Although our present understanding of the fundamental forces and particles in the universe (the Standard Model) is remarkably successful in many respects, there are still significant questions that remain unanswered. One important question has to do with certain differences in how matter and anti-matter behave (so-called CP-violating effects). Such effects have been observed in the decays of exotic particles called mesons, but the differences that have so far been observed are much too small to explain, for example, why there is much more matter in the universe than anti-matter. For this and other reasons, it is believed that the Standard Model is incomplete. Thus, experimental searches for other types of CP-violating effects can provide information that might help to extend the Standard Model. The goal of this award is to design and build a new instrument to search for CP-violating effects in the annihilation of electrons and their anti-particles, positrons. This project will provide research training opportunities for undergraduate students at Wittenberg University, where the proportion of women physics majors is well above the national average. These students will work closely with faculty to develop, assemble, and test state-of-the-art instrumentation and will learn how to do research at the frontiers of science. The proposed apparatus will also enable a research project for at least one graduate student at Michigan State University.
This award is to develop a highly efficient, compact, symmetric, and high rate capable instrument which will enable the search for CP violation in the decay of tensor polarized Positronium (Ps). CP violation is known to occur in the weak decays of the K and B mesons and is described in the Standard Model (SM) through the CP-violating phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Since the discovery of neutrino oscillations, a similar mechanism is also expected to exist in the neutrino sector. Searches for new mechanisms of CP violation in observables where SM CP-violating effects are strongly suppressed are considered among the most sensitive probes to search for new physics. They are also expected to provide possible microscopic explanations for the origin of the dominance of matter over anti-matter in the universe. The instrument to be developed will make possible an order of magnitude improvement in the measurement of a CP-violating correlation involving the spin of positronium and the momenta of the two most energetic gammas in the 3-gamma decay of positronium. The instrument consists of a positron source sandwiched between two thin plastic scintillators surrounded by a high porosity medium where positrons efficiently form positronium. This positronium production and tagging system will be surrounded by 48 LYSO scintillation crystals covering a large solid angle and segmented so as to select the most sensitive decay events. The entire detection system will be located inside the warm bore of an existing superconducting magnet available at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The extraction of the correlation term requires the determination of the tensor polarization of positronium, which will be deduced from the decay time spectrum of positronium. The start signal will be given by one of the thin scintillators and the stop signal by a pair of LYSO crystals in coincidence. The merit of the project resides in both the statistical and systematic sensitivity of the system. The group expects to get the current limit of 0.002 in a few hours and to improve it by one order of magnitude within a reasonable measuring time. In addition, the symmetry of the proposed instrument, and the fact that all configurations are measured simultaneously, eliminate the dominant systematic effects in the most precise previous measurement.
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.
