Flavor-Dependent Long-Range Neutrino Interactions in DUNE and T2HK: Synergy Breeds Power

Discovering new neutrino interactions would provide evidence of physics beyond the Standard Model. We focus on flavor-dependent long-range neutrino interactions mediated by ultra-light mediators (masses below ${10}^{-10}$ eV) from lepton-number gauge symmetries $L_e-L_{\mu }$, $L_e-L_{\tau }$, and $L_{\mu }-L_{\tau }$. These interactions, sourced by electrons and neutrons in the Earth, Moon, Sun, Milky Way, and the local Universe, could modify neutrino oscillation probabilities. The upcoming long-baseline experiments, DUNE and T2HK, with their large statistics, reduced systematic uncertainties, and well-characterized neutrino beams, will probe these interactions. We forecast that, while individually DUNE and T2HK could constrain these long-range neutrino interactions, their combination lifts parameter degeneracies that weaken individual sensitivity and provides stronger constraints.