Neutrino Physics in Present and Future Kamioka Water‐Čerenkov Detectors with Neutron Tagging

Abstract

This thesis is focused on the upgrade of the Super-Kamiokande detector, consisting in the addition of a salt of gadolinium into the water of the detector to enable a very high efficient capability to detect the neutrons produced in the detector: the SuperK-Gd project (former GADZOOKS!). This feature will improve largely the scientific power of SK because the neutron production is related to the matter-antimatter character of the interacting neutrino; charged current interacting neutrinos tend to produce less neutrons than their analogous antineutrinos. It also provides a new and powerful selection criterion for proton decay searches due to the fact that in a proton decay, should it exist, basically no neutrons are produced in the final state. In addition to these anticipated benefits, in this work other features are explored finding out new improvements due to neutron-tagging in various physics analyses. For the realisation of SuperK-Gd, an exhaustive R&D program has been carried out. It pivoted on the EGADS prototype and included many studies of different nature to ensure the feasibility and performance of Gd-doped water-Cherenkov detectors. This thesis covers works on EGADS, construction, calibration and monitoring, Gd-neutron tagging implementation, capability and impact on the neutrino physics on a wide variety of physics phenomena within a broad energy range and from different origins, including detectability of Diffuse Supernova Neutrino Background (DSNB), supernova burst neutrinos, pre-supernova neutrinos (from Si-burning phase), reactor antineutrinos and solar neutrinos, and also neutrino oscillation parameters using atmospheric and long baseline neutrinos. Since in the not-oo-distant future experimental neutrino physics will need to build even larger detectors to address the remaining or new arisen unknown properties of neutrinos, this thesis also studies their sensitivities for atmospheric and long baseline neutrinos for the Hyper-Kamiokande project with neutron tagging. In order to proof the relevance of neutron tagging in large water-Cherenkov detectors and confirm the studies with actual data, a complete and global analysis of SK atmospheric neutrino data is performed including the previously developed neutron tagging tools applied to the 4th phase of Super-Kamiokande, which is already capable of detecting a low, but already useable, fraction of the neutrons produced through hydrogen-neutron captures. It provides the most updated oscillation analysis on the atmospheric oscillation parameters of Super-Kamiokande (Note: results are preliminary).