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Ab initio theory for two-neutrino and neutrinoless double-beta decay

University of British Columbia

Although the complexity behind nuclear physics problems is often deemed too intensive even for modern supercomputers, non-perturbative ab initio (meaning "from first principles") many-body technologies have made significant strides towards efficiently modelling low-energy QCD structure. In particular, the similarity renormalization group, along with an "in-medium" reference state normal ordering (IM-SRG), has proven useful for making nuclear problems computationally manageable. This IM-SRG method translates chiral Hamiltonians, nuclear properties, and decays into a numerically tractable framework, whilst capturing bulk effects of forces via normal ordering. A decay that has generated much interest in the nuclear community is double-beta decay. The two-neutrino mode has evaded a proper theoretical treatment, due to a disconcerting puzzle known as "quenching." The neutrinoless mode (though still hypothetical) could unveil fundamental properties of the neutrino, such as its absolute mass and potential Majorana nature. In this dissertation, we will use IM-SRG, in a valence space shell model construction, to compute both the two-neutrino and neutrinoless double-beta decay of the doubly magic nucleus, Calcium-48. We conclude that the use of a fully ab initio method that models many-body effects, via IM-SRG, have decreased the two-neutrino double-beta decay nuclear matrix element by a factor of 3 (without any quenching factor), and the neutrinoless counterpart by roughly 20%, compared to the standard phenomenology for Calcium-48. This result has concerning experimental implications, since the half-life of a decay is proportional to the square of the inverse of the nuclear matrix element.

Text

2018-01-19

Attribution-NonCommercial-ShareAlike 4.0 International

http://hdl.handle.net/2429/64417

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-sa/4.0/