An intermediate water Cherenkov detector for the Hyper-Kamiokande experiment: overview and status

Abstract

The proposed Hyper-Kamiokande (Hyper-K) is a next generation underground large water Cherenkov (WCh) detector. Hyper-K will act as a far detector to measure the oscillated neutrino flux from the long-baseline neutrino experiment using 0.6 GeV neutrinos produced by a 1.3 MW proton beam at J-PARC in Japan. It has a broad program of physics and astrophysics mainly focusing on the precise measurement of the lepton mixing matrix and the leptonic CP asymmetry. The main systematic uncertainties on these measurements arise from uncertainties on the modelling of neutrino-nucleus interactions. To minimise these uncertainties an intermediate WCh detector, E61, has been proposed. E61 aims to investigate the relation between the neutrino energy and the lepton kinematics using measurements of final state muons at many different off-axis angles. The detector will use a new technique to enhance its performance, neutron tagging. The water in the tank will be doped with Gd that allows to tag neutrons. Tagging events by the presence and number of final-state neutrons provides a unique capability to discriminate between different (anti)neutrino interactions. A neutron multiplicity spectrum will be measured aiming to improve the current knowledge of neutrino nucleon interactions and thus help in the background reduction as well in the systematic uncertainty reduction at Hyper-K. We will discuss the expected physics reach and the status of the E61 experiment focusing on the benefits from the off-axis spanning and neutron tagging techniques.