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. In this thesis project, the cross section for coherent neutrino-nucleus scattering is modeled and its importance for astrophysical neutrinos investigated. The influence of nuclear parameters as e.g. the strange-quark content of the nucleon will be examined.
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 energy 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 correlations between the nucleons in the nucleus on the reconstructed energy. Work on this thesis involves a mix of weak interaction physics, nuclear scattering theory and numerical work.
Neutrinos interact with matter via the weak interaction. To make any progress in the determination of neutrino-oscillation parameters one needs to understand the neutrino-nucleon reaction mechanisms. For that, it is essential to have a good knowledge of the weak properties of the nucleon : weak charge, axial and vector form factors, structure of the currents, etc. The use of neutrinos as probes to study weak properties of matter has a huge drawback : monochromatic neutrino beams are not available. This makes it difficult to extract precise information from data due to the lack of control on the kinematics involved in the experiments. Alternatively, electron beams have been used for decades with unbelievable precision and success. Electrons interact via the electromagnetic (EM) interaction (mediated by the exchange of virtual photons) and via weak neutral current (WNC) interaction (mediated by the exchange of Z bosons). The EM interaction is approximately 5 orders of magnitude stronger than WNC interactions, which makes it truly challenging to demonstrate effects related to the weak interaction. For this, one needs observables whose presence is unequivocally due to the weak interaction. In this project the student will study one of these observables: the "parity violating asymmetry" for electron-nucleon scattering in the pion-production region. If one only considers the EM interaction, the electron-nucleon scattering probability for a positively polarized electron beam (positive helicity) is exactly the same as the scattering probability for a negatively polarized electron beam (negative helicity). Thus, the process is said to conserve parity. However, when taking into account the WNC interaction this probability is not the same, i.e., the process does not conserve parity. The parity violating asymmetry uses this property of the weak interaction to reveal information about the axial structure of the matter. In particular, this project focuses on the kinematic region around the pion production threshold. This will allow one to study the axial form factors of the Delta resonance which are essential input for any theoretical model which aims at predicting neutrino nucleon (neutrino-nucleus) cross sections at intermediate energies. First, the student will have to deal with the formalism of electron-induced pion production on nucleons considering both EM and WNC interactions. This will provide all the ingredients for the investigation of the parity violating asymmetry. Then, the student should study the current experimental and theoretical status of the topic. The last step in this thesis work will be the implementation of this model in a C++ code in order to produce theoretical predictions to be compared with recent experimental data.
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, Minerνa 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.