# A New Concept for Searching for Time-Reversal Symmetry Violation Using Pa-229 Ions Trapped in Optical Crystals

## TL;DR

This paper proposes a novel method to search for time-reversal symmetry violation by trapping Pa-229 ions in optical crystals, leveraging their nuclear properties and potential for enhanced sensitivity, despite current sourcing and coherence challenges.

## Contribution

It introduces a new concept utilizing Pa-229 ions in optical crystals for symmetry violation searches, including surrogate experiments with Pr-141 to address current limitations.

## Key findings

- Pa-229 is highly sensitive to symmetry-violating interactions.
- Embedding Pa-229 in optical crystals could enable large field interactions.
- Using Pr-141 as a surrogate demonstrates feasibility of the approach.

## Abstract

Certain pear-shaped nuclei are expected to have enhanced sensitivity to time-reversal and parity-violating interactions originating within the nuclear medium. In particular, Protactinium-229 is thought to be about 100,000 times more sensitive than Mercury-199 which currently sets some of the most stringent limits for these types of interactions. Several challenges would first have to be addressed in order to take advantage of this discovery potential. First, there is not currently a significant source of Pa-229 (1.5 day half-life); however, there are plans to harvest Pa-229 at the Facility for Rare Isotope Beams at Michigan State University. Second, the spin-5/2 nucleus of Pa-229 limits its coherence time while also making it sensitive to systematic effects related to local electric field gradients. On the other hand, this also give Pa-229 an additional source of signal in the form of a magnetic quadrupole moment (MQM) which violates the same symmetries as an EDM but is not observable in spin-1/2 systems. Third, in order to compensate for the small atom numbers and short coherence times, the Pa-229 atoms would have to be probed with exceptionally large electric and magnetic fields that may be possible if Pa-229 ions are embedded inside an optical crystal. We will describe some aspects of this concept using the stable Praseodymium-141 isotope as a surrogate which has the same nuclear spin and similar atomic structure of Pa-229.