Outer Detector of Hyper-Kamiokande

TL;DR

The paper discusses the selection and evaluation of photomultiplier tubes for Hyper-Kamiokande's Outer Detector, demonstrating effective cosmic-ray muon background reduction crucial for neutrino and nucleon decay studies.

Contribution

It compares PMT candidates and presents simulation results showing the background rejection performance of the Outer Detector.

Findings

Hamamatsu R14374 PMTs chosen for their efficiency and stability.

Cosmic-ray muon reduction inefficiency reaches 10^{-6} with OD cuts.

Combined cuts reduce background to 10^{-9}, negligible for analyses.

Abstract

Hyper-Kamiokande (HK) is the world's largest water Cherenkov ring-imaging detector, planning to start data taking in 2028. The Outer Detector (OD) surrounds the Inner Detector and plays a critical role in rejecting background events entering from outside, particularly cosmic-ray muons. We report on the selection of $8\,\mathrm{cm}$ diameter photomultiplier tubes (PMTs) for the OD, comparing Hamamatsu R14374 and NNVT N2031 candidates, and present the evaluation of cosmic-ray muon background reduction performance using a full detector simulation. Hamamatsu PMTs were adopted for their superior in-water detection efficiency in deep-UV and stability. The cosmic-ray muon reduction inefficiency reaches $O(10^{-6})$ with OD-based cuts alone, and $O(10^{-9})$ is expected when combined with fiducial volume cuts, which is sufficiently negligible for nucleon decay and atmospheric neutrino analyses.