Towards Measuring the CP Phase with Atmospheric Neutrinos

Stephan Meighen-Berger, PhD

One of the highest-priority goals in neutrino physics is testing for CP violation in neutrino mixing. Accelerator long-baseline experiments will measure the CP phase through neutrino versus anti-neutrino comparisons that are sensitive to sin δCP. We propose a new approach, using atmospheric
neutrinos, that is sensitive to cos δCP. Employing an up-down ratio that cancels many uncertainties, and accounting for realistic detector effects in the Hyper-Kamiokande experiment, we define a robust observable that depends on cos δCP at the ∼5% level. We demonstrate that this approach can enable accurate measurements of δCP, even where accelerator experiments have lower resolution, particularly near δCP = π/2 and 3π/2.