Impact of nuclear structure on elastic scattering of weakly interacting particles with nuclei

Abstract

The problem of dark matter (DM) has puzzled physicists for decades, with its nature and interactions yet to be characterised fully. Particle dark matter theories have been proposed as potential candidates for this mysterious matter, where weakly interacting massive particles (WIMPs) constitute a particularly popular potential candidate, inspiring a range of experimental efforts towards its detection. These DM direct detection experiments, concerned with measuring and characterising the elastic scattering of WIMPs off target nuclei, have produced some discrepancies in the reported results - the DAMA/LIBRA collaboration in particular has consistently reported an annually modulating signal consistent with DM that is in disagreement with other experiments' null results in the relevant DM mass region. This has encouraged a plethora of efforts to enhance experimental setups, as well as diversify the theoretical modelling approaches employed for experimental analysis and interpretation, and better quantify the uncertainties present.

The calculation of WIMP-nucleus elastic scattering observables depends on several inputs - these include the experimental parameters; the high energy particle physics content; the DM halo velocity distribution; and nuclear structure information. In this thesis, we focus on the nuclear physics component of the theoretical modelling, contained in the nuclear form factors, and investigate the effect of nuclear structure modelling on WIMP scattering predictions and observables, quantifying the theoretical nuclear uncertainties present. A non-relativistic effective field theory (NREFT) framework is employed, which accounts for the nucleon velocities within the nucleus through nuclear operators that are momentum-dependent in nature. This provides a more comprehensive set of nuclear scattering channels to be employed for modelling, which builds on approaches that typically only employ the standard spin-independent and spin-dependent operators. We perform large-scale nuclear shell model calculations to evaluate the ground-state nuclear wave functions for a range of targets relevant for direct detection searches. To better quantify the impact of these nuclear uncertainties and their propagation to observables associated with WIMP elastic scattering, we evaluate said observables for a range of direct detection experimental parameters of both a modulating and non-modulating nature.

Through our analysis, we comment on the nuclear operators and scattering channels most impacted by nuclear structure modelling for each of the considered nuclear targets, and outline how the unique structure of each impacts uncertainty quantification. To better understand the interplay between the nuclear uncertainties and the other rate components, we employ several approaches for the latter - including three DM halo distributions and two different approaches to dealing with the particle physics coefficients. The impacts of these various models on nuclear uncertainty propagation is discussed for the different experiments considered. Given that DM is yet to be confirmed through direct detection, it is beneficial to test the theoretical nuclear modelling approaches employed in this scattering formalism on other processes which have confirmed experimental data and signals. One such process is coherent elastic neutrino-nucleus scattering (CEvNS), which employs some of the same EFT nuclear operators as the WIMP counterpart. We study the impact of nuclear structure on the CEvNS-specific form factors, and compare theoretical predictions with experimental expectations in order to discuss the validity of the theoretical nuclear modelling employed for the dominant nuclear channels. We also comment on the validity of application of this specific CEvNS nuclear study to the WIMP nuclear uncertainties, with regards to conclusions about the accuracy of nuclear modelling employed for the WIMP-specific form factors. Show more