Collaborative Research: Novel Plasma Physics of Trapped Antimatter

The prevalence of matter over antimatter is one of the most important unexplained observations in physics. As currently understood, the laws of physics predict that there should be an equal amount of matter and antimatter, which is at odds with our everyday experience and detailed astronomical observations. Such an inconsistency suggests that our current understanding of the laws of physics may be incomplete. ALPHA is an interdisciplinary antimatter experiment at the European Organization for Nuclear Research, known as CERN, that tests this notion by producing antihydrogen and sensitively measuring its properties in comparison with the hydrogen atom. These experiments are improved by the efficient conversion of collected antiprotons and positrons into antihydrogen. Trapping antimatter to produce antihydrogen is a plasma physics problem, consisting of collecting and manipulating large collections of charged particles using electric and magnetic fields. This award supports a joint effort between the University of Michigan and Marquette University, in collaboration with Brookhaven National Laboratory, that will advance the understanding of the novel plasma physics processes expected in antimatter traps and will conduct experiments using ALPHA to test the predictions. This project will also contribute to developing the next generation of the science and technology workforce by supporting the training of undergraduate and graduate students, and will contribute to local education and science engagement activities, including development of a plasma physics exhibit for the Discovery World Museum in Milwaukee, WI. Trapped antimatter is novel from a plasma physics perspective, as well as a particle physics perspective. These plasmas are so cold, and the magnetic field in the trap is so strong, that they exist in a state that is not well described by the usual models of plasma physics. Specifically, the low temperature causes the plasma to be strongly coupled, which means that it behaves more like a supercritical fluid or a liquid, than the more common dilute-gas-like behavior. The strong applied magnetic field, in combination with the low density, causes the plasma to be strongly magnetized in the sense that the circular gyromotion that charged particles make in response to the magnetic field is much smaller than the scale over which particles interact. Currently understood methods of plasma theory do not apply in either of these circumstances. This award will enable continued development of theoretical approaches that extend plasma theory into these domains, and then testing them by conducting two specific experiments on ALPHA that will measure (1) the rate of temperature relaxation between electrons and antiprotons, which is predicted to be delayed by strong magnetization of the electrons, and (2) the sympathetic cooling rate of positrons with other particles, such as Beryllium ions and protons. An expected outcome is that a better understanding of the underlying plasma physics may be used to improve ALPHA experimental operations by increasing production rate of antihydrogen atoms. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.