Role of Nuclear Structure in One- and Two-Body Nuclear Response for WIMP Direct Detection

Abstract

Most of the matter content of the universe is expected to consist of "dark matter", a form of matter which does not interact with light. While there are multiple lines of evidence for the existence of dark matter, its exact identity remains a mystery. There are a number of hypotheses, of which Weakly Interacting Massive Particles ("WIMPs") constitute one of the most attractive candidates. There are a number of methods the search for WIMPs is using, one of which is direct detection, which attempts to observe collisions between WIMPs and nuclei, or alternatively between WIMPs and electrons, although the latter possibility was not considered in this thesis.

To interpret a signal from a direct detection experiment, or lack thereof, the interaction between WIMPs and nuclei must be understood. This dependence of this interaction on nuclear structure can be encapsulated in terms of "structure factors", which are functions of momentum, specifically, matrix elements of nuclear observables that depend on the momentum exchanged in the collision. The calculated structure factors for a particular nucleus may depend on the nuclear interaction used to calculate its structure. Although the WIMPs are often assumed to only couple to single nucleons, in which case the WIMP-nucleus interaction can be described in terms of "one-body structure factors" (expectations of one-body observables) recent work based on Chiral Effective Field theory predicts possible interactions of WIMPs with pairs of nucleons, which two-body structure factors (expectations of two-body observables) are necessary to describe.

The object of this thesis was to investigate the dependence of the one- and two-body structure factors on the interaction between nucleons used to calculate the nuclear structure. Nuclear responses were calculated for isotopes of fluorine, sodium, silicon, argon, germanium, iodine and xenon, with between 2 and 4 interactions per nuclide. Although some one-body structure factors may vary significantly with nuclear interaction, two-body structure factors were found to depend only very weakly on the interaction. Thus, for comparing data with theory, the two-body structure factors can be calculated with a particular interaction without concern that it will give a significantly inaccurate value. Show more