Ph.d. degree
University of Bern, Swiss, 2021

Patrick Koller

Supervisor: Prof. Dr. Michele Weber

A Method to Improve the Neutrino Energy Reconstruction in LArTPCs

Keywords:

• Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)

Abstract:

Precision measurements of the neutrino oscillation parameters allow to determine the potential violation of CP-symmetry in the leptonic sector. If the violation of CP-symmetry in the leptonic sector can be experimentally confirmed, this could become the preferred explanation in the Standard Model for the matter-antimatter asymmetry of the universe. The Deep Underground Neutrino Experiment (DUNE), which is being built by the Fermi National Accelerator Laboratory (FNAL) in the USA, aims to perform these measurements of the neutrino oscillation parameters using Liquid Argon Time Projection Chamber (LArTPC) detectors. Four massive multi-kt Far Detectors (FDs) are deployed together with a Near Detector (ND) that uses the same detector technology as the FDs. As a consequence of the high-intensity neutrino beam and the short distance to the neutrino source the ND will need the ability to disentangle multiple overlapping events that happen within a single readout cycle. This is achieved with a LArTPC following the ArgonCube concept, which was developed at the University of Bern. The ArgonCube concept divides the detector volume into a number of identical, optically isolated and electrically isolated Time Projection Chamber (TPC) modules. The optical isolation reduces the pile-up of the scintillation light and allows for better localization of light signals. The ND will need to measure the neutrino energy because neutrino oscillations occur as a function of the neutrino energy. Neutrons emerging from the neutrino interaction vertex can carry away more than 25 % of the neutrino energy, which is missed by applying the calorimetric method, because neutral particles are not reconstructed in LArTPCs. I have determined the bias and the uncertainty of the reconstructed neutrino energy due missed primary neutrons, based on simulations using the GENIE event generator. At the DUNE beam spectrum, primary neutrons will be involved in about 80 % of the neutrino-argon interactions. In those events with primary neutrons involved, a mean energy corresponding to ∼10 % of the parent neutrino’s kinetic energy is carried away by primary neutrons, and the relative uncertainty on the reconstructed neutrino energy can exceed 13 %. I have developed a method to identify neutrons in LArTPCs by secondary charged particles that are produced if a neutron interacts with an argon nucleus. These neutrons appear as detached energy deposits, which prevents a trivial assignment to the correct interaction vertex, given the high event multiplicity at the ND site. Therefore, I further developed a method to assign detached energy deposits to neutrino interaction verices, exploiting the fast response of the light-readout systems. With a timing resolution at the O (1) ns, the light-readout systems easily allow to separate the light signals of individual events, which have a mean separation time of 179 ns. If the presented methods are used to veto neutrino interactions with primary neutrons, then the respective uncertainty on the reconstructed neutrino energy can be reduced to < 10 %.