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 δCP=90∘ and 270∘. 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.
@article{arxiv.2605.16721,
title = {Towards Measuring the CP-Violating Phase with Atmospheric Neutrinos},
author = {John F. Beacom and Nicole F. Bell and Matthew J. Dolan and Stephan A. Meighen-Berger and Ho Man Yim},
journal= {arXiv preprint arXiv:2605.16721},
year = {2026}
}