Overview

A plethora of disparate cosmological observations have established the existence of a non-baryonic component to matter, called dark matter. The origin and nature of this dark matter, which makes up 25.8 ± 0.4% of the energy composition of the Universe (Plank experiment, 2013), remains a mystery as there is no candidate in the Standard Model of particle physics that has the properties of being stable over the lifetime of the universe, void of net electroweak charges, and could explain the observed dark matter densities that are roughly five times that of baryonic matter. For this reason, roughly two dozen dedicated dark matter experiments are currently under development or actively taking data, along with several dozen cosmological experiments exploring cosmic rays and high-energy neutrinos that can yield dark matter signals in their results. However, for all of these experiments, nuclear and astrophysics play a crucial role in the feasibility of discovery in these experiments.

The goal of this workshop is to explore, discuss, and debate the current state-of-the-art methods for determining nuclear backgrounds and enhancements/suppressions in dark matter experiments and clarify if better methods exist or could be derived. This workshop will produce a concrete plan of theoretical needs and future calculations to better interpret results from future dark matter experiments. This requires exploring several topics spanning multiple fields of nuclear and particle physics.

Several key topics are as follows:

1. Dominant interaction response for large nuclei: The calculation of any direct detection experimental event rate requires a detailed calculation of the cross-section depending on the dominant nuclear operators. Each of these nuclear responses are unique for each element and isotope present in the composition of the detector. Also, the strength and behavior of the resulting cross-section can be enhanced based on the nature of the interaction of the dark matter particle (higgs exchange, Z-exchange, magnetic moments, etc.). Galilean effective field theory has recently given us a framework for a unified analysis of the low-energy consequences of these theories in direct detection experiments. In certain cases tools like lattice gauge theory can help relate the parameters of the ultraviolet theory to the effective couplings that experiment can constrain.

2. Single and multi-nucleon matrix elements: While the direct effects of dark matter interactions are most easily observed in recoils of nuclei, dark matter models are usually build on top of the relativistic framework of the Standard Model, in particular, interactions with the quarks and gluons within the nucleon. For this reason, single and multi-nucleon matrix elements (such as the sigma term or gluonic content of the nucleon) are of large interest to dark matter experiments. However, two key steps are required to take first-principle calculations of these matrix elements and apply them to large nuclear systems. First, after defining the operators, one must calculate the matrix elements, either through effective Lagrangians or through first-principles calculations, via non-perturbative lattice QCD calculations. Second, mechanisms to map the effects of these single and multi-nucleon matrix elements to large nuclei are required to predict possible enhancements/suppressions.

3. Dark matter effective field theories and collider constraints: One promising avenue to putting constraints on possible dark matter theories is to map collider constraints of couplings of beyond-the-Standard-Model dark matter candidates to the Standard Model. In particular, this method can put bounds on smaller mass candidates where direct detection experiments are often not sensitive. To accomplish this one must connect this EFT at the LHC scale to the scale of single and multi- nucleon matrix elements at the nuclear scale. Also, EFTs in certain dark matter limits can acquire robust bounds in terms of Standard Model processes.

4. Non-standard nuclear cross-sections and search channels: There are a variety of ways that dark matter can interact with large nuclei. Some interactions that are dominant in this sector are either nonexistent or greatly suppressed in the typical interactions of Standard Model particles we observe in experiment (with which nuclear response functions are measured). One such example is a composite dark matter theory with charged constituents in symmetric combinations where candidates primarily interact via electromagnetic polarizabilities, which interact via two-photon exchange and lead to a quark loop in the usual cross-section Feynman diagrams. This leads to the need for detailed knowledge of both the on-shell and off-shell behavior of the nuclear operators. Other directions that can put significant bounds on dark matter include systems in nuclear/astrophysics, such as neutron stars. These alternative directions will also be discussed.

Workshop Organization

The 5-day workshop will stimulate conversations between nuclear and particle theorists about the aforementioned nuclear physics issues and develop a concrete plan addressing these issues going forward. The desired structure of the workshop would be to have 2 or 3 talks each morning (10 to 15 talks total), with the afternoon left open for collaboration and organized discussions.

There is a registration fee of $40 to attend this workshop.