The FOPI Experiment

FOPI - A Detector for the Physics of Nuclear Reactions

Nuclear matter, of which atomic nuclei are composed, is a system of strongly interacting particles called nucleons. In its ground state, it behaves in a manner analogous to a fluid, and strongly resists attempts at compression. To double the density of nuclear matter, for example, requires 2x1033 Pascals, a pressure equivalent to twenty times the mass of the earth resting on an area one square millimeter in size. However, a compressed state is required before some of the fundamental properties of nuclear matter reveal themselves. New studies have shown that it is possible, and perhaps even likely, that the properties of the constituents vary according to the density of their surrounding environment. Referring to this phenomenon, scientists talk of a so-called "in-medium effect." The properties of these constituents-elementary particles-are well-known when the particles are surrounded by vacuum. However, their masses may vary due to a particular symmetry of the fundamental interaction-the chiral symmetry of Quantum Chromodynamics. In general, such a supposition is true for all hadrons, i.e. particles subject to the strong interaction. Detecting and understanding the processes at work here should help physicists tackle one of the central problems of modern physics-the inability of theoretical physics to provide an explanation of the principle determining the masses of the elementary particles.

Tracking down this kind of behavior, however, requires the investment of considerable effort. Another problem is presented by the fact that not all particles behave in the same way. One group of theoretically-promising candidates is provided by the kaons, which are distinguished by the fact that they contain a strange quark or antiquark. Kaons, however, are relatively difficult to generate in compressed nuclear matter, which in the laboratory can only be created in relativistic heavy-ion collisions, and then only for a relatively short time. For example, twice normal density is achieved in the collision of heavy ions (gold against gold) at an injection energy of around 1 GeV per nucleon, i.e. at an energy where the kinetic energy is almost equal to the rest mass of the nucleon. In this case, the velocity is 267000 km/s or 89 percent of the velocity of light. Within 10-22 seconds of the impact, the action is all over. All that remains are the fragments, and possibly particles newly-created during the reaction. An analysis of these remnants provides the basis for a reconstruction of the reaction. In addition, these observations reveal that the density in the heavy-ion reaction is also dependent on other quantities. During the attempt to compress nuclei together, part of the energy applied is inevitably converted into disordered motion, i.e. the system is heated. This thermal motion also generates pressure, so that in general, there is an overall relationship between pressure, density, and temperature. Analogous to classical thermodynamics, this relationship is referred to as an equation of state. Exact knowledge of this equation is a precondition for the characterization of the conditions present during the heavy-ion collision. The search for "in-medium properties" will only be successful if sufficient exact knowledge of the surrounding conditions is available. It is thus immediately obvious that the more completely the remnants of the reaction can be measured, the better this search can proceed.