The experimental confirmation of neutrino oscillations -that was
rewarded with a Nobel prize in 2015 !- sparked off an enormous
experimental and theoretical interest in the oscillation properties of
these elusive particles. Worldwide, several collaborations are
working on extending our knowledge about neutrino masses and mixing
Oscillation experiments are essentially about counting : the
difference in number of neutrinos of a certain flavor between a
detector near the neutrino source and a far detector, several hundred
kilometers away, allows one to collect information about the
neutrinos' oscillation characteristics.
One of the main neutrino-detection mechanisms that is used in these experiments is the scattering of neutrinos off atomic nuclei. Experimentally, these neutrino interactions suffer from a number of complications in comparison with the related electromagnetic scattering reactions : neutrino cross sections are very small, making it challenging to collect sufficient data ; the neutrinos contributing to the signal in the detector have widely varying energies and in a specific event the precise incoming energy is unknown ; and in neutral-current reactions the outgoing lepton is again a neutrino that cannot be detected, complicating the reconstruction of the energy balance at the weak-interaction vertex. These limitations moreover result in the fact that the incoming flux tends to be only poorly known, hampering the normalization of reaction rates.
The analysis of neutrino-oscillation experiments hence strongly depends on a good understanding of the underlying reaction mechanisms. Modeling neutrino-nucleus interactions is of crucial importance for the progress in our understanding of neutrino-oscillations. An important reaction mechanism is quasi-elastic scattering, where the neutrino scatters off a nucleon bound in the nucleus, transfering energy and momentum to the nucleon that subsequently leaves the nucleus to be detected in coincidence with the outgoing lepton.
In coherent processes, low-energy neutrinos are scattered off the nucleus as a whole, without resolving the individual nucleons. The lack of detectable reaction products hampers experimental studies of the process as these have to rely on measurements of the (small) recoil energies of the target nuclei. On the other hand, the coherent reaction mechanism has the advantage that the cross section is relatively large, and dominates the 'standard' inelastic neutrino-nucleus scattering processes for incoming energies up to a few tens of MeVs. This makes the coherent process important for astrophysical neutrinos where the large cross sections make it an important instrument for the transfer of energy from the neutrino to the surrounding material. This is in particular the case for supernova neutrinos, both for their interactions within the collapsing and exploding star core as for their detection on earth. The difficulties met by experiments measuring these coherent cross sections, make theoretical simulations all the more important. The theoretical description of the target nucleus is non-trivial. Each nucleus is constructed of protons and neutrons which are constantly interacting with each other through nuclear forces. This thesis project has following goals:
A central issue in current neutrino
experiments is the lack of a monochromatic neutrino beam. This stands
in stark contrast with experiments that use charged leptons as
projectiles, where the incoming energy is well-known. However, since
neutrinos are produced out of decaying pions, their energy is not
known a priori.
This thesis topic focuses on a central issue in the synergy between theory and experiment in the understanding of neutrino-nucleus scattering : the reconstruction of the incoming energy of the neutrino in an interaction.
This distribution of reconstructed energies is a model-dependent quantity. It is the goal of this thesis work to examine the role of multinucleon knockout processes on the reconstructed energy. Work on this thesis involves a mix of weak interaction physics, nuclear scattering theory and numerical work.
In neutrino-nucleus experiments, neutrinos span a
broad energy range. The models used in our group (e.g. the CRPA
approach) have shown great success in the prediction of experimental
data, but use a relativization scheme that only partially accounts for
most relativistic effects as long as the energies are not too
extreme. Furthermore, the incorporation of final-state interactions in
these models is only partial, with the possible absorption and
rescattering processes being neglected. By necessity, for higher
incoming energies, one is drawn to the use of theoretical models that
are fully relativistic. These become highly relevant at the energies
e.g. MINERvA and Dune employ.
Examples of such models are the relativistic local and global Fermi gas (RFG) and a relativistic mean field (RMF) approach, where one employs a fully relativistic description of the neutrino-nucleus interaction as well as the nuclear dynamics.
The goal of this thesis is, in the first place, to investigate the role of relativistic effects in neutrino-nucleus scattering. Secondly, we wish to take a look at the modeling of final-state interactions (FSI). This will need to be implemented into the code, after which a comparative study can be made into its effects on the model predictions. The goal of this thesis subject is to start a comparative study between these Monte Carlo simulations and the models developed in our own group. Afterwards attention will be given to experiments such as ArgoNeuT and MicroBooNe. Contrary to experiments from the previous generation these ones also measure, besides the muon, the knocked-out nucleons. Are the Monte Carlo simulations also suitable to predict these semi-exclusive measurements? Experimental results aren't yet available, but our own models are suitable for a comparison. This thesis subject offers the opportunity to spend a research stay in the neutrino research group of Wroclaw University, originators of the NuWro generator.
A thorough understanding of the interaction between neutrinos and nuclei is essential in the interpretation of oscillation experiments. In our research group theoretical models are developed to explain experimental results, but in the scientific process, besides experiment and theory, there's a third aspect: computational simulations. These simulations form a bridge between theoretical models and experimental results. GENIE (Generates Events for Neutrino Interaction Experiments) and NuWro are two state-of-the-art neutrino event generators that are freely available. These are used by several experimental collaborations such as MiniBooNE, MINERvA and T2K to assess the feasability of the proposed experiments, to design the detectors and to determine the efficiency of these detectors.
The goal of this thesis subject is to start a comparative study between these Monte Carlo simulations and the models developed in our own group. Afterwards attention will be given to experiments such as ArgoNeuT and MicroBooNe. Contrary to experiments from the previous generation these ones also measure, besides the muon, the knocked-out nucleons. Are the Monte Carlo simulations also suitable to predict these semi-exclusive measurements? Experimental results aren't yet available, but our own models are suitable for a comparison.
This thesis subject offers the opportunity to spend a research stay in the neutrino research group of Wroclaw University, originators of the NuWro generator.