38248008
Helac-Phegas: A generator for all parton level processes
Abstract The updated version of the Helac-Phegas 1 event generator is presented. The matrix elements are calculated through Dyson–Schwinger recursive equations using color connection representation. Phase-space generation is based on a multichannel approach, including optimization. Helac-Phegas generates parton level events with all necessary information, in the most recent Les Houches Accord format, for the study of any process within the Standard Model in hadron and lepton colliders. New version program summary Program title: HELAC-PHEGAS Catalogue identifier: ADMS_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADMS_v2_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.: 35 986 No. of bytes in distributed program, including test data, etc.: 380 214 Distribution format: tar.gz Programming language: Fortran Computer: All Operating system: Linux Classification: 11.1, 11.2 External routines: Optionally Les Houches Accord (LHA) PDF Interface library ( http://projects.hepforge.org/lhapdf/ ) Catalogue identifier of previous version: ADMS_v1_0 Journal reference of previous version: Comput. Phys. Comm. 132 (2000) 306 Does the new version supersede the previous version?: Yes, partly Nature of problem: One of the most striking features of final states in current and future colliders is the large number of events with several jets. Being able to predict their features is essential. To achieve this, the calculations need to describe as accurately as possible the full matrix elements for the underlying hard processes. Even at leading order, perturbation theory based on Feynman graphs runs into computational problems, since the number of graphs contributing to the amplitude grows as n !. Solution method: Recursive algorithms based on Dyson–Schwinger equations have been developed recently in order to overcome the computational obstacles. The calculation of the amplitude, using Dyson–Schwinger recursive equations, results in a computational cost growing asymptotically as 3 n , where n is the number of particles involved in the process. Off-shell subamplitudes are introduced, for which a recursion relation has been obtained allowing to express an n -particle amplitude in terms of subamplitudes, with 1-, 2-, …  up to ( n − 1 ) particles. The color connection representation is used in order to treat amplitudes involving colored particles. In the present version HELAC-PHEGAS can be used to efficiently obtain helicity amplitudes, total cross sections, parton-level event samples in LHA format, for arbitrary multiparticle processes in the Standard Model in leptonic, p p ¯ and pp collisions. Reasons for new version: Substantial improvements, major functionality upgrade. Summary of revisions: Color connection representation, efficient integration over PDF via the PARNI algorithm, interface to LHAPDF, parton level events generated in the most recent LHA format, k ⊥ reweighting for Parton Shower matching, numerical predictions for amplitudes for arbitrary processes for phase-space points provided by the user, new user interface and the possibility to run over computer clusters. Running time: Depending on the process studied. Usually from seconds to hours. References: [1] A. Kanaki, C.G. Papadopoulos, Comput. Phys. Comm. 132 (2000) 306. [2] C.G. Papadopoulos, Comput. Phys. Comm. 137 (2001) 247.
14317912
Polarizing the dipoles
We extend the massless dipole formalism of Catani and Seymour, as well as its massive version as developed by Catani, Dittmaier, Seymour and Trocsanyi, to arbitrary helicity eigenstates of the external partons. We modify the real radiation subtraction terms only, the primary aim being an improved efficiency of the numerical Monte Carlo integration of this contribution as part of a complete next-to-leading order calculation. In consequence, our extension is only applicable to unpolarized scattering. Upon summation over the helicities of the emitter pairs, our formulae trivially reduce to their original form. We implement our extension within the framework of \textsc{Helac-Phegas}, and give some examples of results pertinent to recent studies of backgrounds for the LHC. The code is publicly available. Since the integrated dipole contributions do not require any modifications, we do not discuss them, but they are implemented in the software.
17496843
Multi-parton cross sections at hadron colliders
We present an alternative method to calculate cross sections for multi-parton scattering processes in the standard model at leading order. The helicity amplitudes are computed using recursion relations in the number of particles, based on the Dyson–Schwinger equations, whereas the summation over colour and helicity configurations is performed by Monte Carlo methods. The computational cost of our algorithm grows asymptotically as 3n, where n is the number of particles involved in the process, as opposed to the n!-growth of the Feynman diagram approach. Typical results for the total cross section, the differential distributions of the invariant masses and the transverse momenta of the partons are presented and cross checked by explicit summation over colours.
122772869
Quantum dynamical aspects of two-dimensional spaces of constant negative curvature
The authors study the quantum mechanics of two-dimensional spaces of constant negative curvature. The connection between different formulations is analysed. They derive the propagator and find that in the semiclassical approximation, the time evolution of observables exhibits similar features as in the classical case.
16415075
Studying trilinear gauge couplings at LEP2 using optimal observables
Abstract We study the sensitivity of the processes e + e − → l ν l q q ′ at LEP2 energies on the non-standard trilinear gauge couplings (TGC), using the optimal observables method. All relevant leading logarithmic corrections to the three-order cross section, as well as experimental resolution effects have been studied. Taking into account correlations among the different TGC parameters we show that the limits on the TGC can reach the level of 0.15 (ld) at 161 GeV with 100 pb −1 , a challenge for the first LEP2 phase. At higher energies this can be improved drastically, reaching for the level of 0.02 (1 sd).
15878456
The Fermion loop scheme for finite width effects in $e^{+} e^{-}$ annihilation into four fermions
We describe the gauge-invariant treatment of the finite-width effects of W and Z bosons in the fermion-loop scheme and its application to the six-fermion (LEP2) processes et-e+ → four fermions, with massless external fermions. The fermion-loop scheme consists in including all fertnionic one-loop corrections in tree-level amplitudes and resumming the self-energies. We give explicit results for the unrenormalized fermionic one-loop contributions to the gauge-boson self-energies and the triple gauge-boson vertices, and perform the renormalization in a gauge invariant way by introducing complex pole positions and running couplings. A simple effective Born prescription is presented, which allows for a relatively straightforward implementation of the fermion-loop scheme in LEP1 and LEP2 processes. We apply this prescription to typical LEP2 processes, i.e. et-e+ → μt-νμud, et-e+ → scud, and et-e+ → et-νerd, and give numerical comparisons with other gauge-invariance-preserving schemes in the energy range of LEP2, NLC and beyond.
118327429
Au-197(n,gamma) cross section in the resonance region
The (n,gamma) cross section of Au-197 has been measured at n_TOF in the resolved resonance region, up to 5 keV, with the aim of improving the accuracy in an energy range where it is not yet considered standard. The measurements were performed with two different experimental setup and detection techniques, the total energy method based on C6D6 detectors, and the total absorption calorimetry based on a 4 pi BaF2 array. By comparing the data collected with the two techniques, two accurate sets of neutron-capture yields have been obtained, which could be the basis for a new evaluation leading to an extended cross-section standard. Overall good agreement is found between the n_TOF results and evaluated cross sections, with some significant exceptions for small resonances. A few resonances not included in the existing databases have also been observed.
23902126
The University of Maryland Electron Ring (UMER) enters a new regime of high-tune-shift rings
Beams with a high phase space density are useful for many modern applications such as free electron lasers, pulsed neutron sources, high-energy-density physics, and high-luminosity colliders. Production of such beams requires understanding the complex space charge dynamics at the low-energy end of the accelerator. The University of Maryland Electron Ring (UMER) has been designed and built with the purpose of investigating space charge effects using scaled low-energy electron experiments. We have recently circulated the highest- space-charge beam in a ring to date, achieving a breakthrough both in the number of turns and in the amount of current propagated. We have propagated a beam with an integer tune shift for over 100 turns, and other, even higher-current beams, for 5-50 turns albeit with some beam loss. One beam had a tune shift at injection of 5.0, which is several factors higher than anything propagated in the past. We report here as well on other interesting aspects of the UMER work.
59270303
High Accuracy 233U(n, f) Cross-Section Measurement at the White-Neutron Source n_TOF from Near-Thermal to 1 MeV Neutron Energy
The U-233(n, f) cross section has been measured at the white neutron source n_TOF in a wide energy range with a dedicated fission ionization chamber. We report here the results from similar to 30 meV to 1 MeV neutron energy. The U-233(n, f) cross section has been determined relative to a reference sample of U-235(n, f) measured simultaneously with the same detector. The very high instantaneous neutron flux and the intrinsically low background of the n_TOF installation result in an accuracy around 3 range, while the energy resolution of the neutron beam allows for an accurate description of the fission cross section by means of R-matrix analysis over a wide energy range. The results are, in general, in good agreement with the most recent high-accuracy measurement of this fission cross section, over the more limited range of the previous measurements, and indicated that even the latest evaluations underestimate the cross section in the epithermal region. The present high-quality data provide the basis for a more precise evaluation of the U-233 fission cross section and for improving the reliability of databases needed for the design of new energy systems based on the Th/U cycle.
118856700
Numerical investigations of mismatch induced halos in intense charged particle beams
In this paper, we discuss the parametric resonance model of halo creation, and compare it with self consistent simulation results. In particular, we employ two different initial distribution functions, and we find agreement with the particle‐core model, within the limitations of the latter. Furthermore, using a simple particle tracking algorithm, we are able to follow the trajectories of the halo particles, noting that a large number of them go through the core and re‐emerge later.
123786004
Neutron physics of the Re/Os clock. III : Resonance analyses and stellar (n, gamma) cross sections of Os-186,Os-187,Os-188
Neutron resonance analyses have been performed for the capture cross sections of Os-186, Os-187, and Os-188 measured at the n_TOF facility at CERN. Resonance parameters have been extracted up to 5, 3, and 8 keV, respectively, using the SAMMY code for a full R-matrix fit of the capture yields. From these results average resonance parameters were derived by a statistical analysis to provide a comprehensive experimental basis for modeling of the stellar neutron capture rates of these isotopes in terms of the Hauser-Feshbach statistical model. Consistent calculations for the capture and inelastic reaction channels are crucial for the evaluation of stellar enhancement factors to correct the Maxwellian averaged cross sections obtained from experimental data for the effect of thermally populated excited states. These factors have been calculated for the full temperature range of current scenarios of s-process nucleosynthesis using the combined information of the experimental data in the region of resolved resonances and in the continuum. The consequences of this analysis for the s-process component of the Os-187 abundance and the related impact on the evaluation of the time duration of galactic nucleosynthesis via the Re/Os cosmochronometer are discussed.