Dissertation: Novel Theoretical Developments in Neutrino-Nucleus Scattering (Hellgren)
Neutrinos and their properties are of particularly high interest in contemporary physics research, as there are several unresolved and fundamental problems and questions associated with them. These include their masses, mass hierarchy, possible Majorana character and the existence of the hypothetical sterile neutrino, and solutions to them are pursued in several large-scale multinational neutrino experiments.
- The experimental detection of neutrinos relies on their interactions with matter, mainly atomic nuclei. Theoretical predictions for scattering cross sections are therefore of high importance when designing new experiments and for the data analysis of measurement data, says doctoral researcher Matti Hellgren of the Ä¢¹½Ö±²¥.
Neutrino-nucleus scattering as the central focus of the study
The research in the dissertation of Matti Hellgren focused on the scattering of neutrinos off experimentally relevant target nuclei. He developed novel theoretical methods for the modelling of these processes. The scattering off the stable iodine and caesium nuclei as well as thallium isotopes received particular attention, as these nuclei are present in the target material of the detectors in several neutrino experiments.
- I found that by using a realistic microscopic quasiparticle-based nuclear model and taking into account the quenching of the weak axial-vector coupling constant in sufficient detail the theoretical result for the charged-current scattering cross section off iodine is in good agreement with the corresponding experimental result reported by the COHERENT Collaboration, states Hellgren.
A novel hybrid model was developed for the scattering studies involving the thallium isotopes. This hybrid model combines the strengths of two individual models and achieves a more accurate description of the nucleus.
- In addition to neutrino scattering, the hybrid model can be used in the future in nuclear structure calculations of other weak nuclear processes as well, tells Hellgren.
Novel recoil energy formalism supporting experiments
Previous theoretical research on neutrino-nucleus scattering is mostly focused on the cross section as a function of the incoming neutrino energy. From an experimental perspective, however, the quantity of interest is often the cross section as a function of the nuclear recoil energy, but existing literature regarding this is mostly on elastic processes. To bridge this gap a novel recoil energy formalism was developed in the dissertation research, which enables the computation of the scattering cross section as a function of the recoil energy in inelastic processes.
- Inelastic neutrino scattering is typically subdominant to elastic scattering, but it can potentially overlap with hypothetical physics phenomena beyond the Standard Model. The developed recoil energy formalism then has the potential to serve as a useful tool in the data analysis of experiments that search for these phenomena, explains Hellgren.
The role of the quenching of the axial-vector coupling constant in neutrino scattering
The quenching of the weak axial-vector coupling constant in semileptonic nuclear processes when compared to its bare value is a long-standing puzzle, which was recently solved in the case of beta decay through highly accurate nuclear structure calculations and the inclusion of many-body currents.
- I studied the quenching of the axial-vector coupling constant in the neutrino scattering off iodine. The detailed analysis of the quenching and individual contributions to it can serve as a starting point for calculations involving other important weak nuclear processes, such as the hypothetical neutrinoless double beta decay, says Hellgren.
The obtained results can also potentially serve as inputs for astrophysical simulations of events where the role of neutrinos is important, such as in core-collapse supernovae.
The public examination of the dissertation "Novel Theoretical Developments in Neutrino-Nucleus Scattering" by M.Sci Matti Hellgren is held on Friday 25.4.2025 at 12.00 in the lecture hall FYS1 in Ylistönrinne. Professor Gabriel MartÃnez-Pinedo (Technische Universität Darmstadt) serves as the opponent and Associate Professor Markus Koretlainen of the Ä¢¹½Ö±²¥ will serve as the custos. The language of the examination is English.
The dissertation "Novel Theoretical Developments in Neutrino-Nucleus Scattering" is available in the JYX publication archive:
