There are a number of outstanding questions facing first-principles calculations of double-beta decay nuclear matrix elements and determining the most productive ways to interface with experiment and other established theoretical methods for double-beta decay. We list those most relevant for our workshop, and we ask that participant presentations address one or more of these in order to facilitate discussion, drive progress, and foster new collaborations.
Key Workshop Questions and Goals:
What is the status of ab initio methods, and what are the best benchmarks for large-space ab initio vs shell-model calculations?
What is the sensitivity to different starting chiral Hamiltonians?
What is the best strategy to move ab initio approaches towards 76Ge and 136Xe (and milestone calculations along the way)?
What are the outstanding questions from the standard shell model, and can these uncertainties be quantified?
Do we need to "quench” gA in neutrinoless double-beta decay as is done in single-beta and two-neutrino double-beta decay?
What can the experiences of other phenomenological approaches (DFT, QRPA, IBM) teach us, and how can we best capture the strengths of these methods?
What is the status of electroweak current calculations and the prospect for matrix element inputs for available many-body methods?
Other than currents, how can we quantify other uncertainties in the bare operator (short-range correlations, closure…)? Is everything else under control?
How do we best assign theoretical uncertainties, including both the transition operator and the nuclear structure of the initial/final nuclei?
Is there anything we can learn by studying different mechanisms, e.g. heavy neutrino exchange?
What experimental data should theory reproduce so we trust neutrinoless double-beta decaypredictions?
(How) does neutrinoless double-beta decay correlate with other (measurable) observables indifferent theoretical methods?
What existing experimental data is most useful for constraining the various theory ingredients above, and what are the most crucial unmeasured quantities?
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