Speaker
Description
Neutrinoless double-beta ($0\nu\beta\beta$) decay is a key probe of lepton-number violation, the Majorana nature of neutrinos and their absolute mass scale. As upcoming experiments extend half-life sensitivities by up to two orders of magnitude, reliable calculations of nuclear matrix elements (NMEs) are essential to constrain neutrino masses and underlying decay mechanisms. The standard decay mechanism, in which light Majorana neutrinos are exchanged between two decaying neutrons, has been extensively studied within nuclear theory, however short-range contributions described by contact operators in the low-energy effective field theory are less explored. These contributions are crucial for interpreting experimental results in terms of exotic scenarios, such as heavy Majorana neutrinos arising in seesaw mechanisms. We compute short-range NMEs within an ab initio framework, starting from chiral EFT and using the valence-space in-medium similarity renormalization group (VS-IMSRG), a many-body method that maps the full nuclear Hamiltonian to an effective one targeted to the valence-space of the nucleus. Short-range operators exhibit enhanced sensitivity to regularization and similarity renormalization-group evolution compared to long-range light-neutrino exchange operators, motivating a systematic study of different schemes. We find that maintaining consistency between the Hamiltonian and decay operators is critical, as mismatches in regulators and operator evolution (ubiquitous in nuclear theory) can significantly impact results. We also find that operators involving magnetic currents, while formally suppressed in chiral EFT, are enhanced to leading order due to the large isovector magnetic moment of the nucleon and short-range operator structure. Finally, we apply our NMEs to a toy 3+1 sterile-neutrino model to obtain constraints in the heavy sterile-neutrino mixing-mass parameter space.