Neutrinoless double-beta decay in calclium-48 from first principles with deformed coupled cluster theory

Neutrinoless double-beta decay is a theoretical electroweak process in which a nucleus undergoes two simultaneous beta decays and the resulting neutrinos annihilate one another as Majorana particles. While this process has not yet been observed, the related two-neutrino double-beta decay has been confirmed by several experiments[1]. An observation of this rare decay would not only prove the Majorana nature of neutrinos[2], but would confirm a lepton-number violating (LNV) mechanism beyond the standard model and provide insight into the neutrino mass hierarchy[3] and leptogenesis[4]. This presentation will show results of the neutrinoless double-beta decay nuclear matrix element in calclium-48 calculated from first principles using microscopic nucleon-nucleon forces and electroweak operators derived from chiral effective field theory. The parent and daughter ground states are computed in both spherical and deformed bases, each utilizing a similarity-transformed coupled-cluster Hamiltonian and the double charge exchange equation-of-motion method. Then, the matrix element is calculated with an effective neutrinoless double-beta decay operator constructed with the same coupled cluster transformation. To bolster these results, similar calculations in light nuclei are compared with the quasi-exact no-core shell model. These new calculations show a significant reduction of the calcium-48 neutrinoless double-beta decay matrix element compared to previous ab initio and phenomenological approaches. [1] A. Barabash, Nuclear Physics A 935, 52 (2015). [2] J. Schechter and J. W. F. Valle, Phys. Rev. D 25, 2951 (1982). [3] R. N. Mohapatra and G. Senjanović, Phys. Rev. Lett. 44, 912 (1980). [4] S. Davidson, E. Nardi, and Y. Nir, Physics Reports 466, 105 (2008).