Towards Precision Measurements of Atomic Parity Violation Using Two-Pathway Coherent Control

Purdue University, West Lafayette IN

Investigators

Abstract

The four fundamental forces of the physical universe are gravitational, electromagnetic, weak (responsible for radioactive decay of particles), and strong (the binding force that holds the nuclei of atoms together). A great deal about these forces has been learned over the years, and a theoretical model that unifies and summarizes our understanding of three of these forces (electromagnetic, weak, and strong), known as the Standard Model, has been extremely precise in many of its predictions. There still persist, however, several very important open questions that cannot be explained within the Standard Model, or which fall outside the energy range in which the Standard Model is expected to be valid (such as conditions that existed during the very early stages of the universe). One of these is the existence and properties of dark matter: matter within our universe that we know exists (because of the slowing expansion of the universe), but which does not interact with regular matter in the universe through any means that we have been able to detect. Another is the possible existence of particles that are so massive that they have not yet been generated or observed at the large high-energy particle accelerators (such as the Large Hadron Collider in Switzerland). Yet a third area in which to search for physics beyond the Standard Model is to look for the indirect influence of the proposed extensions on extremely precise measurements of weak optical transitions in atoms. This is the focus of this research effort, which can help guide the answers to these fundamental questions about the universe. The principal investigator and his team will carry out new, higher-precision measurements of weak-force-induced transition amplitudes in atomic cesium. This atom was the focus of prior measurements by the group of Carl Wieman in Boulder in the 1990's, and these measurements of the parity violating amplitude are still the most precise reported for any element. There exists, however, a need to carry out atomic parity violation measurements at an even higher precision. Such a measurement will allow a more precise determination of the weak charge of the nucleus, and from that, an improved determination of the electroweak mixing angle at low momentum transfer. The energy dependence (or "running", as it is called) of this mixing angle, as measured through scattering measurements at various energies, places important constraints on conjectured massive bosons in theories that extend the standard model. These measurements also guide searches for dark matter candidates. Atomic parity violation measurements can also be used to determine the anapole moment of the atomic nucleus. This moment, resulting from weak interactions within the nucleus, provides the leading contribution to the nuclear spin dependence of the parity violating amplitude. To date, the Boulder group's measurement of the anapole moment of cesium is the only successful measurement in any element. Since this result is about twice as large as expected, and its magnitude is still not understood, there is a need for a new measurement to either verify or refute its magnitude. The goal of the present project is to return to cesium for a set of new, high-precision measurements that will address these goals. The principal investigator will apply a two-pathway coherent control technique to these measurements. One set of measurements are centered on the 6s - 7s transition, visited previously by Wieman, while a second set will examine similar effects in the ground state transition between hyperfine components.