Ab Initio Neutrinoless Double-Beta Decay Matrix Elements for ^{48}Ca, ^{76}Ge, and ^{82}Se.

Area of Science:

• Nuclear Physics
• Quantum Many-Body Theory
• Astroparticle Physics

Background:

• Neutrinoless double-beta decay (0νββ) searches for physics beyond the Standard Model.
• Accurate nuclear matrix elements (NMEs) are crucial for interpreting experimental results.
• Ab initio calculations offer a path to more reliable NMEs.

Purpose of the Study:

• To calculate basis-space converged NMEs for 0νββ decay in ^{48}Ca, ^{76}Ge, and ^{82}Se.
• To employ the ab initio in-medium similarity renormalization group (IM-SRG) method.
• To provide a first-principles approach for 0νββ decay calculations.

Main Methods:

• Utilizing two- and three-nucleon forces as initial inputs.
• Applying the ab initio IM-SRG method to derive valence-space Hamiltonians.
• Consistently transforming the relevant ββ-decay operators within the IM-SRG framework.

Main Results:

• Basis-space converged NMEs were calculated for ^{48}Ca, ^{76}Ge, and ^{82}Se.
• The tensor component of the decay operator was found to be non-negligible for ^{76}Ge and ^{82}Se.
• Calculated NMEs are 25%-45% smaller than those from phenomenological shell models.

Conclusions:

• This work establishes a first-principles approach for calculating 0νββ decay NMEs.
• The results highlight the importance of the tensor component and ab initio methods.
• This methodology is applicable to all nuclei relevant for ongoing 0νββ decay searches.