Dissertation: Doctoral research refines nuclear-structure calculations and provides new information on the double beta decay (Kauppinen)

Theoretical nuclear physics is used to search for answers to the universe’s unsolved puzzles by employing the knowledge from both nuclear and particle physics. One of the key questions is the determination of the neutrino’s mass and nature, which is related to fundamental laws of nature and cosmological phenomena. For this reason, nuclear-structure calculations on the evaluation of the neutrinoless double beta decay have risen to a central role in international research. 

- With the use of nuclear-structure calculations, the behaviour of the nucleus, for example, in different decay processes, or studying excitation energies, can be modelled without experimental means. Theoretical calculations are relevant when considering building large research infrastructures and when studying interesting nuclei, says doctoral researcher Elina Kauppinen. 

New information on neutrinoless double beta decay 

In her thesis, Kauppinen has calculated the nuclear matrix element values for known and potential double-beta-decay nuclei. These values are needed for calculating and estimating decay half-lives. Recent studies have revealed that in the calculation of the neutrinoless double beta decay, a term caused by the short-range interaction has been neglected. However, this term has been shown to be an essential part of the matrix element calculation. 

- I have studied the effect of a short-range matrix element on the total matrix element of decay by using a microscopic interacting boson model as a nuclear model. Although producing exact values for the matrix elements is extremely challenging due to uncertainties involved in the calculations, systematic research on multiple nuclei and comparing the results obtained with different nuclear models, a lot of information can be obtained, Kauppinen explains. 

The study strongly supports the notion that the short-range matrix element is an essential part of the calculation of the total matrix element. 

Giant resonances provide information on the workability of the nuclear model 

Nuclear-structure calculations are continuously improved to be as accurate as possible so that the theoretical predictions would be reliable. The second part of Kauppinen’s dissertation was focused on testing the quasiparticle random-phase approximation. In the thesis, the high-energy excitations were studied by calculating the strength functions for the multipole giant resonances.  

- We were able to compare experimentally measured results about the giant resonances with our calculated ones. Experiments and theory mainly corresponded, although some differences were noticed, says Kauppinen. 

M.Sc. Elina Kauppinen defends their doctoral dissertation "Nuclear-structure calculations of double beta decay and giant resonances” on Friday 5.12.2025 at 12:00 in Ylistönrinne auditorium KEM1. Opponent is Professor Fedor Šimkovic (Comenius University in Bratislava) and custos is Senior Researcher Jenni Kotila (University of Jyväskylä). The event is held in English.

The dissertation "Nuclear-structure calculations of double beta decay and giant resonances" is available in the JYX repository: https://jyx.jyu.fi/jyx/Record/jyx_123456789_106935