Collaborative Research: RUI: PM:High-Z Highly Charged Ions Probing Nuclear Charge Radii, QED, and the Standard Model

Appalachian State University, Boone NC

Investigators

Abstract

This project is jointly funded by Atomic, Molecular, and Optical Experimental Physics, the Established Program to Stimulate Competitive Research (EPSCoR), and Experimental Nuclear Physics. When many of the outer electrons are removed from an atom, it becomes a highly charged ion. Such highly charged ions (HCI) are interesting as they have exotic properties compared to neutral atoms. In these ions, the remaining electrons are those that overlap significantly with the small central nucleus. Precision measurements of the light emitted as the electrons change orbits thus yields information about the nucleus. These include the finite nuclear charge radius, nuclear deformations, and magnetic properties. Using an electron beam ion trap (EBIT), this project aims to conduct an experimental study of HCI chosen to have simple, theoretically calculable electronic configurations in order to understand nuclear effects. By measuring the emitted radiation in the extreme-ultraviolet (EUV) and x-ray region, the PIs and their collaborators recently conducted a series of benchmark experiments using Na-like and Mg-like ions and determined the nuclear charge radii differences of high-Z isotopes. In the present project, they will expand these studies, investigate its limitations, and explore its sensitivity to beyond the standard model (BSM) physics. The study will also be used to improve the existing atomic theories of complex atomic systems. Graduate and undergraduate students will be involved in setting up the experiment, data collection, analysis, interpretation, scientific report writing, and presentation at conferences. The research work will integrate with the interdisciplinary educational programs taken by the students such as the “creative inquiry” program at Clemson University and the “Methods of Experimental Physics” at Appalachian State University. Students from underrepresented populations will be encouraged to join the research work and graduate students will be trained to mentor undergraduates. Only a few methods exist to measure the absolute nuclear charge radius, which is a key property of the nucleus that provides information about the onset of nuclear deformation, the structure of exotic halo nuclei, and the interaction between nucleons. In astrophysics, the nuclear charge radius enters in the determination of stellar elemental abundances and is an important parameter in dark matter searches. Atomic spectroscopy of Na-like and Mg-like HCI in an EBIT offers a new method to pursue the measurement of root-mean-square nuclear charge radii that supplement only a handful of available nuclear and atomic physics-based techniques. In addition to the strong electron-nuclear overlap, relativistic and quantum electrodynamics (QED) effects are also more pronounced in high-Z ions compared to neutral atoms or few-times ionized systems. The experimental precision provided by the spectrometer resolution and high statistics of the Na/Mg-like systems complemented by highly accurate state-of-the-art ab-initio calculations thus allows for the study of atomic structure effects such as hyperfine splitting, nuclear deformation, nuclear polarization, and higher-order QED. The experiment will measure the isotope shift between isotopes with some of the smallest nuclear charge radius uncertainties such as tungsten and osmium, serving as ideal candidates to test the limits of the technique and to look for the signs of BSM effects. Nuclear charge radii of isotopes with a large uncertainty such as rhenium will be conducted using osmium as an anchor. Prior work by the PIs have demonstrated the method in xenon and the reduction of the previously reported uncertainty of iridium isotopes by an order of magnitude. 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.