The fast Monte Carlo procedure of hadron generation developed in our previous work is extended to describe noncentral collisions of nuclei. We consider different possibilities to introduce appropriate asymmetry of the freeze-out hypersurface and flow velocity profile. For comparison with other models and experimental data, we demonstrate the results based on the standard parametrizations of the hadron freeze-out hypersurface and flow velocity profile assuming either a common chemical and thermal freeze-out or the chemically frozen evolution from chemical to thermal freeze-out. The ++ generator code is written under the framework and is available for public use at http://uhkm.jinr.ru/.

Fast hadron freeze-out generator

We have developed a fast Monte Carlo procedure of hadron generation that allows one to study and analyze various observables for stable hadrons and hadron resonances produced in ultrarelativistic heavy ion collisions. Particle multiplicities are determined based on the concept of chemical freeze-out. Particles can be generated on the chemical or thermal freeze-out hypersurface represented by a parametrization or a numerical solution of relativistic hydrodynamics with given initial conditions and equation of state. Besides standard spacelike sectors associated with the volume decay, the hypersurface may also include nonspacelike sectors related to the emission from the surface of expanding system. For comparison with other models and experimental data, we demonstrate the results based on the standard parametrizations of the hadron freeze-out hypersurface and flow velocity profile under the assumption of a common chemical and thermal freeze-out. The C++ generator code is written under the ROOT framework and is available for public use at http://uhkm.jinr.ru/.

Models for cosmic ray interactions

Contemporary models of hadronic interactions are reviewed. Basic phenomenological approaches are compared, with an emphasizes on the predicted air shower characteristics. Special attention is payed to the remaining discrepancies between present hadronic MC generators and cosmic ray data. Finally, future prospects concerning model improvements are discussed, in particular, regarding the possibilities to discriminate between different models on the basis of accelerator or cosmic ray measurements.

Energy spectrum of cosmic rays and multiparticle generation of hadrons at ultrahigh energies

None of the presently known local sources of photons whose energy exceeds 1012 eV is incompatible with the universal energy spectrum F(>Eγ) ∼ Eγ−1.36±0.15. The power of extragalactic sources is 106 to 1012 times higher than the power of galactic sources since the respective distances are longer in the former case, while the observed flux intensities are approximately identical in the two cases. A much higher power of extragalactic sources is indicative of an extragalactic cosmic-ray origin and of the existence of a universal (for all energies of protons and cosmic-ray nuclei) process that is responsible for the energy loss in the Metagalaxy and which forms the observed energy spectrum of protons and nuclei (∼E0−2.72). It is shown that there is no break in the energy spectrum of primary protons in the energy range 1015–1016 eV and that the break in the spectrum of extensive air showers with respect to the number of electrons is due to a change in the process of multiparticle hadron generation in the first event of extensive-air-shower production, this being confirmed by a change in the extensive-air-shower absorption length from λa 150 g/cm2 after the break.

Hadron multiplicity in lepton–nucleon interactions

Multi-hadron production in inelastic neutrino–nucleon interactions is investigated within the framework of the quark–gluon string model. The contributions of the planar (one-reggeon exchange) and cylindrical (one-pomeron exchange) graphs to different observables is computed using a Monte Carlo program for the generation of hadrons produced from the decay of colorless quark–antiquark strings. The suggested approach results in a satisfactory description of the experimental data on ν(ν̄)N→μ−(μ+)hX reactions obtained recently at CERN by the NOMAD Collaboration. The data extends over a wide range of initial neutrino energies Eν<200 GeV/c and momentum transfers 1<Q<7 GeV/c, well into the region where perturbative QCD calculations are not applicable.

Statistical hadronization model and transverse momentum spectra of hadrons in high energy collisions

A detailed analysis of transverse momentum spectra of several identified hadrons in high energy collisions within the canonical framework of the statistical model of hadronisation is performed. The study of particle momentum spectra requires an extension of the statistical model formalism used to handle particle multiplicities, which is described in detail starting from a microcanonical treatment of single hadronising clusters. Also, a new treatment of extra strangeness suppression is presented which is based on the enforcement of fixed numbers of ssbar pairs in the primary hadrons. The considered centre-of-mass energies range from ~ 10 to 30 GeV in hadronic collisions (pion-p, pp and Kp) and from ~ 15 to 35 GeV in e+e- collisions. The effect of the decay chain following hadron generation is accurately and exhaustively taken into account by a newly proposed numerical method. The exact p_T conservation at low energy and the increasing hard parton emission at high energy bound the validity of the presently taken approach within a limited centre-of-mass energy range. However, within this region, a clear consistency is found between the temperature parameter extracted from the present analysis and that obtained from fits to average hadron multiplicities in the same collision systems. This finding indicates that in the hadronisation process the production of different particle species and their momentum spectra are two closely related phenomena governed by one parameter.

Hadronic shower models in Geant4 — the frameworks

Optimal exploitation of hadronic final states played a key role in successes of all recent collider experiment in HEP, and the ability to use hadronic final states will continue to be one of the decisive issues during the analysis phase of the LHC experiments. Monte Carlo programs like Geant4 (1994) facilitate the use of hadronic final states, and have been developed for many years. We give an overview of the Object Oriented frameworks for hadronic generators in Geant4, and illustrate the physics models underlying hadronic shower simulation on examples, including the three basic types of modeling; data driven, parametrization driven, and theory driven modeling, and their possible realizations in the Object Oriented component system of Geant4. We put particular focus on the level of extendibility that can and has been achieved by our Russian dolls approach to Object Oriented design, and the role and importance of the frameworks in a component system.

Transverse momentum spectra of identified particles in high energy collisions with statistical hadronisation model

A detailed analysis is performed of transverse momentum spectra of several identified hadrons in high energy collisions within the framework of the statistical model of hadronisation. The effect of the decay chain following hadron generation is accurately taken into account. The considered centre-of-mass energies range from ∼10 to 30 GeV in hadronic collision (ρp, pp and Kp) and from ∼ 15 to 45 GeV in e +e − collisions. A clear consistency is found between the temperature parameter extracted from the present analysis and that obtained from fits to average hadron multiplicities in the same collision systems. This finding indicates that in the hadronisation, the production of different particle species and their momentum spectra are two closely related phenomenons governed by one parameter.

Hadronic models and experimental data for the neutrino beam production

The predictions of meson production by 450 GeV/c protons on Be using the Monte Carlo FLUKA standalone and GEANT-FLUKA and GEANT-GHEISHA in GEANT are compared with available experimental measurements. The comparison enlightens the improvements of the hadronic generator models of the present standalone code FLUKA with respect to the 1992 version which is embedded into GEANT-FLUKA. Worse results were obtained with the GHEISHA package. A complete simulation of the SPS neutrino beam line at CERN showed significant variations in the intensity and composition of the neutrino beam when FLUKA standalone instead of the GEANT-FLUKA package is used to simulate particle production in the Be target.

On the contribution of off-nuclei electroproduction to the generation of energetic hadrons in electromagnetic showers

The calculation of high-energy hadron cascades in extended materials induced by electron beams in the GeV energy range is reconsidered. In particular, we estimate the contribution from hadronic interactions of high-energy electrons on nuclei as compared to hadron production by photons. Our estimate relies on the equivalent photon approximation. The interaction of the resulting quasireal photons with nuclei is described by the vector-meson dominance model and a Monte Carlo realization of the multistring fragmentation model, as discussed in ref. [1]. For thin targets (as compared to the radiation length), hadronic interactions of the incident high-energy electrons on nuclei dominate the generation of high-energy hadrons.

A quantum field theory in momentum space of constant curvature

The model of a polarization operator in the “rainbow” diagram approximation in momentum space of constant curvature is studied. An integral equation is obtained for the absorptive component of the hadron polarization operator in a stereographic parametrization of the de Sitter space. The solution of the integral equation is found for the case of zero mass of the exchange particle. The sections and mean multiplicity of hadron generation in e+e− annihilation are calculated. The infrared divergences which develop in an exact summation of the class of diagrams considered here manifest themselves as a multiplicative renormalization constant.

Masses of baryons and mesons

Although there arc several theories of heavy-mass generation, one of the central problems of elementary-particle physics is the calculation of the particle masses. The mechanisms of the gauge theories (1) and others (2) do not yield the mass spectra for baryons and mesons. On the other hand the e+e colliding-beam experiments suggest generation of hadrons by a mechanism based on photon collisions, which implies a simpler Lagrangian We consider the Lagrangian for a spinor and vector field