Understanding neutrino oscillations and interactions with off-axis neutrino beams

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 is to reduce the systematic uncertainties of Hyper-Kamiokande to ensure the success of its oscillation measurements. Addressing this challenge has been the focus of my fellowship to date and is the continued focus of this renewal.

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 IWCD 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 IWCD to directly relate neutrino energy to what is seen in the detector.

This link enables IWCD 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 IWCD method requires a detailed understanding of the water Cherenkov detector, in particular the detector fiducial volume. In the first fouyr years of this fellowship I have designed, produced and characterised a novel optical calibration system that is currently being deployed at the WCTE experiment at CERN. The data from this calibration system and WCTE will demonstrate the ability to achieve percent-level uncertainties in neutrino event reconstruction using a water Cherenkov detector.

WCTE will also provide new measurements of lepton scattering on oxygen, a crucial input to producing more the accurate neutrino interaction models required for Hyper-Kamiokande.

This fellowship originally proposed to address the key challenges in neutrino oscillation physics in two new ways: development of a novel calibration system to understand water Cherenkov detectors and the use of off-axis beams to understand neutrino interactions. I have accomplished the first of these goals during the initial four years of the fellowship and this renewal will allow me to accomplish the second.

Together these will produce the world's most sensitive search for CP violation in neutrino oscillations.