Near detector simulation studies for CP violation discovery with ESSnuSB

Abstract: The neutrino oscillation experiment ESSnuSB aims to measure the CP violation phase in the lepton sector with an intense neutrino beam produced at ESS. If the CP violation is discovered, then the matter-antimatter asymmetry of the Universe can be possibly explained. In order to do this, a design study of a water Cherenkov near detector was carried out, and its geometrical parameters such as the tank size and the PMT size were evaluated using a dedicated simulation software and a previously developed reconstruction algorithm. The reconstruction results show that the electrons are less likely to be misidentified than the muons, and the misidentification percentages for electrons were highly dependent on the tank size rather than the PMT size. On the other hand, the misidentification percentages for muons were independent of both the tank size and the PMT size. Instead, other geometrical effects played a more dominant role in particle misidentification. In order to lower the misidentification percentages, more sophisticated selection cuts are suggested. Finally, 2D mappings of the input lepton energies versus the reconstructed lepton energies, known as migration matrices, were constructed for both muons and electrons. Dedicated tunes of the modified detector geometries may address a discrepancy between the input lepton energy and the reconstructed energy, while improving the particle identification accuracy. These results provided guidance on more comprehensive design studies of the near detector.