Session JL: Fundamental Symmetries II

Precision studies of single molecular ions for fundamental physics

Precision molecular experiments provide a unique tool for measuring electroweak nuclear properties. Compared to atoms, certain molecules can offer more than eleven orders of magnitude enhanced sensitivity to symmetry-violating nuclear effects, enabling precision tests of the Standard Model. Being able to trap the molecules of interest can significantly enhance the coherence time of the experiment and hence the measured signal. Therefore, ionic molecules offer a major advantage, as they can be trapped and manipulated using electric and magnetic fields. In this talk I will present a proof-of-principle experiment aiming to measure parity-violating nuclear properties of ²⁹Si, by trapping individual SiO⁺ ions in a Penning trap. The trap’s strong magnetic field can bring molecular levels of opposite parity close to degeneracy, thus enhancing the sensitivity to parity-violating, nuclear spin dependent effects. The experiment is expected to measure the anapole moment of ²⁹Si with a relative uncertainty of 10%. Preliminary results and future perspectives will be discussed.   

Precision studies of radioactive molecules relevant to fundamental physics

Certain molecules that contain heavy, deformed radioactive nuclei are predicted to be exceptionally sensitive laboratories to examine the fundamental symmetries of nature [1-4]. Despite their recognized potential, technical challenges prevented experimental studies of these systems until very recently. The first landmark study of a short-lived molecule was able to determine the rovibronic structure of different isotpologues of radium monofluoride [5,6] , a promising system in which to search for signatures of symmetry violations.

This contribution presents results from subsequent experimental campaigns in which a massive improvement in resolution enabled an unprecedented glimpse into the structure of the radioactive ^223,225,226RaF. This allowed the laser cooling scheme of ²²⁶RaF to be established which will be critical for realizing future high-precision studies. Furthermore, the hyperfine structure in ²²⁵RaF is shown to be highly sensitive to the Bohr-Weisskopf effect, laying the foundation for the first studies of the distribution of nuclear magnetization in a molecule. Additional results characterising a plethora of newly observed excited electronic states as well as an accurate determination of the molecule's ionization potential will also be presented.

[1] Altuntaş, E. et al., Phys. Rev. Lett. 120, 142501 (2018)

[2] ACME Collaboration, Nature 562, 355–360 (2018)

[3] Flambaum, V. V. et al., Phys. Rev. Lett. 113, 103003 (2014)

[4] Berger, R. et al., WIREs Comput. Mol. Sci. 9, e1396 (2019)

[5] Garcia Ruiz, R. F. et al., Nature 581 396–400 (2020)

[6] Udrescu, S. M. et al. Phys. Rev. Lett. 127 03301 (2021)   

Measurement of Neutron Polarization and Transmission for the nEDM@SNS Experiment

The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS) will implement a novel method, which utilizes polarized ultra-cold neutrons (UCN) and polarized ³He in a bath of superfluid ⁴He, to place a new limit on the nEDM down to 2-3x10^-28 e·cm. The experiment will employ a cryogenic magnet and magnetic shielding package to provide the required magnetic field environment to achieve the proposed sensitivity. This talk will describe the design and implementation of a ³He polarimetry setup at the SNS to measure the monochromatic neutron polarization and transmission losses resulting from passage through the magnetic shielding and cryogenic windows. Preliminary results of monochromatic neutron flux measurements and engineering design and assembly of the experimental setup will be presented.   

3He polarization and injection system for the nEDM@SNS SOS apparatus

The Systematic and Operations Studies (SOS) for the neutron electric dipole moment (nEDM) experiment at the Spallation Neutron Source (SNS) will measure the trajectory correlation functions of ³He and neutrons in order the determine the expected frequency shift from the geometric phase effect in the nEDM@SNS experiment. To this end the SOS apparatus will utilize Metastability Exchange Optical Pumping (MEOP) to polarize ³He to 80% polarization at room temperature. The ³He is then injected into measurement cell inside the cryovessel where the experiment is performed with concentrations of ³He as low as 10^-10 and a temperature of 0.4 K. We describe the polarization and injection system as well as report on results from tests of the MEOP system, simulations of ³He injection, and our calculations of trajectory correlation functions.

    

Electrical Breakdown Engineering for the Cryogenic High-Voltage Multiplier for SNS nEDM

The nEDM@SNS experiment will employ a system of voltage amplification electrodes called a Cavallo Multiplier to produce the high voltage necessary for the neutron electric dipole moment's sensitivity goal of 10^-28 e cm. These electrodes must be fabricated out of copper-coated PMMA (resin) to reduce eddy current heating and magnetic field disturbances in the sub-kelvin measurement region. Electrical breakdown is a big concern for these delicate electrode surfaces. Strategies to prevent spark damage include loading the apparatus under pressurized liquid helium and designing a sacrifical disposable button. These strategies will be discussed in the context of the electric breakdown engineering for the nEDM@SNS experiment.   

Cancelling the geometric phase in the nEDM@SNS experiment

A non-zero neutron electric dipole moment (EDM) produces a tiny precession frequency shift proportional to the strength and direction of the electric field. The nEDM@SNS experiment uses Helium-3 as a comagnetometer. Three frequency shifts (“geometric phase”, |E|² and |B|²) all of which depend on the same trajectory correlation functions, are significant at the goal sensitivity (2-3x10^-28 e*cm). These effect both neutrons and comagnetometer in any neutron EDM experiment. Helium-3 has a unique advantage as a comagnetometer in that its trajectory correlation functions, and therefore its frequency shifts, are strongly temperature dependent. This allows the experiment temperature (T) and  magnetic field strength (B) to be tuned to cancel the geometric phase. We present a simulation demonstrating how a measurement of the |E|² frequency shift can be used to experimentally determine values for B and T where the geometric phase is effectively cancelled and place a limit on this key systematic error.   

Wall effects on neutron storage lifetime simulation in Geant4 for the nEDM experiment

The nEDM experiment at the Spallation Neutron Source utilizes UltraCold Neutrons (UCNs) to measure a permanent electric dipole moment of the neutron. For the nEDM experiment, the UCN storage lifetime is directly affected by the loss of UCNs to the walls of the measurement cell. Using Geant4, we are able to build a simulation that allows us to see how these UCNs behave in our cell by utilizing microroughness surface functions for non-specular reflections. This talk will cover the ongoing simulation work in Geant4 as well as comparing it to other programs such as penTRACK or starUCN.   

Addressing the impact of magnetic field fluctuations on the critical dressing mode of nEDM@SNS

The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS) proposes to measure the nEDM using the spin-dependent capture cross section of neutrons on helium-3. The critical dressing mode of this experiment uses an oscillating magnetic field to dress the gyromagnetic ratios of neutrons and helium-3 to the same effective value. While this technique grants increased sensitivity to the nEDM by improving the signal-to-noise ratio, randomly fluctuating magnetic fields and magnetic field gradients can cause the spins of the neutrons and helium-3 atoms to drift apart over time, potentially erasing these gains. Here we use second-order time-dependent perturbation theory to estimate the relaxation and frequency shifts due to magnetic field instabilities in terms of the noise power spectrum and compare these calculations to numerical solutions obtained by integrating the Bloch equations. We then investigate several mitigation strategies for this type of noise. In particular, we demonstrate that modulating the dressing field can reduce the impact of magnetic field fluctuations.