2020

Jokiniemi, Lotta

In this thesis, it is shown how charge-exchange reactions and nuclear muon capture can be utilised to probe neutrinoless double-beta (0νββ) decay, a beyond-Standard-Model process that, for the time being, has not been observed despite massive experimental efforts. If detected, 0νββ decay would not only prove the existence of physics beyond the Standard Model but also provide precious information on the yet unknown nature and mass-scale of neutrinos. Hence, improving the theoretical description of the related nuclear-structure physics is a crucial aid in planning the future experiments. The 0νββ decay proceeds through virtual states of an intermediate nucleus to the ground or excited states of the decay daughter. The decaying, the intermediate and daughter nuclei form a so-called double-beta-decay triplet. One way to improve the description of the related nuclear structure is finetuning the nuclear-model parameters by exploiting available data on relevant measured processes. To that end, one can study complementary nuclear processes for which experimental data exist or are being or will be measured. In this thesis, it is proposed that one can probe the 0νββ decay by studying the strength distributions of charge-exchange reactions and ordinary muon capture in the double-beta-decay triplets. By studying these nuclear processes one can not only probe the intermediate states of the double-beta decay, but also eventually shed light on the highly debated effective values of the weak couplings in wide excitation-energy and momentum-exchange regions relevant for 0νββ decay. All the computations presented in the thesis were performed in the protonneutron quasiparticle random-phase approximation (pnQRPA) framework. pnQRPA allows accommodating large no-core single-particle bases including all the relevant spin-orbit-partner orbitals, hence providing access to wide excitation-energy regions. Since pnQRPA has shown to be capable of describing the gross features of the distributions of nuclear states, it is an excellent tool for simultaneous consistent description of double beta decay, charge-exchange reactions and muon capture. The thesis consists of five publications and an introductory part. Articles [I, II] cover probing the 0νββ matrix elements by isovector spin-multipole transitions in the key double-beta-decay triplets, and articles [III, IV, V] probing the 0νββ decay in various ways by ordinary muon capture. In the introductory part, the study of this thesis is set in the wider frame of weakinteraction processes, and the theoretical formalism and results of all five publications are twined together.