Measurements of Beyond Standard Model Interactions with the UCNA and nEDM@SNS Experiments

Citation

Sun, Xuan (2022) Measurements of Beyond Standard Model Interactions with the UCNA and nEDM@SNS Experiments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n87j-gh29. https://resolver.caltech.edu/CaltechTHESIS:12032021-231746989

Abstract

Ultracold neutrons (UCNs) are neutrons that have been cooled such that their kinetic energy is on the order of their gravitational potential energy. Experimentally, ultracold neutrons are valuable because at these energies they are trappable and provide experimenters with long observation times. In fact, their observation times are on the order of the free neutron decay lifetime --- allowing direct observation of neutron β-decay. Many contemporary experiments measuring high-precision processes involving neutrons use UCNs. Two such experiments are UCNA and nEDM@SNS, both of which form the basis of this work.

UCNA is an experiment that took place at Los Alamos National Laboratory. In this work, we analyze the 2010-2013 UCNA datasets on neutron β-decay using UCNs. These datasets were originally designed to measure the asymmetry parameter, A, in neutron β-decay. However, there was also sensitivity to another physical parameter in the neutron β-decay rate: the Fierz interference term, b. The Fierz interference term in neutron β-decay acts as a probe of beyond Standard Model (SM) physics interactions, specifically scalar and tensor couplings in the weak interaction. Due to the vector - axial-vector nature of the weak interaction in the SM, any non-zero measurements of b would be indicative of new, beyond SM couplings. In this work, we present the extraction of the Fierz interference term as measured by neutron β-decay for the 2010, 2011-2012, and 2012-2013 UCNA datasets. We present these measurements using two methods: a direct extraction by measuring shape distortions in the β-decay electron spectrum, and an energy dependence in the asymmetry, A₀. These two methods across the three datasets yield six new measurements of b from neutron β-decay data. Our final result is the weighted average of the three asymmetry-extracted b results.

The UCNA datasets were also sensitive to another type of beyond SM interaction: neutron decaying to dark matter with an accompanying positron-electron pair (first proposed in [FG18a]. This decay channel was originally proposed in order to resolve the discrepancy between two measurement methods of the neutron lifetime: bottle experiments which measure neutron population as a function of time, and beam experiments which measure the decay protons from conventional neutron β-decay. Due to the experimental setup of the UCNA apparatus, the UCNA dataset was sensitive to such a decay channel. Using the 2012-2013 UCNA dataset which had functioning timing data, we effectively rule out this decay channel as the sole explanation for the neutron lifetime discrepancy for ≈ 84% of the available decay phase space. Furthermore, we set branching ratio limits on this decay channel as compared to the conventional weak interaction mediated decay.

The last project in this work is the construction of a large scale magnet for the nEDM@SNS experiment. The nEDM@SNS experiment is an experiment designed to measure the neutron electric dipole moment (nEDM) and will take place at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory (ORNL). This experiment will take place in 2027 and make a leading precision measurement on the nEDM. Part of the experiment is the magnetic system and, within that system, the B₀ magnet which will provide a DC holding field to UCNs within the experiment's measurement volume. The assembly procedure for constructing the B₀ magnet is detailed and intermediate quality checks as well as a post-construction room temperature magnetic field map are presented. The preliminary results indicate that the completed B₀ magnet satisfies the specifications and will be useable in the nEDM@SNS experiment.

Thesis Files