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 ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>3</a:mn> <a:mo>+</a:mo> <a:mn>1</a:mn> </a:mrow> </a:math> ) framework, consisting of three active and one sterile neutrino state. We focus on the regime where the new mass-squared splitting satisfies <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi mathvariant="normal">Δ</c:mi> <c:msubsup> <c:mi>m</c:mi> <c:mn>41</c:mn> <c:mn>2</c:mn> </c:msubsup> <c:mo>≲</c:mo> <c:mn>1</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:msup> <c:mrow> <c:mi>eV</c:mi> </c:mrow> <c:mrow> <c:mn>2</c:mn> </c:mrow> </c:msup> </c:math> , 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.