Understanding neutrino interactions and oscillations 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 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.

Water Cherenkov detectors are a key tool in many areas of high energy physics, allowing researchers to instrument large volumes at a reasonable cost. These detectors led to the discovery of neutrino oscillations and the Nobel prize in Physics in 2015. This has had a worldwide impact on the high energy physics community, leading to new theoretical models describing the fundamental particles and forces of nature alongside new experiments probing neutrino phenomenology.

These detectors are now being used to pursue some of the biggest questions in science today: testing the fundamental symmetries of nature, uncovering the evolution of stars and searching for dark matter, the mysterious substance that makes up most of the matter in the universe. Pushing the boundaries of detector performance will impact research in all of these fields and a new discovery in any one of them would have huge scientific and societal impact.

The unique potential of the E61 experiment can also benefit a wide section of the neutrino community. First, precision neutrino cross-section measurements will drive the development of interaction models, which will be used by both next-generation long-baseline oscillation experiments, DUNE and Hyper-Kamiokande. Beyond this, E61 can reproduce the atmospheric neutrino flux, allowing it to constrain the major background to proton decay searches at Hyper-Kamiokande. Finally, E61 has a good sensitivity to sterile neutrinos with masses near 1eV (the LSND anomaly). Observation of a sterile neutrino, a new class of particle, would be a major discovery in fundamental physics. The E61 experiment will begin data taking around 2025, and so the benefits to other parts of the neutrino community will be realised then. The E61 test beam and calibration work will provide impact within the community much faster, with results expected within the first four years of this fellowship.

Precision mapping of 3D objects is essential in many fields, from archaeology to medical research, geography to automated manufacturing. Photogrammetry provides a very cost effective and accurate tool for these fields, but it has limitations, mainly to do with the objects being mapped. Improvements to the mapping of reflective and transparent objects has the potential to extend the use of photogrammetry to new academic disciplines and industrial projects, providing the benefit of a quick, low cost 3D mapping solution. This work is expected to be largely complete after 4 years of the fellowship but will provide impact throughout the development process.

Outside academia, fundamental research into the nature of our universe is a driving force behind the technological development that continues to advance society. This type of research inspires the scientific interest of students, leading them to develop and apply analytical skills to difficult problems. These skills are necessary and highly sought after in industry as well as across wider society. The observation of neutrino oscillations has enthused young people about the importance and excitement of science, the impact of which is being realised now. As experiments make further discoveries this effect will strengthen providing ongoing impact beyond the scale of the fellowship.

Openness within research is essential for the health of the field. As such the work produced during this fellowship will be published in open-access, peer-reviewed journals, making it available to everyone.