Quark and gluon structure of nucleons and nuclei

Phiala Shanahan, Massachusetts Institute of Technology

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Understanding the nature of dark matter is a defining challenge for contemporary particle and nuclear physics.

The interaction of a broad class of dark matter candidates with the nuclei used in direct detection experiments is governed by nuclear scalar matrix elements. In recent work, the PI and collaborators determined the scalar matrix elements of light nuclei with A 4 via first-principles calculations of the underlying interactions with the Standard Model, albeit with larger-than-physical values of the quark masses used in the study (allowing computationally cheaper calculations).

These calculations revealed significant, and unexpected nuclear effects in the scalar interactions in these light nuclei. These significant effects in small nuclei potentially indicate even larger effects and uncertainties in the scalar matrix elements of the much larger nuclei, for example Xenon with A = 131, typically used in direct detection experiments. If these effects persist in a controlled study at the physical values of the quark masses, it will have significant implications for the interpretation of the results of current and future dark matter direct detection experiments around the world. As such, it is essential that such a study is undertaken.

The computations we are pursuing here will determine the scalar matrix elements of light nuclei with A 4 via first-principles calculations of the underlying Standard Model interactions with controlled uncertainties for the first time. These results are essential to constrain unknown parameters in effective field theory expansions that can then be used to perform calculations for experimentally relevant nuclei,  providing critical input to current and future dark matter direct detection programs.

The calculations obtained will provide necessary theoretical input for interpreting dark matter searches that are currently being undertaken at laboratories around the world and will allow optimal design of the next generation of such experiments.