Studying the internal structure of the proton
The research in Yrjänheikki’s thesis is fundamental research by nature, and is a part of a larger endeavor to chart and understand the structure of the proton. A typical answer to the question "What is the proton made of?" is that it is made of two up-quarks, one down-quark, and the gluons that bind them. In reality the answer is more complicated, and there are other kinds of quarks in the proton.
The number density of these quarks and gluons in the proton is described by parton distribution functions. At the moment, these distribution functions cannot be precisely calculated directly from theory. Thus, the most accurate picture of the internal structure of the proton is obtained by fitting parton distributions to available experimental data. Parton distributions cannot be directly measured, but they are related to experimentally measurable quantities through perturbative calculations. Theoretical modeling is therefore needed for using experimental data, even if parton distributions cannot be calculated directly.
From neutrino-nucleus collisions to the strange-quark distribution
The thesis focuses on the theoretical modeling of neutrino-nucleus collisions where a pair of muons is produced. Such collisions probe particularly the strange-quark content of the proton and atomic nuclei, which is still poorly known. The theoretical treatment of these data has been lacking before. The purpose of Yrjänheikki’s research is to improve this treatment and to let go of previously used approximative assumptions.
The main outcome of the research is an improved theoretical modeling of dimuon production neutrino-nucleus collisions, and a computer code that enables the practical use of this new modeling method in the study of parton distributions. The improved method describes existing experimental data well. The higher perturbative accuracy helps give a clearer and more reliable picture of the strange-quark distribution. This is particularly important when new data from the neutrino experiments of the upgraded Large Hadron Collider (LHC) at CERN become available. Even though the research focuses on the treatment of a specific neutrino-nucleus collision process, the method developed in the thesis can also be used to model other such processes.
The public examination of M.Sc. Sami Yrjänheikki's doctoral dissertation “Dimuons from Neutrino-Nucleus Collisions” will be held on Friday, 4 September 2026 at 12 o'clock in the lecture hall KEM1 on Ylistönrinne. The Opponent is Professor Michael Klasen (University of Münster) and the Custos is Senior Lecturer Hannu Paukkunen (University of Jyväskylä). The defense will be held in English.
