Sensitivity of Hyper-Kamiokande to sub-eV Sterile Neutrinos

In this work, we investigate the sensitivity of Hyper-Kamiokande (Hyper-K) to light sterile neutrinos within the $(3+1)$ framework, consisting of three active and one sterile neutrino state. We focus on the regime where the new mass-squared splitting satisfies $\mathrm{\Delta }{m}_{41}^2\lesssim 1$ eV${}^2$, a parameter space complementary to short-baseline sterile-neutrino searches. Using both accelerator and atmospheric neutrino samples, we evaluate the expected capability of Hyper-K to constrain active-sterile mixing. Our results show that Hyper-K can significantly improve current bounds on sterile-neutrino parameters and achieve sensitivity that is competitive with that of future dedicated experiments.