Opportunities for a master's thesis in neutrino physics and nuclear astrophysics
A double differential neutrino-nucleus scattering cross section

The experimental confirmation of neutrino oscillations -which 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 angles. 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.

The neutrino group at Ghent University is part of the worldwide NuSTEC (Neutrino Scattering Theory-Experiment Collaboration) network, and is involved in vivid collaborations with other research groups working in neutrino-nucleus physics. In the context of the thesis work in this group, there is ample opportunity for research stays at foreign institutions (e.g. IRFU Saclay, University of Madrid, University of Geneva, Wroclaw University)


Coherent scattering of supernova neutrinos
Supervision : Alexis Nikolakopoulos, Natalie Jachowicz

The coherent cross section is important for supernova physics

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. In particular, this is 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:

  • Model the cross section for coherent neutrino-nucleus scattering.
  • Examine its importance for astrophysical neutrinos.
  • Investigate the influence of nuclear parameters (e.g. the strange-quark content of nucleons).


Nuclear effects in energy reconstruction for neutrino-oscillation studies
Supervision : Alexis Nikolakopoulos, Natalie Jachowicz
The effect of multinucleon knockout in energy reconstruction.
From Nieves et al. [Phys. Rev. D 85, 113008]

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. The incoming neutrino energy has to be reconstructed from experimentally measurable variables which do not fully specify the kinematics of the process. This means that the interpretation of reconstructed variables strongly depends on the understanding of the interaction of the neutrino with the nucleus.

The reconstructed variables are usually interpreted by using Monte Carlo generators, which typically employ simplified interaction models. Using detailed microscopic models we are in the position to interpret the model-dependent relation between real and reconstructed variables. The effect on measurements of neutrino oscillation parameters can be large. Therefore this subject attracts a lot of interest from within the experimental community.

For example, the unrealistically large charged-current quasi-elastic cross section measured in the MiniBooNE was partially explained by including the effect of multinucleon knock-out processes. This reaction channel skewed the distribution of reconstructed energies leading to an excess of events for low values of the reconstructed energy. Using a mean-field approach for the description of the nuclear structure tends to skew the distribution in a similar way, which might be important in the interpretation of the electron-neutrino excess found at low reconstructed energies.

Who knows how many more of these reconstruction artifacts are present in current data?


Neutral current single pion production at high energies
Supervision : Alexis Nikolakopoulos, Natalie Jachowicz

In neutrino-nucleus scattering experiments, neutrinos span a broad energy range. Neutrino-induced single pion production constitutes a large part of the signal in present and future oscillation experiments. Depending on the energy of the process, characterized by the hadronic invariant mass (W), different approaches are used.

At low hadronic invariant mass, single pion production can be modeled by combining background diagrams for the pion-nucleon system with the excitation and subsequent decay of the Delta resonance. For higher energies, an increasing amount of resonances, and higher-order background diagrams would have to be taken into account. An approach that quickly becomes intractable.

We therefore describe electroweak pion production at sufficiently high energies by making use of an approach based on Regge theory. In general, a transition amplitude for a hadronic process can be described by an infinite summation of all partial waves in the t-channel. This infinite sum is cast into a contour integral in the complex angular momentum plane. By doing this, Regge theory provides one with the s-dependence of the amplitude, but does not predict the t-dependence of the residues.

The “Reggeization” procedure used in the high energy model, dubbed the ReChi model, is an approximation in which the t-dependence of low energy background diagrams is used by replacing the t-channel propagators of exchanged mesons by a Regge propagator.

The charged vector current, and axial current are Reggeized by identifying the pion, and rho-meson exchange as the leading Regge trajectories. This model is not complete, for example neutral current pion production is still lacking. Additional meson exchanges should be taken into account to fully describe electroweak pion production on the nucleon.

This thesis involves a study of hadron physics and Regge theory, and aims at extending the electroweak single pion production model by implementation of the procedure in a computational model.

Monte Carlo study of neutrino scattering off atomic nuclei at intermediate energies
Supervision : Natalie Jachowicz, Kajetan Niewczas
Two-particle knockout: experiment and theory

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 feasibility 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.


References:
[1] MiniBooNe website : http://www-boone.fnal.gov/.
[2] MINERνA website : http://minerva.fnal.gov/.
[3] T2K website : http://t2k-experiment.org/.
[4] Deep Undergound Neutrino Experiment (DUNE) website : dunescience.org.
[5] MicroBooNE website : http://www-microboone.fnal.gov/.
[6] Neutrino Scattering Theory-Experiment Collaboration (NuSTEC) website : nustec.fnal.gov.
[7] L. Alvarez-Ruso et al., NuSTEC White Paper, Progr. in Part. and Nucl. Phys. (2018).

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