Thomas D. Gottschalk

A realistic model for e+e− annihilation including parton bremsstrahlung effects☆

Abstract The recently proposed model of Field and Wolfram for e+e− annihilation, which describes hadronization without using fragmentation functions, is examined and enlarged. Extensions of the original scheme include (i) heavy quark mass effects in the QCD parton shower and the explicit formation of final state heavy hadrons, (ii) a modification of the cluster formation algorithm to combine small mas clusters prior to hadronization and (iii) an enhanced rate of soft gluon production at early times in the parton shower. The last two modifications are fundamental and provide a more sensible separation of long and short distance physics. Initial results of the model agree well with PETRA/PEP data. Some predictions are made for LEP energies.

Implementing distributed synthetic forces simulations in metacomputing environments

A distributed, parallel implementation of the widely used Modular Semi-Automated Forces (ModSAF) Distributed Interactive Simulation (DIS) is presented, with scalable parallel processors (SPPs) used to simulate more than 50,000 individual vehicles. The single-SPP code is portable and has been used on a variety of different SPP architectures for simulations with up to 15,000 vehicles. A general metacomputing framework for DIS on multiple SPPs is discussed and results are presented for an initial system using explicit Gateway processes to manage communications among the SPPs. These 50K-vehicle simulations utilized 1,904 processors at six sites across seven time zones, including platforms from three manufacturers. Ongoing activities to both simplify and enhance the metacomputing system using Globus are described.

A New Model for Hadronization and

Abstract A new string/cluster model for e+e− annihilation is presented. Event evolution is described using a simple, 3-stage formalism: (i) coherent parton shower generation, (ii) evolution of elementary strings formed from the parton final state, and (iii) parameterized hadronization of small mass clusters formed during string evolution. Significant improvements over previous cluster models arise from the use of full, massless relativistic string motion in the second stage of the model, so that the formalism is naturally insensitive to soft perturbative radiation. Parameter fitting and sensitivity of model predictions to variations in the cluster formation parameters are discussed in detail. The model is shown to reproduce a variety of e+e− annihilation data over a wide range of ECM. Specific suggestions are made for future improvements in the perturbative QCD and cluster decay stages of the model.

e^+ e^-

A simple cluster model for limited-pt jets is developed. The formalism combines a string model description of color screening in non-perturbative QCD and an accurate parameterization of hadron production from low-mass clusters. The non-perturbative description of soft color evolution is found to provide a substantial improvement on present, purely perturbative QCD-cluster models. An extended model including hard perturbative QCD radiation is also presented. Results of this composite model are found to be in good agreement with experimental results for e+e− annihilation over the entire energy range 3 GeV ⩽ ECM ⩽ 35 GeV. Shortcomings in the present formalism are examined and specific suggestions for further improvements are made.

Annihilation

Abstract A Monte Carlo model for initial state parton showers is presented. Showers are generated by working backwards from the hard scattering to unevolved structure functions at a reference scale Q 0 2 . This backward evolution formalism allows the incorporation of exact kinematic constraints at all stages of shower generation. The physical regions for deep-inelastic showers and hard scattering showers (with an additional fixed time-like scale s ) are different, leading to kinematic differences in scale-dependent evolution. Shower estimates of these kinematic differences are shown to be compatible with the O( α s ) corrections for the Drell-Yan process. Transverse momentum distributions are discussed, with attention given to the merging of shower model results and exact, low-order QCD cross sections.

An Improved Description of Hadronization in the {QCD} Cluster Model for

Abstract Results of a new evaluation of the O(αs2) dressed three-jet cross section for e+e− annihilation are summarized. For vanishing cutoffs, the different results in published three-jet cross section formulae are due to differing definitions of resolvable four-jet events. Subleading terms which were ignored in previous calculations are evaluated and found to be sizeable in the phase space region where three-jet events become more two-jet-like. A simple calculation demonstrates that αs values determined according to the three-jet cross section formulae of Gutbrod, Kramer and Schierholz are systematically too large.

e^+ e^-

Abstract The differences among recent calculations of the O(αs2) QCD corrections to shape variable distributions for e+e− annihilation are shown to arise from different underlying definitions of jets and jet observables. In particular, the Caltech (large corrections) and DESY (small corrections) results are not obviously contradictory. We raise questions on the validity of all such calculations for the precise empirical determination of αs from data for e+e−→ hadrons.

Annihilation

On large Linux clusters, scalability is the ability of the program to utilize additional processors in a way that provides a near-linear increase in computational capacity for each node employed. Without scalability, the cluster may cease to be useful after adding a very small number of nodes. The Joint Forces Command (JFCOM) Experimentation Directorate (J9) has recently been engaged in Joint Urban Operations (JUO) experiments and counter mortar analyses. Both required scalable codes to simulate over 1 million SAF clutter entities, using hundreds of CPUs. The JSAF application suite, utilizing the redesigned RTI-s communications system, provides the ability to run distributed simulations with sites located across the United States, from Norfolk, Virginia, to Maui, Hawaii. Interest-aware routers are essential for scalable communications in the large, distributed environments, and the RTI-s framework, currently in use by JFCOM, provides such routers connected in a basic tree topology. This approach is successful for small to medium sized simulations, but faces a number of constraining limitations precluding very large simulations. To resolve these issues, the work described herein utilizes a new software router infrastructure to accommodate more sophisticated, general topologies, including both the existing tree framework and a new generalization of the fully connected mesh topologies. The latter were first used in the SF Express ModSAF simulations of 100,000 fully interacting vehicles. The new software router objects incorporate an augmented set of the scalable features of the SF Express design, while optionally using low-level RTI-s objects to perform actual site-to-site communications. The limitations of the original MeshRouter formalism have been eliminated, allowing fully dynamic operations. The mesh topology capabilities allow aggregate bandwidth and site-to-site latencies to match actual network performance. The heavy resource load at the root node now can be distributed across routers at the participating sites. Most significantly, realizable point-to-point bandwidths remain stable as the underlying problem size increases, sustaining scalability claims.

BACKWARDS EVOLVED INITIAL STATE PARTON SHOWERS

Abstract A distributed, parallel implementation of the widely-used Modular Semi-Automated Forces (ModSAF) Distributed Interactive Simulation (DIS) is presented, with Scalable Parallel Processors (SPPs) used to simulate more than 50,000 individual vehicles. The single-SPP version is described and shown to be scalable. This code is portable and has been run on a variety of different SPP architectures. Results for simulations with up to 15,000 vehicles are presented for a number of distinct SPP architectures. The initial multi-SPP (metacomputing) run used explicit Gateway communication processes to exchange data among several SPPs simulating separate portions of the full battle space. The 50K-vehicle simulations utilized 1904 processors on SPPs at six sites across seven time zones, including platforms from three computer manufacturers. (Four of the SPP sites in the large run used the single-SPP code described in this work, with a somewhat different single-SPP ModSAF implementation used at the other two sites.) Particular attention is given to analyses of inter-SPP data rates and Gateway performance in the multi-SPP runs. An alternative, next-generation implementation based on Globus is presented, including discussions of initial experiments, comparisons to the Gateway model, and planned near-term extensions. Finally, comparisons are made between this work and ongoing mainstream DIS activities.

A reassessment of the O(αs2) three-jet cross section for e+e− annihilation☆

A simple parameterization of hadron production from low-mass clusters is presented. Parameters of the model are determined from analyses of nucleon-antinucleon annihilation at rest and charm and baryon production in e+e− annihilation. The model is shown to provide a good description of multiplicities and inclusive energy distributions in low-energy e+e− annihilation. The cluster hadronization model developed here is intended to be used as the final stage in more general models for e+e− annihilation and other hard processes - models in which color separation is locally screened before hadrons actually form.