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    Elementary Particle Physics

Text "Elementary Particle Physics"	Klaus Schilling, NIC Research Group for High Energy Physics
Structure of Matter
The Femto-World under the "Computer Microscope"	Klaus Schilling, NIC Research Group for High Energy Physics
Monopoles	Thomas Lippert, Theoretical Physics, University of Wuppertal
Numerical Simulation of Supersymmetric Quantum Field Theories	Robert Kirchner, Istvan Montvay, Jörg Westphalen, DESY Hamburg; Silke Luckmann, Klaus Spanderen, Institute of Theoretical Physics I, University of Münster

Text "Elementary Particle Physics"

When we step down to the smallest length scale accessible to us, molecules and atoms become giant structures and we even regard protons and neutrons "from inside". These nuclear constituents belong to the class of the so-called hadrons, which are subject to very strong interaction with a range of 1 femtometre = 10^-15 m. We currently imagine that this femto-world is described by quantum chromodynamics (QCD) within the framework of the standard model of elementary particles. QCD is the fundamental theory of the strong interaction of matter. The fundamental constituents of hadronic matter are the so-called quarks. They carry a novel form of charge (colour charge). In contrast to atoms or atomic nuclei, they are not directly detectable as free objects since they are always bound in "colour-neutral" hadronic states.

Computers of the highest power class are required to evaluate the basic equations of QCD with a view to predictions. The second picture shows the field distribution around a static, bound quark-antiquark pair, as calculated in a computer simulation at NIC. The resolving power of the "computer experiment" can be seen here from the plotted mesh and, similar to a real experiment, is significantly determined by the computing power and memory size of the apparatus (here of the computer). The spatial resolution attained 1/20 femtometres in the NIC system. For comparison: a proton has a diameter of about 1 femtometre.

The evaluation is stochastic so that the measuring points are affected by errors as in a real laboratory experiment. The positions of the two particles can be recognized from the peaks of field distribution. An elongated flux tube is formed between these peaks, which is characteristic of the above-described confinement phenomenon. It has been possible for the first time at NIC to observe the formation of this flux tube with sufficient precision over a distance of 2 fermis.

In searching for insights into the physics of the femto-world, theoretical elementary particle physics has become a motor for the development of supercomputers worldwide. One example is the massively parallel APEmille computer developed by physicists from the Italian INFN, which is now being completed in cooperation with NIC. Consuming only 20 - 30 kW of electricity it will provide a real power of about 500,000 MFLOPS, which is about the 5000-fold of a modern workstation.

Essential insights into the nature of a quantum field theory can be obtained from the study of its ground state, i.e. vacuum. The deeper we look into the world of the microcosm, the more complicated appears the structure of vacuum. Macroscopic theories like Newtonian mechanics still understand vacuum - according to the origin of the name - as an absolutely empty space. However, vacuum is already characterized by quantum fluctuations on the atomic scale.

The third and fourth pictures provide information about possible vacuum structures in the range of the smallest length scales in two quantum field theory models related to QCD. In these cases, too, the supercomputer acts as a "microscope" with which we can make the complex structure of vacuum visible.

Within the framework of the SESAM project, QCD vacuum configurations are calculated at NIC with the aid of APE computers. The corresponding computations are extremely sophisticated: they would require about 2,000,000 hours of computing time on a modern workstation. Other supercomputers like CRAY T3E are used for evaluation.

(Klaus Schilling, NIC Research Group for High Energy Physics)

Structure of Matter

Almost the entire matter of the atoms is concentrated in the atomic nucleus, which consists of protons and neutrons and is held together by strong interaction. Protons and neutrons (hadrons) are again composed of three quarks held together by the exchange of gluons (from 'glue'). As far as we know today, quarks and gluons have no further substructure.

The Femto-World under the "Computer Microscope"

It can be seen that the static interaction between quark and antiquark is effected by the formation of a narrow flux tube. It prevents quarks being isolated from antiquarks.

(Klaus Schilling, NIC Research Group for High Energy Physics)

Monopoles

The current lines of magnetic monopoles form closed loops in the four-dimensional Euclidean lattice world of quantum electrodynamics. Although magnetic monopoles have not yet been discovered in classical electrodynamics, they can appear as field fluctuations in the quantized theory. In the vicinity of the so-called confinement-deconfinement phase transition they show extreme density fluctuations on the lattice. At this point there is a great chance of discovering a new exotic phase of electrodynamics with unusual physical properties. In simulation calculations on the CRAY T3E parallel computer we intend to clarify the role of magnetic monopoles in this context. Visualization helps us to develop efficient simulation algorithms which directly attack the monopole currents. The fourth dimension is colour-encoded: if the monopole migrates in the direction of time, the associated arrow changes its colour. In this way, the closed current loops can be identified in the toroidally closed space.

(Thomas Lippert, Theoretical Physics, University of Wuppertal)

Numerical Simulation of Supersymmetric Quantum Field Theories

The standard model of elementary interactions is a quantum field theory which basically explains all known physical phenomena except gravitation. A prerequisite for incorporating gravitation into this successful theoretical framework is based on the assumption that the physical processes exhibit a very high degree of symmetry, so-called "supersymmetry", above the achievable energy range.

Supersymmetry does not exist on the energy scales accessible with present-day accelerators. This apparent contradiction is resolved by the existence of different vacuum states in supersymmetric quantum field theory according to which the physical states observed are only built up on one of these vacuum states. Supersymmetry is violated by this restriction, which explains its absence in previous observations.

Within the framework of the "simplest" model of a supersymmetric gauge theory it has been possible to prove the existence of various ground states with the same energy. In this model, exactly two vacuum states occur in spatially separated regions. They are shown in the picture as a snapshot during a dynamic simulation. In this way, it has been possible for the first time in a computer simulation to identify an essential element of supersymmetric theories which, in the past, has only been based on hypothetical assumptions.

(Robert Kirchner, Istvan Montvay, Jörg Westphalen, DESY Hamburg; Silke Luckmann, Klaus Spanderen, Institute of Theoretical Physics I, University of Münster)

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