Towards Measuring the CP-Violating Phase with Atmospheric Neutrinos

We propose a new approach to measuring the CP-violating phase in neutrino mixing using atmospheric neutrinos, differing significantly from prior work. We develop an up-down flux ratio for sub-GeV atmospheric neutrinos that incorporates realistic detection effects and reduces systematic uncertainties. For the example of Hyper-Kamiokande – the first experiment with sufficient atmospheric-neutrino statistics in this energy range – our approach can surpass the sensitivity of the Tokai to Hyper-Kamiokande (T2HK) long-baseline experiment near ${\delta }_{\mathrm{C}\mathrm{P}}={\mathit{90}}^{\circ }$ and ${\mathit{270}}^{\circ }$. Realizing this potential will require additional, but realistic, work to reduce theoretical uncertainties. Success will provide an important, complementary probe to multi-$1B accelerator-based experiments.