Understanding neutrino interactions and oscillations with off-axis neutrino beams

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Explaining the observed excess of matter compared to antimatter in the early universe is one of the biggest questions in physics. Neutrino oscillations can violate the CP symmetry, potentially by an amount large enough to produce this excess. Understanding neutrino oscillations is an essential step to understanding the universe we see today.Hyper-Kamiokande will use a 200-kiloton water Cherenkov detector to measure neutrino oscillations with unprecedented statistical precision. The challenge now is to reduce the systematic uncertainties of Hyper-Kamiokande to ensure the success of its oscillation measurements. Addressing this challenge is the focus of my fellowship.The dominant systematics in long-baseline oscillation experiments are due to the difficulty in relating what is observed in the detector to the neutrino energy. The E61 experiment has been designed to measure neutrino interactions over a range of angles off the J-PARC neutrino beam axis. The peak energy of the neutrino beam decreases as the off-axis angle increases, allowing E61 to directly relate neutrino energy to what is seen in the detector. This link enables E61 to produce a data-driven mapping between neutrino energy and the signatures observed in the detector, significantly reducing the systematic uncertainty associated with this.The E61 method requires a detailed understanding of the E61 detector, in particular the detector fiducial volume. To achieve this I will produce an optical calibration system for the E61 detector that will both map the detector volume and measure the detector response to a known signal. The system will deploy calibration sources and high-resolution cameras within the detector to build a 3D model of the full apparatus, producing an in-situ measurement of both the detector response and geometry, rather than relying on ex-situ measurements of one to calculate the other. This will be developed using a staged approach, with a prototype system installed in the E61 test beam experiment. The prototype will provide essential feedback for the full calibration system, whiile the test beam will provide valuable physics data for the calibration of water Cherenkov detectors.The research that this fellowship enables will address the key challenges in neutrino oscillation physics in two new ways: the use of off-axis beams to understand neutrino interactions and a novel calibration system to understand water Cherenkov detectors. Together these will produce the world's most sensitive search for CP violation in neutrino oscillations.