Neutrino physics in present and future kamioka water-cherenkov detectors with neutron tagging

This thesis is focused on the upgrade of the Super-Kamiokandedetector, 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- antimattercharacter of the interacting neutrino; charged currentinteracting neutrinostend to produce less neutrons than their analogous antineutrinos. It also provides a new and powerful selection criterion for proton decaysearches 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 neutrinophysics on a wide variety of physics phenomena within a broad energy range and from different origins, including detectability of Diffuse Supernova NeutrinoBackground (DSNB), supernova burst neutrinos, pre-supernova neutrinos(from Si-burning phase), reactor antineutrinos and solar neutrinos, and also neutrino oscillationparameters using atmospheric and long baseline neutrinos. Since in the not-oo-distant future experimental neutrinophysics 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 neutrinosfor the Hyper-Kamiokande project with neutron tagging. In order to proof the relevance of neutron tagging in large water- Cherenkov detectorsand confirm the studies with actual data, a complete and global analysisof SK atmospheric neutrinodata 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).