GENERAL PHYSICS COLLOQUIUM _____ Title: Slow antiprotons and the secrets of atomic collisions Speaker: Helge Knudsen, Department of Physics and Astronomy, Aarhus University Time: Wednesday, 11 May, 2011 at 3:15 p.m. Place: Physics Auditorium, 3rd floor, Department of Physics Abstract One of the first successes of quantum theory was the accurate description of the structure of the isolated hydrogen atom. Due to difficulties with the inclusion of the interaction between its two electrons, it took years to reach a similar accuracy for the helium atom. heliumioniantiprotToday, 90 years later, similar progress has not yet been achieved in the case of atomic collisions, where a description of dynamic systems involving several electrons is needed. Even for the "theorists dream system", namely the collision of a slow antiproton on a helium atom leading to the liberation of one electron, for which a large number of advanced theoretical calculations exist, these differ by up to 30%. Since 1986, our group has been investigating such processes at CERN, starting with very fast projectiles, but recently we developed a new scheme for the creation of a DC beam of slow (a few keV) antiprotons at CERN, and we have now obtained benchmark data stretching over many decades of projectile energy for the development of more accurate theoretical calculations. The use of antiprotons instead of other ions is important, since antiprotons, due to their negative charge, do not support the process where electrons are transferred to the slow projectile. This simplifies the already difficult task of the theorists. _____ Coffee/tea and cake will be served at 3 p.m. David Field