In this project the most basic atomic fragmentation processes will be studied. By monitoring the motion of all fragments produced in the reaction complete information about the atomic system will be obtained. These studies directly address one of the most fundamentally important, and yet unsolved problems in Physics, the so-called few-body problem. Its essence is that any system with more than two mutually interacting particles cannot be described analytically by theory even when the initial conditions and the forces acting within the system are precisely known. Therefore, detailed experimental data on the most basic systems are crucially important to guide theoretical efforts in finding numeric solutions. This work addresses systems of 3 to 5 active particles and stringently tests theoretical models which numerically treat few-body interactions. Understanding such simple systems is a prerequisite for describing more complex systems on a fundamental level.
Advancing our understanding of the few-body problem, and then making the transition from few to many particles, has far-reaching ramifications in almost all areas of modern research. For example, modeling chemical or biological processes requires simulating interactions involving many particles. Understanding such reactions has major impacts on developing new chemical compounds and on future medical applications like e.g. new cancer therapy methods. Interactions with or within e.g. clusters, plasmas, solids, and Bose-Einstein condensates also involve a large number of particles. For such complex systems theoretical models developed for basic 3 to 5 particle systems can serve as a base to provide a more detailed description. As a last example, theoretical few-body models developed for atomic systems can be applied to elementary particle reactions and thereby help to advance our understanding of the underlying nuclear forces in such systems, which at present is still rather incomplete.
The major goal of the research performed with funding of this grant was to advance our understanding of the so-called few-body problem (FBP). The essence of the FBP is that the equations of motion describing systems containing more than two mutually interacting particles are not analytically solvable. An illustrative example is our solar system. If it consisted only of the sun and the earth then the classical laws of motion (based on Newton's laws) could be solved analytically. It would then be possible to predict the position and velocity of the earth with virtual certainty for any time in the future. However, if only one more body is added to the solar system, like another planet, the equations of motion can only be solved numerically, which does not always lead to exact results. In the case of the solar system powerful munerical methods are available and, in spite of the FBP, very accurate predictions can be achieved. Another prominent few-body system is a charged particle (e.g. a proton, a positron, or an electron) colliding with an atom or molecule. Here, understanding the evolution of the system under the influence of the mutual interactions between the particle in the system is much more challenging because of the extremely small size of the system. Particles of such microscopic size behave not only like particles, but simultaneously also like waves. As a result, the equations of motion are not based on Newton's laws, but on the Schrodinger equation, the most fundamental equation in quantum-mechanics. In the last decade enormous progress was made in developing powerful methods to solve the Schrodinger equation numerically. Nevertheless, until today the achievable accuracy cannot remotely compete with the numerical methods used for classical systems of macroscopic size (like the solar system). The quantum-mechanical FBP remains one of the most fundamentally important, unsolved problems in the natural sciences! In order to advance theoretical numerical approaches it is important to test these methods by detailed experimental data. To this end we have performed some of the most detailed experiments studying collisions between a broad variety of charged particles with atoms and molecules. Various proccesses were investigated like ionization (i.e. an electron is ripped of from the target in the collision), capture (i.e. an electron is transferred from the target to the projectile) or dissociation (i.e. a molecule is torn apart into two atoms). Using three different, unique aparatuses the complete kinematic information of every particle in the collision system was obtained. These experiments yielded several surprizing results. For proton and heavy-ion impact we demonstrated that the evolution of the collision sensitively depends on the coherence of the projectile beam, an important point which has been overlooked in theoretcial studies of the past. This outcome led to the resolution of one of the biggest puzzles in atomic collision physics of the last decade. We also found an unexpected breaking of symmetry in the fragmentation pattern of collisions between heavy ions and polarized target atoms. Symmetries play an important role in physics. Our experimental observation has triggered significant theoretical activities and the breaking of the symmetry can now be reproduced by theory. Our positron and electron impact studies show that there are important differences in the probabilities that an interaction will occur and how the target atom or molecule will respond if the sign of the incoming particle changes, i.e. if the direction of the electric field between the incoming particle and a target electron is reversed. This provides more information about how matter interacts with matter on the atomic scale and it shows how antimatter-matter interactions are similar, or different. The results of our studies have significant broader impacts well beyond our research area. The FBP, addressed by our projects, is of high relevance in many areas of the natural sciences because modern research is usually dealing with systems containing more than two mutually interacting particles (e.g. formation of molecular bonds in chemistry, damage to DNA, Bose-Einstein condensates, etc.). To use a more specific example, to understand nuclear systems is even more challenging than atomic systems because there the forces acting within the systems are not well understood. In nuclear physics it is therefore not always clear whether experiments test the theoretcial description of the force or odf the few-body aspects. Our studies helped to advance theoretical methods to describe the few-body aspects and this progress can be used in nuclear physics to study the nuclear force in more detail. Our research efforts also have broader impacts on future generations of scientists. The experiments involve a broad spectrum of experimental techniques. Furthermore, the interpretation of the results requires a solid understanding of the underlying physics concepts. Therefore, numerous graduate and undergraduate students received extensive training in modern scientific methods and are well prepared to meet the scientific needs of our society.
